FROM THE SUN TO THE STARS

The OLLI Lectures

By Jim Kaler

Department of Astronomy, University of Ilinois

From the STARS pages

For more detailed discussions, see The Natures of the Stars and Spectra.

Table of Contents

WHAT IT'S ALL ABOUT

The following is a narrative to the eight OLLI lectures given at the University of Illinois in 2010-12. The presentation of a lecture with slides is not quite the same as a written version. Nevertheless, the overall pattern follows what was done in voice and images. We start with the Sun, including its nature, spectrum, activity, and effect on Earth. We then expand outward into the Solar System to visit the planets, their orbits, and the leftovers of their formation. From the outermost debris, we launch ourselves to the stars to examine their natures, their lives, and their deaths. This discussion is introduced with a brief foray into HOW we learn about celestial things through our telescopes. At the end, see that our Earth is a product of the Universe at large, whereupon we turn around to search for planets and life elsewhere while at the same time appreciating ourselves and what we have been given.

I. SUN

The Sun. A glowing ball everyone appreciates (especially when missing it on a rainy day) and no one dares look at except perhaps when rising or setting in horizon murk or when behind sufficiently thick clouds. It's just too bright. But without its intense illumination, there would be no life, as nearly all the energy we live by comes from solar power, from farm products to fossil fuels that descend from ancient plants. Its simple facade hides a magnificent nuclear engine that has glowed at close to its current brightness for nearly five billion years and has another five billion to go before it begins to die away. Which is a story for later, as is its place among its billions of siblings, seen at night as all the other stars in a blackened sky.

Stats

The Sun looks solid, but it's gaseous throughout. The apparent razored edge is caused by highly opaque solar gases: you just can't look very far into the apparent solar "surface" any more than you can into a fair weather cloud. Everything about the Sun is huge, to the point that seemingly dry numbers take on lives of their own. The opaque "surface" -- the "photosphere" -- glows with a temperature of some 5500 degrees Celsius (almost 10,000 degrees F), or 5780 degrees on the absolute "Kelvin" scale, which counts Celsius degrees upward from absolute zero, -273 degrees C. It's 150 million kilometers (93 million miles) away, 400 times farther than the Moon, so far that at freeway speeds it would take 150 years to drive there. Some 1.4 million km (860,000 miles) across, once you got there in your highly insulated car, it would take you another decade to drive around it looking at the sights: which include bubbling gases, gigantic magnetic ropes, immense explosions, and deep dark magnetic valleys with slippery slopes, many of which can dwarf the Earth (which is little more than a speck 100 times the solar diameter away and but a hundredth the solar size).

Many of the numbers are so large that we can't easily write them out. We need a shorthand. Big numbers are usually expressed through exponents, written here as **. As examples, 4 = 2**2 (2 X 2, two squared), 100 = 10 X 10 = 10**2. A number like 10,000 is 10**4, 50,000 then being 5 X 10**4. Using the shorthand, on a pretend Earthly scale the Sun weighs in at 2 X 10**30 kilograms (2 followed by 30 zeros), or 2 X 10**27 (two thousand trillion trillion) metric tons. It radiates at a rate of 4 X 10**26 (four hundred trillion trillion) watts. To run the Sun for one second, you would have to pay your power company the gross domestic product of the US for a million years. Even at its great distance, with the Sun overhead it delivers energy at a rate of nearly 1400 watts per square meter of ground, shining to us here on Earth half a million times brighter than the nearby Moon.

Very unlike the Earth and excluding the nuclear engine at the core, the Sun (like most stars) is made of 90 percent hydrogen, 10 percent helium, and a tiny fraction of everything else, including the iron, silicon, and carbon of which the Earth is composed, making us actually rather special in spite of our small size. How do we know? To find out, we must peel sunlight apart.

A Sun of Many Colors

Sunlight looks to be a bit on the yellowish side of white. It actually shines an amalgam of continuous colors from red through orange, yellow, green, blue, violet, and all the shades between, as demonstrated by Isaac Newton in the 17th century when he passed a sunbeam through a refracting prism onto a cloth to reveal the solar spectrum. The eye then re-assembles the colors, merging them back into a visual yellow-white.

Light consists of a flow of electromagnetic energy that can be visualized as electric and magnetic waves moving at the "speed of light," in vacuum at 300,000 kilometers (186,000 miles) per second. It's the speed limit of the Universe. Waves to which the eye is sensitive have very short distances ("wavelengths") between their crests, under 0.0001 centimeter or so (2.5 centimeters to the inch). Those with the shortest wavelengths appear as violet, those with double that wavelength as red, with the progression of other colors in between. When passing from air into glass or water, the waves slow and bend, or "refract," toward the perpendicular, shorter violet waves more than longer red ones. Emerging, they speed up and bend again, separating the colors even more: hence Newton's spectrum.

Up in the Air

The solar spectrum plays out in many attractive ways, most notably as the rainbow. An afternoon thunderstorm blows off to the east, allowing sunlight to fall upon its raindrops. Single one out. As a sunray enters the droplet, it bends and splits into its component colors. They all reflect off the drop's backside, then exit and separate yet more, the reversal in direction sending the split ray into your eye. The combination of all drops, combined with the properties of water, creates a circular colored bow 42 degrees in radius around the point below the horizon opposite the Sun, with red on the outside, blue or violet (because they refract more) on the inside. Two reflections inside the drops produce a second, larger bow outside the bright one with colors reversed. Sometimes you can see a series of pink "supernumerary" bows inside the main one that are caused by light waves interfering with one another, in one direction adding up, in another canceling each other out. If the Sun is too high in the sky, the point opposite the Sun will be too far below the horizon, and the bow cannot be seen. Rainbows thus appear in the early morning or late afternoon when the Sun is low enough to loft the bow into visibility against the sky.

Separation of solar colors is a natural part of the day. Why is the sky blue? Another view of light is that it consists of speeding particles, "photons." The modern concept in the weird world of quantum mechanics is that light is both a wave and a particle at the same time. You might think of a photon as a chunk of a wave, though that's not a very accurate description. Some of the incoming solar photons bounce off air molecules (mostly nitrogen and oxygen), which changes the photons' directions. The process is far more efficient at shorter wavelengths that are closer to the molecules' sizes. Violet photons (they are not colored; that is just the effect they have on the eye) are fiercely scattered, while longer red photons are not affected much at all. Blue and violet photons then get knocked all over the sky, resulting in some coming into your eye from any direction. But there are relatively few violet photons in sunlight, nor is the eye very sensitive to them, so most of the scattered photons we see are blue. Hence the glorious azure sky.

The complement to a blue sky must be a red sunset. The sky looks as if it is a dome over your head. It isn't. The air is actually a thin layer that hugs the ground and fits to the curvature of the Earth, the density and pressure dropping off quickly with height (as anyone who climbs mountains will tell you). Toward overhead, you see to the outside through the thinnest part of the layer. As your gaze drops downward, you have to look through more and more air. At 30 degrees up from the horizon you see through double the overhead thickness, while at the horizon you look through 38 times more air than when you look directly up. As a result, the light of the setting Sun has a longer pathway for it to be scattered and, with more blue light removed, the Sun turns redder. The effect is strongly enhanced by the absorption of solar photons by the air, watery haze, and pollution, both natural (by volcanos) and artificial. We thus see wonderful red sunsets, which increase in intensity as the Sun drops below the horizon.

Ice also refracts. Ice crystallizes into six-sided hexagons (remember the snowflake on your mitten?) that form tiny prisms. Put light ice clouds in front of the Sun, and they can create a colored ring 22 degrees (the bending angle through the sides of the hexagon) in radius with the Sun in the middle. Because the bending angle increases with decreasing wavelength, the halo is red on the inside and blue on the outside. (Be sure to hide the Sun behind something so as not to look at it directly.) As the Sun sets, spots on the ring on a line through the Sun parallel to the ground grow in intensity, until these "sundogs, "mock suns," dominate. A white ring parallel to the ground caused by reflection can in rare instances extend through the sundogs and encircle the entire sky. Since people can readily look at the Moon, the ring and its "moondogs" are more commonly seen around the lunar disk. Refraction through the square sides of the icy prisms can produce a rare ring 47 degrees in radius. Various arcs tangential to the rings can paint the blue canvas as well. Most are related to the 22-degree ring, though one, the "circumhorizontal arc," can sometimes be seen attached to the lower edge of the 47-degree ring even when 47 itself is not visibly there, leaving a sort of rainbow floating in the sky beneath the Sun.

Among the more familiar spectral phenomena is a tight, often- colored ring contiguous with the Moon's disk again caused by the interference among the wave-like photons as they pass among the water or ice particles on their way to you. The colors are reversed from that of the large halo, with blue now on the inside, red on the outside. You see the same thing through foggy glasses around bright lights. Great fun can be had by looking out an airplane window. Here we might see especially intense sundogs, as well as the pure reflection of the Sun off light icy clouds below the craft, making a "subsun" that follows along with you, the clouds sometimes so transparent as to be practically invisible.

If you are positioned right, you might see the shadow of the plane cast on the clouds below. Interfering light waves combined with reflection can produce a colorful halo, the "glory" (the "pilot's rainbow"), around it. If the clouds are far away, the airplane's shadow becomes lost, leaving nested rings floating in the air below.

Back to White

Now blend the colors back to the yellow-white glow of pure sunlight. As the Sun rises or sets across a lake or ocean, it throws forward a "glitter path," making it look as if you could walk into space. It's caused by reflection of sunlight on the irregular waves. Tip it upside down, where the "ocean" is made of atmospheric ice crystals that form light clouds. Now the glitter path seems to rise upward into the sky as a "sun pillar."

Clear air not only scatters, but as a "substance" (albeit a light one), also refracts. As sunlight enters the layer of air above us, it bends downward. All celestial objects as viewed from Earth are therefore lifted upward, appearing higher in the sky than they really are. As we look toward the horizon, where the angle between incoming sunlight and the air layer decreases and the path length through which we look increases, the effect is powerfully magnified. At the horizon, 90 degrees from overhead, things are lofted upward by half a degree, which is the angular diameter of the Moon and Sun. See the Sun (when safe, through haze or clouds) sitting on a flatland or ocean horizon. If you could magically remove the Earth's air, the Sun would drop from sight! The effect extends daylight hours by several minutes, depending on latitude.

Again seat the Sun on the horizon. The bottom of the solar disk is refracted upward more than the top, which squashes the solar disk into an oval that gets rounder as the Sun rises. Since the lower part of the rising or setting Sun must pass through more air, it will have more short waves removed and will be redder than the upper part. The Moon behaves the same way, but is harder to see when just rising or setting. (Along with refraction goes the dispersion of colors. The short-wave "image" of the Sun will be refracted upward more than the longer. But violet and blue are scattered out, leaving the setting or rising Sun with a brilliant green upper rim. As the last thing to disappear over a flat horizon (best over the ocean), it produces the "green flash" beloved of bar loungers in Key West.)

Our atmosphere is filled with watery haze that varies mightily from place to place and over time. Complementing it is ever- present dust blown upward from the ground by winds. The dust and water vapor are lit by reflected sunlight. A cloud in front of the Sun casts shadows. The effect is that sunrays seem to project through holes in the cloud or are cast into the sky from the cloud's ragged edges. The Sun is so distant that the sunrays are parallel to one another. But from the ground, perspective makes them seem to converge into the distance, and if conditions are right to create a glorious "sunburst" effect that appears to emerge from the solar disk. With the drab name of "crepuscular radiation," the effect is especially pretty when seen at or before sunrise when the air is moist and the rays seem to climb from Earth to Heaven. You'll see it a lot in grade-school art. Add to it a hidden Sun shining through the thin layer at the cloud's edge to create a "silver lining" and the results are, well, celestial. When the sunrays (really the intervening cloud shadows) project to the ground, folk philosophy says that the Sun is "drawing water" back into the sky. On the contrary, it is mostly the water already in the air that makes the effect possible.

Shadows are everywhere. What is night after all, but our being in the shadow not of clouds, but of the entire Earth. And you can watch it grow. As the Sun drops below the western horizon, the Earth's shadow rises as a gray band in the east that climbs higher until it sweeps over the entire sky like a dusky hood. Night does not "fall"; it rises. Above the band, the upper atmosphere still catches some sunlight and scatters it downward, giving us a peaceful period of twilight. You will see night "fall" in the morning during dawn, and then the Sun comes up and the grey band sets in the west at the moment the first glimmer of glorious sunlight comes over the horizon.

Moon and Sun

Sunlight also plays on the Moon, which orbits Earth just under once a month and shines strictly by reflected sunlight. Keep in mind that the Sun is 400 times farther than the Moon. The phases are caused by our seeing varying portions of the daylight lunar hemisphere as the angle between the Sun and the Earth-encircling Moon changes. When the Moon is between us and the Sun, we are presented with the nighttime side, and the Moon is "new" and invisible. Just after new, as the Moon orbits us we get our first glimpse of lunar daylight and the Moon appears night after night as a "waxing" (growing) crescent in western twilight. When the Moon and Sun are at right angles to each other (at the "first quarter" of the orbit) we see half the sunlit face, while the other half is in night. As we see more and more of the sunlit face, the Moon fattens into a "gibbous" shape, and when the Moon is finally opposite the Sun, we see all of the sunlit disk and the Moon is "full." Occasionally, the full Moon passes through the Earth's extended shadow, and we can admire an eclipse. It only happens one or two times a year, as the lunar orbit is tilted, and the full Moon usually passes above or below the shadow. Full Moon rising in the Earth's shadow is strikingly lovely. The apparent "hugeness" of the rising full Moon is but an illusion, as it is always the same half of a degree in diameter. The whole scene is then repeated in reverse, the Moon going through its waning gibbous shape to "third quarter," then to its shrinking morning crescent, until it returns again to new.

When the Moon is new, the Earth as seen from the Moon is full. When the Moon is early in its waxing crescent phase (in the evening seen during or just after twilight), a lunar visitor would see a large, highly reflective Earth that is so bright as to light the lunar night, allowing us here on Earth to view the whole ghostly lunar outline, the Moon's nighttime side aglow with "earthlight." We see the same thing in the morning near dawn when the Moon approaches new once again. And are there few more pleasing sights than the near full Moon tempting us with its glittery path across the sea: climb aboard.

Expanding the Spectrum

Look to the red end of Newton's rainbow, which deepens in color until it seems to evaporate. "Is that all there is?" Does the spectrum stop or do we just not see it? William Herschel found out in 1802 when he accidentally placed a thermometer off the red end of the spectrum and watched the temperature rise. He had discovered the INFRAred. (Herschel, a founder of modern astronomy, was a Hannoverian musician who emigrated to England in the late 1700s. Fascinated by astronomy, he built the finest of telescopes, discovered the planet Uranus, and with his sister Caroline catalogued thousands of celestial objects.) The infrared's longer wavelengths just do not register to the eye. The "IR" extends from off the red toward much longer waves until, at a wavelength of a millimeter or so, we arbitrarily call it "radio," a portion of the "electromagnetic spectrum" that was independently being explored in the early 20th century. With wavelengths that extend to kilometers, radio has no known outer limit.

Given Herschel's discovery, one might expect the deep violet to morph into something else as well. Sure enough, shorter than violet lies the ULTRAviolet (UV), to which our eyes are again insensitive. At wavelengths a few thousandths of times shorter than violet, we re-name the radiation "X-rays," and for those much shorter yet, "gamma rays," to which again there is no known short end.

The energy of a photon, be it X-ray, yellow light, or 10-km radio, depends strictly upon wavelength: the shorter the photon's wavelength, the more energy it carries. Visual photons, to which our air is blessedly (mostly) transparent, are energetic enough to heat the Earth, while infrared ones are more benign (felt as heat). Radio waves are almost totally harmless. The ultraviolet though can be damaging, while all know that X-ray and gamma-ray photons can be increasingly deadly.

If placed in a colder environment, bodies above absolute zero (- 273 C, -459 F) must radiate their internal energy away. If really cold, all a body can release is low-energy radio (which we see coming from the cold clouds of interstellar space). Warm it up to a few tens of degrees above absolute zero, and infrared radiation pours out. The Sun, at nearly 6000 Kelvin (Celsius degrees above absolute zero), produces copiously in the energetic visual spectrum. Hotter stars, in the tens of thousands of Kelvins, radiate most of their energy in the ultraviolet, and many are the ultrahot, X-ray, and gamma-ray sources. No matter where in the spectrum a body achieves its maximum radiative power, however, it also shines well at lower energies. The Sun, for example, sends us infrared and radio, plus for that matter quite a lot of UV (mostly from magnetic effects) to boot. The effect of temperature is more subtly seen through star colors. Cool stars appear reddish. As the temperature rises, we march through the spectrum toward shorter and more energetic wavelengths, from red to orange, yellow, white (substituting for green), then blue. Especially cool stars radiate just in the infrared, especially hot ones pour out ultraviolet in addition to all the lower wavelengths.

Fortunately, our protective atmosphere is mostly opaque to ultraviolet light thanks to absorption by a high layer of ozone. A little part of the ultraviolet spectrum just off the violet, however, sneaks through to cause tans and sunburns, which get worse the higher the Sun is in the sky thanks to the sightline's decreasing thickness. The infrared, however, is not blocked continuously as is the UV, but at a variety of spectral bands, allowing some natural lower-energy radiation to penetrate to the ground such that we can still see to the outside and practice infrared astronomy. The Earth, heated by sunlight to 300 or so Kelvin, radiates only in the infrared and radio domains. With low-energy radiation absorbed in the infrared by water vapor, carbon dioxide, methane, and other atmospheric ingredients, the air now acts as a blanket that helps keep in the heat, a phenomenon called the "greenhouse effect." It's necessary for life; without it, the Earth would probably be too cold to have sustained or even to have birthed it. Here though is the basis of the argument for human effects on global warming, as increasing the carbon dioxide (and methane) content should act to increase Earth's average temperature and change climate patterns.

Why is the air transparent to some wavelength bands and not others? For the answer we look to the atom.

The Very Small Explains the Very Large

The "chemical elements" are the fundamental substances out of which everything is made, and include such familiar items as hydrogen, carbon, iron, gold, and uranium. It all comes down to protons, tiny positively charged particles. They and neutral neutrons of about the same size and mass make up "atoms," more specifically the atom's core, or "nucleus." One proton and the atom is hydrogen, two and it's helium, 6 carbon, etc. The number of attached neutrons starts at zero for ordinary hydrogen then eventually increases a bit more rapidly than the number of protons. In spite of the repulsion of like charges, the protons are stuck together by a powerful short-range "strong force," as are the neutrons. This central "nucleus" is surrounded by a cloud of negatively charged electrons equal in number to that of protons to keep the whole affair electrically neutral. The 100 or so natural elements are stacked together in the chemist's famed "periodic table."

For a given element, different numbers of neutrons make different "isotopes," while removing electrons produces charged "ions." Normal hydrogen has just one proton and no neutrons, so it's H-1. Stick on a neutron and you have H-2 (deuterium). Too many or too few neutrons and the atom falls apart, becoming "radioactive," whence it emits dangerous particles and high energy electromagnetic radiation. The best known example is uranium- 238, 92 protons, 146 neutrons, which is feared when it is enriched with U-235 (92 and 143) to use in a nuclear power plant or an atomic bomb. Take an electron from an atom and you get a positively charged "ion." One removed from helium for example gives us He+, both removed, He+2.

Atoms of all elements both absorb and emit radiation in the form of photons, the job usually being handled by the electrons. But (and here is the key) at low pressures they do it only at particular energies peculiar to the atom or ion. In a hot high- pressure gas or solid, all the particular energies smear together make a pure unbroken spectrum. Put a low-pressure gas in front of a glowing higher temperature dense body, however, and the rarefied one will absorb particular very narrow colors to produce something of a "barcode" of dark "absorption lines" set against a bright background, the appearance depending on the element or ion that does the absorbing.

The outer layers of the Sun act similarly, with the barcodes of all the elements overlapping each other. After we disentangle and identify them with particular elements or their ions, the absorptions give up all manner of solar properties, including the temperature and the chemical composition, which is 90 percent hydrogen, 10 percent He (the helium content actually found from other means), and about a tenth of a percent all the others elements. Topping the "others" is oxygen, followed by neon and carbon, then nitrogen. Most stars are similar. Of great fascination are the ones that aren't. Molecules, combinations of atoms, produce extraordinarily rich sets of absorptions, whole bands of them, which returns us to the absorption of infrared and ultraviolet radiation by Earth's molecular atmosphere and the greenhouse effect.

Inside the Sun

To make sense of the inside of the Sun, we must look more closely at the outside. The seemingly solid opaque gaseous surface is broken up into state-sized bright "granules" surrounded by dark lanes that come and go over periods of minutes. They are the tops of giant up-and-down convection cells that carry heat and light being passed up from far below. The turbulence plus giant magnetic disturbances cause the Sun to ring like a bell with huge numbers of overtones, from which we can do "seismic" studies to "see" inside much as we do for the Earth with vibrations from earthquakes. The convection layer extends around a third of the way in. Below that, the gas is quiet and moves energy outward by atomic absorption and radiation.

At the heart of things is the seat of solar energy, the "core." Occupying about a quarter of the solar size and half the solar mass, the core is so hot (16 million Kelvin) and dense (a dozen times the density of lead, but still a gas) that lighter elements can be transformed into heavier ones, in our case here four hydrogen atoms into one of helium. A little mass (M), however, 0.7 percent, is lost. But it can't just go away. Instead it's converted into energy (E) via Einstein's renown equation, E=Mc**2, "c" the speed the speed of light, which when squared converts a small amount of mass into a huge amount of energy.

Here is the source of all life. If the Sun were to work off just gravitational energy, staying hot while squeezing down, it could shine for no more than 10,000 years. But the record of radioactive decay (for example the transformation of uranium into lead at a known rate) shows that the Earth, and thus the Sun, must be 4.5 billion years old. And the fossil record reveals that the Sun has been shining at something close to its present rate for nearly all that time.

The Sun runs instead on the "proton-proton" ("pp") reaction. It can operate only at high density and if the temperature is above a few million Kelvin, which makes the protons (hydrogen atoms without attached electrons) slam into each other at speeds so great that they can overcome the electrical repulsion and get close enough together such that they can stick under the strong force and produce heat in the form of a gamma ray. In the process, one proton turns into a neutron with the ejection of positive electron (anti-matter!) and a strange particle called a "neutrino." The result is heavy hydrogen, H-2, or deuterium. The positron then hits an electron, the pair annihilating themselves in the creation of more gamma rays.

Another hit by a proton makes light helium, He-3, and yet more energy. A collision between two He-3 atoms finally creates normal helium, He-4, with a couple protons left over. The Sun cannot blow up because the first reaction is creepingly slow. The energy gradually works its way out through successive absorptions and re-emissions by intervening atoms, the gamma rays gradually turned into sprays of yellow-white light emitted at the convective surface. It's gravity's job not to run the Sun directly but to provide the compressive force needed to get the temperature and density high enough to run the fusion chain, which in turn then provides support and keeps the Sun from collapsing. There is enough hydrogen left in the core to keep the Sun going pretty much as it is now for another 5 billion years. (As we will see, the devil is in the "pretty much.")

Neutrinos are odd. The Sun is so dense inside that energy transfer by photons (in which gamma rays are broken down into the more benign visual radiation that warms us) can take hundreds of thousands of years. The sunlight you see today was created all that long ago. But the neutrinos zip right out. Everything is transparent to them. Billions going near the speed of light pass through you every second, day and night, indeed right through the Earth. But we've learned to catch a few with neutrino telescopes. (Which are unexpectedly weird. The first was a hundred thousand gallon vat of chlorine-based cleaning fluid buried a mile deep in South Dakota to protect it from outside radiation.) From the rates of capture of a very few we find that the numbers coming out are just those expected. We can actually peer deep into the solar core using them.

Outside the Sun

Back now to the opaque surface, the "photosphere," where the radiation is released. Even a quick look at the Sun usually shows dark patches, "sunspots," some of which are much larger than Earth. They possess intense magnetic fields thousands of times stronger than that of our planet. Looking rather like fried eggs, the spots have depressed dark central "umbras" with temperatures about 1500 Kelvin cooler than the surroundings. (The amount of radiation emitted by a dense gas or solid depends critically on the temperature. Cooler bodies are therefore much the darker.) Surrounding the umbras are lighter, grayish, slopes, which rise upward to the bright surface. Sunspots come in pairs of opposing magnetic fields, one positive, the other negative, with the pairs commonly assembled into huge confusing groups.

From the rates at which they seem to march across the solar surface, we find the Sun's rotation period to be 25 days near its equator, but closer to 30 days nearer the poles, the solar gases madly tearing themselves past one another. The movement of electrically-conducting charged gases through rotation and convection produces a global magnetic field that is concentrated into thick subsolar ropes by the shearing rotation. Floating upward, the ropes pop through the surface in giant loops. Where they enter and exit, the magnetism is so powerful that it stops the upward convection, which forms a dark spot.

The loops and magnetic energy create and heat a vast surrounding envelope, the solar "corona," to around two million Kelvin. Yet it is so vacuous that in spite of its high temperature, it is invisible against the blue sky. Only during a solar eclipse, in which the new Moon covers the bright solar disk, is it visible from the ground. People, including scientists, travel thousands of miles to see and study the corona. From space, it is found to be filled with hot, bright, looping structures caused by the same kinds of magnetic fields that produce the sunspots. Sandwiched in between the photosphere and the corona is a thin, red, magnetically active layer, the "chromosphere."

The giant loops retain the corona and inhibit it from expanding, as befits a hot gas. Where the magnetically heated corona is not sufficiently confined, it blows outward in a "solar wind" that tears past the Earth and, with the aid of the Earth's magnetic field, helps create our surrounding "van Allen radiation belts." One 1.5 Earth radii out, the other 4, they in turn protect us from the invading and dangerous wind. Funnelled down the poles of our planet's magnetic field, the wind's particles make the upper air glow as the northern and southern lights (aurorae) that are readily visible throughout Alaska and northern Canada. The loops are terribly unstable. If two of them connect and neutralize each other, they violently collapse, sending atomic particles downward to the chromosphere to create a bright spot, a solar "flare," while other particles move outward at high speed to hit Earth. Released, a freed blob of coronal gas then takes a couple days to get to us, whereupon it disturbs the Earth's magnetic field, producing aurorae that can stretch to lower latitudes far from their normal homes. The electromagnetic effects on the ground are enough to bring down power grids, which can cause millions of dollars in damage. The blast can also destroy satellite electronics, though those (and astronauts) inside the van Allen Belts are at least somewhat protected. Our technological world is terribly subject to "space weather."

The magnetically driven action goes in cycles that average 11 years to complete. At peak activity, the Sun can be quite covered with spots, while at minimum they can entirely disappear. Aurorae, coronal mass ejections, flares, and a variety of other magnetic phenomena follow suit, the whole thing controlled by the time it takes for the deep magnetic ropes to reach their greatest complexity, break down, and dissolve. When they come back, all the fields are reversed, yielding a magnetic cycle of 22 years.

All this impacting energy is somehow absorbed by Earth in its climate pattern. Sunspots (which as we now know are symptoms of solar magnetism) have been related to everything from drought cycles to the stock market. What seems to be real is related to the "Maunder minimum," when between 1650 an 1715, the spots and cycle disappeared, whence the northern hemisphere of Earth was plunged into the "Little Ice Age" (which, some argue, could also have been the result of global volcanic activity). Other stars show such cycles, from which we can get some kind of prediction. We are maybe due for another shutdown. Now wouldn't that mess up the global warming debate!

II. PLANETS

From our parochial perspective the most important feature of the Sun is its planets, one of which belongs to us. Or rather we to it. Then there are all the others, Mercury through (if you will forgive the anachronism) Pluto, but really Neptune, as poor little Pluto is seen as a different kind of body, one belonging to a vast debris belt outside the orbit of distant Neptune. In a very real sense, the planets are the satellites, the "moons," of the Sun.

Here, within the Solar System, we measure distances not in mere kilometers, but in "astronomical units," the AU the average distance between us and the Sun of 93.2 million miles or 150 million kilometers. Terribly tiny compared with the Sun (even giant Jupiter is but a tenth the solar diameter, the Earth a tenth of that), they all go around the Sun in more or less circular orbits, counterclockwise as viewed looking downward from the north, and in similar planes that are tilted to each other by small angles (all clues as to their origins). In order, they are (with average distance from the Sun, diameter, mass, orbital period, and the viewing cycle, that is, the interval between successive oppositions to the Sun):

PLANETARY CHARACTERISTS
Planet Distance Diameter Mass Orbital Period Viewing Cycle
Mercury 0.39 AU 0.38 Earth 0.055 Earth 88 days 116 days
Venus 0.72 AU 0.95 Earth 0.815 Earth 225 days 584 days
Earth 1.00 1.00 1.00 365.2422... days ...
Mars 1.52 AU 0.53 Earth 0.107 Earth 1.88 years 2.13 years
Jupiter 5.20 AU 11.21 Earth 318 Earth 11.9 years 1.09 years
Saturn 9.55 AU 9.45 Earth 95.2 Earth 29.4 years 1.04 years
Uranus 19.2 AU 4.01 Earth 14.5 Earth 83.7 years 370 days
Neptune 30.1 AU 3.88 Earth 17.1 Earth 164 years 367 days
Pluto 39.5 AU 0.18 Earth 0.002 Earth 248 years 367 days

Ecliptic

Imagine the Earth in space with you on top: the way it looks to you personally. Now put the sky, the infinite "celestial sphere," around it. Expand a plane at your feet tangent to the Earth's curve, and it slices the sky in half at the true horizon. Everything above the horizon is visible, all that are below are out of sight. Run the rotation axis of Earth out through the north and south poles, and it sticks through the celestial sphere at the north and south celestial poles. Above and parallel to the Earth's equator is the celestial version, the sky simply the Earth exploded outward. The celestial equator intersects the horizon at due east and west on the compass. Running perpendicular to the equator through the celestial poles and the point overhead (the zenith) is the celestial meridian, which hits the horizon at due north and south. The north celestial pole (closely marked by the star Polaris) is elevated above your horizon by an angle equal to your latitude.

The Earth rotates counterclockwise on this axis every 24 hours. But we eschew reality here for what seems to be. In response to the Earth's rotation, it looks as if the sky is turning about us in the other direction, clockwise (no coincidence), carrying the stars, Moon, Sun, planets, with it. Watch as they rise over the eastern half of the horizon, set past the western half. If near enough to the north celestial pole, celestial objects are always visible, never setting or rising; if they are too close to the south pole, they are forever out of sight unless you travel and change the celestial sphere's north-south orientation.

The Earth in truth orbits the Sun once a year in the counterclockwise direction. Pretend however that we are stationary such that the Sun seems to go about US, also counterclockwise. It will appear to trace a path, the "ecliptic," to the east against the distant stars. If the Earth's axis were bolt upright to the plane of Earth's orbit, then the ecliptic would lie directly on the celestial equator. But it isn't. Instead, it's tilted relative to the orbital perpendicular by an angle of 23.4 degrees. As seen in the sky then, the ecliptic is also so tilted relative to the celestial equator. The Sun follows this tipped path, crossing the equator about March 20 and September 23, which mark the beginning of spring and fall. As the Earth rotates, on these dates the Sun technically rises due east, traces out the celestial equator, and sets due west. Since the Sun is on the equator, and the equator is split evenly by the horizon, the Sun is up for 12 hours, down for 12; days and nights are thus of equal length. (Refraction and the finite angular solar diameter in truth extend daylight hours by a few minutes.)

The tilt then takes the Sun up to 23.4 degrees north of the equator on June 21, the first day of summer, when in temperate latitudes we see it highest in the sky, then brings it as low as 23.4 degrees south of the equator on the first day of winter, December 21. Here is the sole cause of the seasons, as in summer the solar light impacts the Earth from on high, while in winter it strikes more at an angle, spreading itself out, resulting in a low heating rate.

The planets all move, inner ones rapidly, outer ones slowly. But since they, including Earth, are more or less in the same general plane, they will all be found near the ecliptic. It passes through a set of 12 ancient constellations collectively called the "Zodiac," which is the basis for astrology. And move they do. The motions of Mars and Venus can be seen night-to-night. Outer ones go more slowly, Jupiter taking 12 years at a rate of one constellation per year, Saturn nearly twice as long. But since we view them from a moving platform, their apparent motions are rather complex. Given their steady counterclockwise progression, they generally appear (ignoring daily rising and setting) to move to the east against the starry background. But when the Earth goes between an outer planet, or a speedier inner one passes us, it will appear to go backwards, to the west, or "retrograde." Given the ancients' belief that the Sun and planets all went around a central Earth, these movements were a source of great consternation, and they constructed elaborate schemes to explain them. Then came reason.

The Revolution and the Gang of Six

First in the bunch was the Polish cleric, Nicolaus Copernicus. Born in 1473, 19 years before Columbus "discovered" us, he spent much of his life demonstrating that the Earth and planets all went about the Sun. His great work, "de Revolutionibus Orbium Celestium," was published in 1543, the year of his death, which may have saved him from the Inquisition.

Next up is Tycho Brahe. Born just after Copernicus died, Tycho was one of greatest naked-eye observers who ever lived. With giant protractors he measured the positions of stars and recorded the movements of the planets against them. Dying in 1601, the results of his studies were turned over to (purloined by?) his assistant, our number three man Johannes Kepler. Copernicus's heliocentric theory did not predict the motions of the planets any better than did those from the older geocentric one. He had made the logical error of having the planets move in circular orbits, the sphere and circle of course being the perfect figures, ones that would naturally have been chosen by God.

Kepler, in the spirit of modern science, set out to calculate how the planets actually do move so as to replicate Tycho's observations. It took him nearly a decade to work out his first two laws of planetary motion and another for the third. Here they are.

1. A planet moves along an ELLIPSE with the Sun at one focus. (The ellipse is drawn such that the sum of the distances along the curve to two interior points, the foci, is a constant. Bring the foci together and you have a circle, stretch them way apart relative to the size of the figure, and the ellipse comes out long and skinny.) This means that the distance between a planet and the Sun constantly changes. The Earth is nearest the Sun (1.7 percent closer than average) at "perihelion," about January 2, farthest at "aphelion" around July 4. We are closest to the "fire" in the dead of northern winter, farthest in the heat of summer: so much for that theory of the seasons.

2. Planets speed up in a prescribed way as they approach perihelion, slow down as they recede to aphelion. (The radius vector, the line that connects the planet to the Sun, sweeps out equal areas in equal times.) These laws together explain the "inequality of the seasons," that spring and summer are roughly three days longer than fall and winter, known since the time of ancient Greece.

3. Finally, Kepler put them all together in his Harmonic Law: the square of the orbital period of a planet in years equals the cube of its semimajor axis ("a") in Astronomical Units (the semimajor axis being half of the ellipse's long axis), or P**2 = a**3. Try it. Jupiter is 5.2 AU from the Sun. Cube it (5.2 X 5.2 X 5.2) to get 140.6, the square root of which is 11.9 years. Kepler rests his case.

The fourth person in this starry hit parade is Galileo, who, starting in 1609, worked about the time Kepler was producing his first two laws. He did not invent the telescope, but with the best ones he could make turned on to the sky he found: (1) the moons of Jupiter, and that they went around the planet much as Copernicus's planets go around the Sun; (2) craters and other features on the Moon; (3) sunspots; (4) the phases of Venus (not possible in the Earth-centered system); (5) that the Milky Way is made of lots of stars. There's much more including the rings of Saturn, which he mis-interpreted with his small scope as knobs. But what set him apart from others, who were doing similar work, is that he continued with his observations, sought to interpret them, realized that he had given proof to Copernicanism, and wrote extensively about it. Which inevitably got him in trouble with the Church. Called to Rome, he was forced to recant and was placed under house arrest for the remainder of his life. His works were banned until the nineteenth century.

But "the cat was out of the bag" (an old naval phrase having nothing to do with actual cats). And nobody could put it back in. Keplerian theory worked beautifully. But at the same time nobody could not figure out WHY it worked. Kepler thought some mysterious force pushed the planets around. It remained for numbers 5 and 6 in our tale to get it right.

The Last Two Told How It All Works

Isaac Newton is among the most brilliant people who have ever lived. Contentious, reclusive, with few friends, his mind ranged over a vast tract of intellectual space. He, with Leibniz in Germany, invented calculus, which he developed from the concepts of Descartes. (No, I won't write out the joke.) He also invented the reflecting telescope, was among the first to examine the solar spectrum, and in one of his greatest achievements (all described in the "Principia") set down the laws of motion and gravity, and in doing so also explained the tides.

(Tides. The Moon's gravity exerted on the part of the Earth directly under the Moon is higher than it is on the Earth's opposite side. The result is a stretching across the Earth, noticeable mostly in its oceans, which will have a bulge in principle facing the Moon. As the Earth turns, a point on the shore will go under deeper then shallower water roughly twice a day. In reality, high tide and low tide lag behind the position of the Moon in the sky. Two bodies placed in proximity to each other will always raise mutual tides. Tides raised in the solid body of the Moon by the Earth have stopped its rotation relative to us such that we always see the same lunar face.)

"Acceleration" is a change in "velocity," which as a technical term combines both speed and direction. Increase or decrease your speed, or drive around a curve at constant speed, and you are accelerating. Newton postulated three laws of natural motion.

(1) A body's motion stays constant unless acted upon by an outside force.

(2) The acceleration achieved by a body is directly proportional to the force applied to it and inversely proportional to the body's mass (thus defining the meaning of "mass"). Turned around, Force = mass X acceleration, F=MA.

(3) Every action has an equal and opposite reaction (which causes airplanes and rockets to work). He, with Galileo as predecessor, invented physics.

No apple hit Newton on the head, nor did he discover gravity. The first caveman to fall off a cliff did that. He did something far greater by discovering how gravity works. Newton noted that the acceleration of a falling body (which increases its speed for every second of fall and is independent of mass) and the acceleration of the orbiting Moon were in inverse proportion to their distances from the center of the Earth. What makes the apple fall is a universal force that also makes the Moon follow its closed curving path. The force of gravity between spheres is directly proportional to the product of their masses and inversely proportional to the squares of the distances between their centers, or F=G(M1 X M2)/D**2, where G, found in the lab, is a constant that makes the units come out right. Terrestrial gravity works as if the Earth's mass were all concentrated to the center off the planet; in that sense you are 6500 km (4000 miles) from Earth, our planet's radius. As you stand, the Earth pulls you down toward the center, but since the Earth is solid you can't get there. Your "weight" is thus a force against the surface of Earth such that W=G M(Earth) X M(you)/(Earth radius squared). Substitute the mass and radius of a planet for those of Earth, and get your weight on Mars or the Moon. Since F=MA, the acceleration of gravity, the acceleration with which you or anything else falls, is G times the mass of Earth divided by radius squared, or g=G M(Earth)/R**2. Measure "g" and you can find the mass of Earth.

Edmund Halley, he of comet fame, found that Newton's theory predicts elliptical orbits. Newton said that he'd already done that. What exactly then is an "orbit?" Drop a ball and time its fall to the ground. There is no relation between horizontal and vertical motion. Throw the ball horizontally and it immediately starts to fall at same rate at which you first dropped it. The faster you throw it though, the farther it will go before landing. At some speed, the curvature of the Earth comes into play. Throw the ball at 18,000 miles per hour and it drops at the same rate at which the Earth curves. Though falling constantly, it can't catch up with Earth's surface and just keeps going around and around (presuming neither mountains nor Grandma get in the way). It's now in orbit. At the Earth's surface, the circuit would take 88 minutes. Put a Shuttle astronaut into higher orbit, and she and the craft fall with the same acceleration. She thus feels "weightless" even though within the Earth's gravity, which has not dropped off by very much. Following Kepler's third law, P**2 proportional to a**3, the farther the satellite is from Earth (pursuing a circular path), the longer it takes to orbit. At a distance of 41,000 km (25,000 miles), the period is the same as the Earth's rotational period of 24 hours. If you place it above the Earth's equator, the thing then seems to hang motionless in the sky, and we have found the best place to put a communications satellite.

Using his laws, Newton came up with generalizations of Kepler's Laws.

(1) The orbit of one body around another is a "conic section." Cut a cone at different angles. Perpendicular to the axis you get a circle; tilt it some and you get an ellipse. Tilt the cut parallel to the side, and out comes a parabola, while a greater tilt yields a hyperbola. Parabolic and hyperbolic orbits are open-ended, one way: here comes the deadly asteroid; there it goes, never to return.

(2) The conservation of angular momentum. Swing a rock on a string around your head. The angular momentum is the rock's mass times its speed times the string's length. Change one quantity and you automatically change another. As a planet gets closer to perihelion, it speeds up in response to shortening distance. Watch a dancer or skater work the same way as she brings in her arms.

(3). Gravity involves mass, yet mass does not appear in Kepler's third law, P**2 = a**3. Kepler also used units relative to Earth, periods in years and distances in astronomical units. Using his laws of motion and the law of gravity, Newton not only put in mass, but expressed the third law in physical units. He found that the squares of the periods, P**2 ("P" in seconds), equals a constant times a**3 ("a" in centimeters) divided by the sum of the masses of the two bodies, or P**2 = (4 pi**2/G) X a**3)/(M1+M2). If M1 is the mass of the Sun and M2 the mass of a planet, M1+M2 is essentially constant. Kepler got away with his third law because planetary masses are so small. It's a powerful rule. Tip it upside down and you can use the orbit of the Earth to measure the mass of the Sun, the orbit of one of Jupiter's moons to find the planetary mass, or the period of an orbiting satellite at any particular distance.

Indeed, we can get the masses of ANY two bodies (two stars for example) in orbit about each other. Orbits are always mutual, each body going around a common center (the "center of mass") between them whose position depends on the inverse of the mass ratio (a1/a2=M2/M1, where "a1" and "a2" are the semimajor axes of masses 1 and 2 about the center of mass). As the Moon orbits Earth, Earth orbits the Moon, though with a smaller orbital radius of just 3000 miles as derived from the mass ratio. Assuming that the smaller body goes exclusively about the bigger (with a semimajor axis of a1+a2) gives the sum of masses. The location of the center of mass (which can be difficult to find) then gives the mass ratio, hence the individual masses.

Number 6? Albert Einstein, who found the Newton was wrong, though at least under common conditions not by much. Einstein's theory of relativity holds that we live in a web of four- dimensional spacetime in which the speed of light is constant no matter what the observer's velocity (the phenomenon experimentally observed). Einstein's gravity is a distortion of spacetime by mass. Gravity has energy. E = Mc**2, so gravitational energy has a mass equivalent, which changes the rules a bit, though there is far more to it than this. The term "relativity" comes from the realization that the timing of an event is always relative to the position and motion of the observer. We all see the Universe slightly differently and, because of the finite speed of light, at different times. Relativity is essential to modern life. Without relativity, we would have no successful planetary probes and the GPS would not work.

Expanding the Solar System

Now the story jumps back (or, continuing from Newton, is it forward?) to William Herschel. He was introduced as the discoverer of infrared radiation. He made his mark, though, as discoverer of the planet Uranus, in 1781, which nearly doubled the size of the Solar System overnight. He found it while systematically scanning the skies and recognizing it as something rather odd. First thinking it might be a comet, regular observations by him and others clearly showed its planetary nature, a body about a third the size of Jupiter at 19 AU out from the Sun in an 84 year orbit. The new planet caused a sensation. Herschel originally named it "Georgius Sidus," "George's Star," after his monarch, King George III. The French and others thought that not such a good idea, so all agreed eventually to continue the mythological sequence, Uranus being the father of Saturn.

The Sun clearly has major control over the planets. But each planet must influence all the others, both directly and indirectly through its gravitational interaction with the Sun. That's a lot of interconnections when one is to calculate real planetary orbits, which are then not quite perfect ellipses. It's extremely difficult without a computer, and 19th century astronomers became masters at it. The deviations of the positions of Mercury from those predicted by Newtonian theory are among the prime proofs of relativity, which gets it right.

Uranus is not all that faint, and under good conditions can be seen with the naked eye. Calculations of its orbital path showed it had been observed as early as 1690 by others and thought to be merely another faint star, so there was a string of data to work with that included post-discovery observations. And Uranus was never just where it was supposed to be. No relativistic problem here, as Uranus is too far from the Sun and moves too slowly. But if there is a planet beyond Saturn, why not one past Uranus? John Adams in England and Urbain Leverrier in France set out to "look" for it, each trying to predict the position of the trans- uranian on the basis of the deviations from Uranus's calculated positions, which critically included the effects of the known planets. In a long, complicated, and disputed story, Adams's work was not followed up. Leverrier sent his prediction to the Berlin Observatory, where Johannes Galle found what was to be named Neptune right away on September 23, 1846. Thirty AU from the Sun, it has just completed one full 165-year orbit since it was found.

The discovery caused yet another sensation. Newtonian physics works! If you knew the positions and velocities of all the atoms in the Universe, you could predict the future: determinism is king. "Your honor I robbed the bank because Newton's Laws made me do it." The philosophy had some credit until the advent of quantum mechanics in the early 1900s and the discovery of the natures of subatomic particles. Like photons, electrons and protons behave with wave-particle duality such that we have no hope of pinning them down. All we can give is probabilities. Free will now reigns. Or at least accidental will.

Even factoring in Neptune, Uranus did not perfectly behave itself, so the same trick was tried again. Finally, in 1930 Clyde Tombaugh came across Pluto. Terribly faint, the "last planet" (so-called at the time) turned out to be far too small to influence much of anything. The discovery had been fortuitous. Averaging 40 Astronomical Units from the Sun, taking 249 years to go around, Pluto has a highly tilted, eccentric orbit that takes it inside that of Neptune. Moreover, Neptune has a lock on it, as Pluto -- about the size of the western US -- orbits twice for every time Neptune orbits thrice (a word you don't get to use very often). The little one turns out to be the main member of the Kuiper Belt (after Gerard Kuiper), a ring of planetary debris outside the orbit of Neptune, and was thus "de-planeted" to a lower status. Or to a higher one. In any case, we'll encounter it again. (Improved masses of the outer planets from the movements of passing and orbiting spacecraft finally solved Uranus's problem. There are no more actual planets, no "Planet X".)

Touring the Planets

What are planets doing in a review of the Sun and stars? They seem to be a natural part of the star-forming process, so we need look at them rather like a "Reduced Shakespeare" in which all the plays are done in one fast performance. The first four, Mercury through Mars, bear superficial resemblance to one another. Called the "terrestrial planets" after the largest of their membership, Earth, all are rocky with nickel-iron cores.

Earth's core takes up about half the planetary diameter. As found from earthquake vibrations, the core's inner half is solid, the outer liquid. The outer half of Earth, the "mantle," is mostly silicate rock. On top floats the solidified rocky crust, which is divided into lighter-weight continents and low water- filled basins. Though the oceans look huge, they are no more than a thin film a couple miles deep on top of a body 8000 miles wide. We are really very dry, though nowhere near as much as the other terrestrials (with Mars a bit iffy).

The Earth's interior is heated to thousands of degrees largely by the radioactive decay of heavy uranium and thorium into lead. Circulation of the liquid core produces our magnetic field and ultimately our aurorae. The mantle is in a hot plastic state that slowly flows in convection currents that break through the crust mostly at mid-ocean ridges. Fracturing the crust into individual plates, the currents push the continents around, making them crash into one another to create mountains, earthquakes, and volcanos. Volcanos like Hawaii also arise from the mantle's depth well away from plate boundaries. Above it all lies our oxygenated 80-percent nitrogen atmosphere.

Begin with the closest body, the Moon. Circling Earth, it's not considered a "planet," though it surely looks like one. About a quarter the size of Earth with a bit over one percent of Earth's mass, it's a beaten, battered body covered with impact craters and lunar "maria," large basins coated with ancient dark lava that make the face of the "Man in the Moon." Most of the craters were dug out some four billion or more years ago by late impacts of the debris that followed on the heels of Solar System formation. The basins, which are huge craters, came along shortly thereafter through a series of gigantic strikes. They are mostly on the side of the Moon facing Earth (the Moon not rotating relative to us, compliments of tides raised in the Moon by the Earth). Since then the cratering rate has dropped substantially, as witnessed by the relatively few craters within the maria.

If the Moon is so cratered, how did Earth escape? It didn't. The heavy cratering was wiped away by erosion and continental drift. The 200 or so known impact craters are relatively recent. Too small to hold on to any significant atmosphere, the Moon is almost completely dry with just a bit of ice protected in deep polar craters that see no sunlight as well as a small amount locked in rocks. With just a tiny frozen iron core, the Moon has no global magnetic field.

Mercury at first appears similar. Close to the Sun and hot, surface rocks hit 430 C (near 800 F) at noon, dropping to under 300 below in the dead of night. It too has little of any atmosphere, just that formed by atoms kicked off the surface by the solar wind. It's SO close to the Sun that it's hard to see from Earth, rising and setting only in twilight. It's even too close to the Sun for the Hubble Space Telescope to look at. What we know has come from passing and orbiting spacecraft. Like the Moon, it's covered with impact craters (and basins) that go back to the early days of the Solar System, but also has extensive inter-crater volcanic plains. Mercury's biggest physical feature is its huge iron core, which takes up some 70 percent of the planet's radius. The original mantle may have been ripped off in an ancient collision. Nobody knows. Small amounts of ice reside in dark polar craters. The iron core should by now have frozen solid, yet Mercury possesses a weak magnetic field. Like Moon to Earth, Mercury is tidally locked onto the Sun but, because of the rather high orbital eccentricity, with a rotation period of 59 Earth days, exactly 2/3 of the 88-day Mercurian year.

Often called Earth's twin, Venus is just a bit smaller than our planet. Some twin. It's blanketed by a carbon dioxide atmosphere at a pressure 100 times Earth's. The greenhouse effect has gone mad, boosting the surface temperature to close to 470 C (near 900 F). Covered by thick sulfuric acid clouds, the rocky surface is invisible to the visual observer, which has mostly been examined by orbiting craft using radar. The planet displays myriad volcanoes similar to mid-ocean Hawaii and a crater count that suggests the planetary surface volcanically repaved itself no more than a billion years ago, which wiped out any early record. Extreme volcanism may have cooled the core enough that it produces no magnetic field. With no real continents, with lower basins unfilled with absent water, the place is an astronaut's nightmare.

Smaller Mars, about half our size, is far more hospitable. Its rotation and axial tilt, and thus seasons, are similar to ours. At the poles are thick ice caps that during fall and winter advance toward lower latitudes. Some astronomers believed they saw fine lines, "canals," crossing the surface, leading to belief in Martian civilizations. They are illusory. The southern hemisphere is populated by ancient craters, while the northern is known for volcanoes, one of which towers more than twice the height of Everest. You can build bigger mountains on lower- gravity planets, especially if there is no continental drift to limit them. Mars tried to be geologically active, with a rift 3000 miles long, but cooled off too quickly inside for the planet to make continents. The air, just one percent the pressure on Earth, is almost all carbon dioxide. The polar caps are mostly water ice with some dry ice in the south. The low atmospheric pressure cannot support liquid water, but there is plenty of evidence that it once existed. We see branching dry valleys in the south, huge eroded channels in the north, possible shore lines, suspiciously layered rock, and evidence for an aqueous chemistry. The place must once have had a thicker atmosphere and been much like Earth. Was there once life? There is no evidence of any. And if not, why not?

Traveling farther outward, toward the bigger planets, we encounter our first very small ones, the "minor planets," the asteroids. Hundreds of thousands, millions, more, are spread mostly between 2.1 and 3.2 AU from the Sun. The largest, Ceres, is just under 1000 kilometers (575 miles) across, well under 10 percent the size of Earth. Collisions and planetary gravity throw them as far out as the orbit of Jupiter and farther inward than Earth. Small ones that hit us are called meteorites. They come in two basic flavors, rocks and irons, telling us that the asteroids must be similar. Some of the rocks are primitive, going back to the birth of the Solar System 4.5 billion years ago. (Indeed, it's the radioactive dating of primitive meteorites that gives us the age of the Solar System in the first place.) Others, and the irons, must be the fractured remains of asteroids that once had iron cores. Over the aeons big ones occasionally strike us. We are trying to track the dangerous ones all down with the dream of altering the orbit of any predicted to hit Earth.

Then come the lumbering giants, Jupiter, at 5.2 AU the largest, 11 times the size of Earth, carrying more than 300 Earth masses. In homage to the Sun, made largely of hydrogen (though in the molecular form of paired hydrogen atoms) and helium, the planet is covered with ammonia clouds full of noxious hydrocarbons that stripe out through strong winds parallel to the equator. Unexpectedly, it spins with a "day" less than half Earth's. Deep down the compressed hydrogen turns liquid, the circulation of which generates a magnetic field more than a dozen times the strength of Earth's. Filled with trapped solar particles, the "magnetosphere" is not only lethal, it's among the brightest radio sources seen from Earth.

Four large satellites, of Moon-Mercury size, orbit the planet. Discovered by Galileo, entertaining to watch as they go around on the order of days, the satellites can be seen in binoculars. Innermost is Io, just 6 Jovian radii out with a period of only 1.8 days. Gravitationally tugged about by the next two, Europa and Ganymede, it is flexed by tides raised by Jupiter, and heated inside to make it the most volcanically active body known as it spits out plumes of sulfur-laden silicates. It's so close to Jupiter that there is an electrical current running between the two. Europa is quieter, though tidally heated to the point where it may embrace an ocean below a surface made mostly of ice. Icy Ganymede is the largest satellite in the Solar System, Callisto (26 Jupiter-radii out, taking about a fortnight to orbit) not far behind. A common theme out here is ice, which makes up about half the bulk of the outer big satellites. The space around Jupiter is shared by some 60 or more tiny moons.

Then near 10 AU from the Sun we find glorious Saturn, known for its system of flat, ultrathin but bright rings whose diameter is double that of the planet (which is much like Jupiter, though a third the mass and much less dense). Made of particles of ice and rock typically few centimeters in diameter, the rings are embedded with larger bodies that, along with the gravitational action of other satellites, drive a great and beautiful structure of nested ringlets. The easily-visible rings, probably made of a smashed or tidally disrupted satellite, are just the inner part of a vastly extended system. Orbiting outside the obvious rings are some 20 modest satellites (and far more smaller ones). Many have bizarre characteristics, like Iapetus with two distinct faces and Enceladus with watery volcanic plumes. The largest moon, just under Ganymede's size, is strange Titan. With a thick, visually-impenetrable atmosphere, it's covered by methane clouds that rain liquid methane into hydrocarbon streams and lakes (the latter seen by the orbiting Cassini spacecraft).

We might take Uranus and Neptune, at 19 and 30 AU, together, though each has highly distinctive characteristics. Both are much denser than lightweight Jupiter and Saturn, and though filled with hydrogen and helium, contain much more water/methane/ammonia ice. Uranus's chief oddity is that it is tipped over almost perpendicular to its orbit, giving it extreme seasons. Next is probably a set of narrow, dark, widely spaced rings almost the reverse of Saturn's. (Neptune and Jupiter have thin ring systems as well.) While Uranus has a large set of smallish satellites, Neptune has few, though one large moon, Triton, stands out. Orbiting backwards, it's almost certainly a captured body, one that looks a lot like Pluto. So what's Pluto? By modern definition to small to be a real "planet," it's a vastly important transition body to the outer debris belt. Along with a large satellite and four dinky ones, Pluto is orbitally locked to Neptune, it's one-time brother, Triton, now actually owned by Neptune.

Launching Outward

Pluto was the "last planet" until late in the twentieth century, when other smaller bodies in similar locations, even in orbits similar to Pluto, began to turn up. Dozens were found, then thousands. One, Eris, has a diameter and mass very similar to Pluto. If Pluto is a planet, why not Eris, why not the myriad smaller bodies? Pluto is really the harbinger of a vast collection of small, icy rockballs that extend out to about 55 AU (a few scattering well beyond). The Kuiper Belt was predicted as the reservoir of short-period comets, those that take less than 200 years to make a full journey around the Sun.

Comets are icy, dusty bodies on long elliptical orbits. As they approach the Sun, they partially "melt," subliming from ice to gas, which is ionized by sunlight. The solar wind then blows the gas backward into a long, bluish, glowing tail. The melting of the icy matrix releases the dust. Sunlight both reflects off the particles and pushes them backward into a yellow-white curved dust tail. Comets do not streak across the sky, but like planets move in stately order a bit each night, usually getting brighter as they approach the Sun, fainter as they then recede. There are two broad kinds, short and long period. The short stick to the ecliptic plane and orbit counterclockwise like planets. Frequently encountering the Sun, they evaporate quickly and are generally faint.

Collisions and gravitational effects of the outer planets gradually move Kuiper Belt objects inward until they visit closely with the Sun and become actual comets. There must be billions of them out there. That said, Pluto, Eris, and the other bigger ones, are NOT comets, but are evolved bodies that perhaps "tried" to become planets but that just could not gather enough material to grow to decent size through constant collisions.

About once a generation, we are visited by a truly great comet that has an orbit that takes thousands, even millions, of years to complete. Encountering the Sun infrequently, even for the first time, a long-period comet can be amazingly bright, some even visible in daytime. Among their company is Halley's. Its orbit was worked out by Newton's friend, Edmund Halley, who found that different historical comets were actually the same one coming back over and over again. With a rather short 76 year period, it's an exception, its orbit perhaps shortened by a planetary encounter. Halley's lore is deep and wide. It appears on the Bayeux Tapestry and has a dreadful reputation as a bringer of evil. The last time it came by, in the mid-1980s, Mexico City fell down in a great earthquake. It's been accused of carrying off small children. Comets, great and small, have no influence on Earth, as their gravity is far too small. Unless they, like asteroids, hit us, as Comet Shoemaker-Levy did Jupiter in 1994. A lesser comet caught by the giant planet, it was pulled apart into multiple pieces through Jupiter's tidal action, then one after another they struck. If there, why not here? Many of the impact craters on Earth should have been caused by comets. Unlike asteroids, the appearances of the long-period comets are quite unpredictable, giving little time to save the planet. It's widely believed that early collisions with comets (and watery asteroids) brought the cooling Earth its water. Even if they don't hit anything, comets that periodically revisit the Sun are doomed to death by evaporation.

The long periods and huge orbits told Jan Oort that there must be another comet reservoir filled with a trillion icy bodies that could extend outward perhaps halfway to the nearest star. Encounters with stars and interstellar clouds occasionally bring a few inward to appear as great, long-period comets. Back in the Kuiper Belt, Eris goes out to 100 AU, another, Sedna, out to nearly 1000. Are we beginning to see Oort Cloud comets, or things related to them, in place?

From these distant bodies, we look outward to the stars. And perhaps to other planets and other life. But first we have to digress to see how we might learn about them.

III. TELESCOPES

"You can see a lot by looking" (attributed to Yogi Berra, who said he did not say all the things he said). The naked eye tells little. The breakthrough was made in 1609 when Galileo turned his telescope onto the sky. There are two kinds of traditional telescopes. The first, like Galileo's, uses a curved "objective" lens to refract and bend starlight to a focus, which makes a pointlike image of a pointlike object. The larger the objective, the brighter the image or the fainter the star that can be detected. The "light gathering power" of a telescope depends on the objective's area, or diameter squared, while (because of the interference of light waves) the amount of detail that can be seen depends directly on the diameter. Because of its importance, telescopes are in fact named by the objective's diameter. We can "look" at the image with a second lens, an "eyepiece." By picking different eyepieces of different focal lengths, we can change the telescope's magnifying power. Power equals the focal length of the objective (the distance from the lens to the focal point) divided by that of the eyepiece. We can also put a photographic plate or (now) electronic detector at the objective's focal plane, allowing us to permanently record the picture, measure brightness; or we can use an intervening device (a "spectrograph") to create a spectrum.

Refractors are cumbersome. To cut down on "chromatic aberration" (a lens also acts like a prism that sends different colors in slightly different directions), they traditionally have long focal lengths. Moreover, glass can sag, which disturbs the sharpness of the image. The largest refractor in the world, at the Yerkes Observatory in Wisconsin, has a lens but 40-inches (a bit over a meter) in diameter, which is small by today's standards. Moreover still, these older telescopes were built close to where the astronomers lived, and are now seriously light-polluted.

It's far better to use reflection by a curved (essentially parabolic) concave mirror to bring light to a focus, then place the thing on a dark mountaintop with as little disturbing air overhead as possible. Different secondary and tertiary mirrors can bring the light anywhere the designer wants. Newton's original had a flat secondary that was placed just short of the focal position to send light off to the side (the most common arrangement for small amateur scopes). Professional instruments commonly use a curved secondary to reflect the converging beams back through a hole in the primary (objective) mirror. Or we might use more mirrors to send light to a fixed position in the basement.

Telescope mirrors are front-surface reflectors. The reflecting surface is a thin film of aluminum on a curved glass or ceramic base. There seems to be little limit as to how big you can make them. Until 1947, the largest in the world was the 100-inch (mirror diameter) on Mt. Wilson in California. It was superseded by the great 200-inch (5 meter) telescope on Palomar Mountain. The largest single mirrors are around 8 meters across, all in the American southwest, the Chilean Andes, or on Hawaii. We can make them even larger by using multiple mirrors to mimic one big one, the grandest of which are currently the twin 10-meter behemoths on Mauna Kea. Plans for 30-meter and larger instruments are underway. Galileo's was about 2 inches across.

Ground-based astronomy has severe limitations. The Earth's atmosphere makes images shimmer (twinkle), disallowing a sharp view, and also absorbs fiercely at wavelengths shorter than violet light and in much of the infrared. We can overcome both of these problems by orbiting our telescopes above the blanket of air. The Hubble Space Telescope is the best known, but there are numerous others that range in wavelength reception from the infrared into ultraviolet, X-ray, even gamma ray, where there are exquisite things to see. With a 2.4 meter diameter the HST is hardly the largest telescope, but it is in the best observing location. The James Webb Telescope, designed for the infrared, will have nearly triple the diameter, but its flight is far from assured.

The first expansion into other areas of the electromagnetic spectrum, though, took place in the mid twentieth century, when a radio engineer named Karl Jansky working for Bell Labs discovered radio radiation coming from space. Most radio telescopes operate much like optical ones, with a curved "mirror" that sends radio waves to an antenna and amplifier. They work just like satellite dishes. Radio astronomers do not "listen" to anything, but record natural static and can even build up radio pictures of celestial objects like planets and interstellar clouds. Because radio waves are long, to get good images radio telescopes must be similarly large. The biggest steerable radio telescope is 100 meters in diameter, larger than the standard American unit of measure, the football field; the largest fixed-dish is the 305- meter scope in Puerto Rico. By using multiple telescopes in tandem to create an "interferometer" (which uses the interfering properties of electromagnetic waves), we can mimic the effect of one giant telescope. The best known of these is the Very Large Array (VLA, go visit) that covers 27 miles of desert in New Mexico. Using precise timings instead of direct cable connections, interferometers have been made larger than the whole US, which gives us an amazing ability to sense fine detail. One of the great triumphs of the twentieth century was the opening of the electromagnetic spectrum, which allowed celestial sights of all energies to be seen, most of them quite hidden from optical view. With that in mind, let us now take a small look at what our own eyes can see.

IV. A CELESTIAL TOUR

In times past, the children of the old aristocracy took the grand tour of Europe to view the sights, to become more worldly and sophisticated. Here we do the same with the sky, wandering among the stars to sample the celestial sights and to intrigue. Various side tours are taken, after which the main tour continues.

Constellations: A Continuing Theme

A typical autumn evening view shows the stars of the Great Bear (Ursa Major) treading down the northwestern sky. Does it look like a bear? Sure, if you relax the imagination there are stars that mark the body, the tail, the snout, and three feet. It's an example of a "constellation," a sky figure with a name. Most cultures seem to have made them up, our "western" ones coming out of Mesopotamia through Greece. They were later helped along by the Arabs, then sent back to Europe and on to us today. Some are made for storytelling, while others, those that hold the ecliptic, are also sacred. They do not necessarily look like what they are supposed to be. No artist created them. They represent, not portray. If you want to see a lesser Bear (Ursa Minor), the two front bowl stars of the Big Dipper (a beloved informal "asterism" within the Bear that makes the animal's hind quarters and tail) point to Polaris, the North Star. It lies at the end of the handle of the faint Little Dipper, which forms the tail and body of Ursa Minor. Look at the tails! Ever seen a bear? How do you get such a beast into the sky? Sneak up behind it and quickly grab its short tail. Whirling it around your head to toss the bear into the sky, you stretch it out. The tribe's storyteller could stretch that tale out until the campfire died and all the potables were consumed.

Forty-eight constellations descend to us from the ancients. The relatively recent division of Argo, the Ship (of the Argonauts), into three parts gives us 50. In the seventeenth and eighteenth centuries, the ancient sky figures were supplemented by dozens of "modern" constellations made of faint stars and those of the newly-explored southern hemisphere, of which 38 were adopted in our own times, giving us a total of 88. To these add the many informal figures like the Big Dipper.

Brightness and Distance

Two matters now intrude. Brightness first. The Dipper stars are mostly all of "second magnitude." In the second century BC, Hipparchus of Nicea divided the sky's stars according to apparent brightness, first magnitude the brightest of them, sixth the faintest he could see. There are just 22 first magnitude stars. Going to deep sixth we can count around 9000 that are potentially visible to the naked eye. The magnitude system is still in use today, though defined by precise mathematics. Like the audio scale of decibels, it's logarithmic, in which small numbers signify a big change. Sixth magnitude stars are 100 times fainter than first, magnitude 30 stars (about the faintest that can be detected) are four billion times fainter yet. When calibrated and scaled, the brightest celestial bodies go into negative numbers, giving us Sirius at minus first magnitude, Venus at her best at -5, the full Moon -13, the Sun -27. The system is also fully continuous and decimalized. Generic first magnitude runs from 0.50 to 1.49, etc.

Even the nearest of stars is unimaginably far away. It takes light, at a speed of 300,000 kilometers (186,000 miles) per second, just over a second to traverse the distance from the Moon to Earth, eight minutes for a photon to make the journey from the Sun to here. A signal from a spacecraft orbiting Pluto takes four hours. Then the gap hugely widens. The nearest star, Alpha Centauri (third brightest in the sky and the luminary of Centaurus, the Centaur), is four light years away, the "light year" the distance a ray of light covers in a year of some 31 million seconds. The most distant star the naked eye can see is thousands of light years off. The distances are so large that light travel time becomes something of a popular issue. We see Alpha Cen as it was four years ago, not as it is "today." Something could happen to it and we would not know it for four years. But we also see the Sun as it was eight minutes ago, the Moon as it was just over a second ago. It does not matter. You see the people around you as they were a few trillionths of a second ago. Nobody cares. The Universe, from people to stars, is as you see it, while everything is seen at a different time! Here again is one of the foundations of relativity. With distance, we can distinguish between apparent and absolute brightness. The apparent brightness of a star in the sky depends on its wattage and its distance. Move it farther away and it looks fainter. From distance we find that the stars cover an amazing range of true luminosity, from millions of times more luminous than the Sun to tiny fractions solar.

Galaxies

All these stars, and more than 200 billion others, are part of our Galaxy. It's a flat, rotating disk that contains our Sun. More than 100,000 light years across, we see the combined light of the countless stars of the disk around our heads as the Milky Way. Lost among the bright lights of town, from the dark countryside the Milky Way can be a spectacular sight. Our Galaxy is but one of a vast, uncountable number, hundreds of billions of galaxies going off into distances that stretch billions of light years away. Some are shaped like ours, while others are structured very differently. But all are receding from us in direct proportion to their distances and thus seem to result from a "creation event," the "Big Bang," that took place nearly 14 billion years ago. Here, and really only here, among distant galaxies, is light travel time important, as it allows us to look into the truly distant past to see what happened and how the Universe was born and evolved.

Continuing On

As the Dipper is to northern hemisphere spring and summer, Orion (the Hunter) is to winter, his three star Belt prominently crossing the sky close to the celestial equator. He is said to have been poisoned by Scorpius, the Scorpion. The gods then put them in the sky opposite each other so that Orion need not look upon his killer. In another story, he was struck down by an arrow accidentally shot by his lover, Diana. He's most famously depicted as raising his club against charging Taurus, the Bull, which lies to the northwest. Look down from the Belt to the Sword he carries below. Even binoculars will reveal the center of the Sword to be surrounded by the Orion Nebula, a huge cloud of glowing interstellar gas lit by hot stars that is the marker of a great star-forming engine some 1500 light years away. At his right shoulder (he is facing you) is the first magnitude star Betelgeuse. One of the larger stars in the Galaxy, this cool "red supergiant" would nearly fill the orbit of Jupiter. At a distance of nearly 600 light years, the great star shines with the light of 85,000 Suns, implying a mass nearly 20 times that of the Sun. Elsewhere we see stars even larger and brighter.

Down and to the left of Orion, we find the opposite extreme. Here, in Canis Major (Orion's larger Hunting Dog), shines Sirius, the brightest star in the sky. It's not only intrinsically bright, but it's close too, only 9 light years away. Circling around it every half century is a tiny blip of a hot star 10,000 times fainter than Sirius itself, which requires it to be not much bigger than Earth. But from Kepler's laws and an assessment of the system's center of mass we find a mass for faint companion about the same as that of the Sun. The average smeared out density of the Sun is a bit more than that of water, which by definition is a gram per cubic centimeter, about the size of a sugar cube. Stuff that into a body a hundredth the solar size and you get an average density of a ton per cubic centimeter. Sirius B, a "white dwarf," is the end product of stars like the Sun. Betelgeuse, a fine example of a dying massive star that is destined to explode, will produce a remnant far smaller, a 30- kilometer "neutron star," these stars revealing something of the immense range in stellar properties.

Betelgeuse and Sirius are the northwestern and southern apices of the Winter Triangle, which also includes Procyon in Canis Minor (the Smaller Dog) at the northeastern corner. (Oddly, Procyon has a white dwarf companion too.) The Winter Triangle's clone is the Summer Triangle, made of Vega (in Lyra, the Lyre) at the northwestern apex, Deneb (the tail of Cygnus, the Swan) at the northeastern point, and Altair in Aquila (the Eagle) at the southern, the triangle framing the Milky Way. Vega, the third brightest star in the northern hemisphere, is surrounded by an infrared-radiating disk that implies a circulating planetary system, though no planet has ever been seen. First magnitude Deneb is fainter than Vega (25 light years away) only because of its great distance of 1400 light years. If this white supergiant were placed at Vega's distance, it would shine 15 times more brightly than Venus, cast modest shadows, and be easily visible in daytime.

Where there be "supergiants," there must be "giants." A prime example is Arcturus in Boötes, the Herdsman, who drives the Great Bear around the pole. This brightest star of the northern hemisphere can be found by following the curve of the Big Dipper's handle southward. And where there are giants there must be "normal" stars, which like the Sun and Sirius are quiet hydrogen fusers that for historical reasons are called "dwarfs." And there you have the basic kinds of stars. We'll look at the origins of all these later. But in quick summary, stars are born as hydrogen fusing dwarfs. Lower mass ones, those under 8-10 times that of the Sun, become first giants and then white dwarfs, while the more massive stars turn into supergiants and then explode.

Star Names

Where do the star names come from? Many derive from the Greek words for the star's character or position. "Sirius," the name of the brightest star in the sky, comes from the Greek for "searing," while "Arcturus" means "the Bear watcher." Admiring Greek astronomy, the ancient Arabs added names appropriate to the star's location within its constellation (adding names from their own indigenous constellations as well), which were commonly mistranslated back into Latin. The large majority of star names are thus of at least Arabic origin, even if unrecognizable. "Betelegeuse" comes from "bet al Jauza," meaning "the hand of the central one." Most straightforward is "Deneb," Arabic for "tail," which is applied to the end of Cygnus, the Swan.

In the early 1600s, Johannes Bayer, who constructed a magnificent star atlas from Tycho's positions (plus others in the southern hemisphere), gave Greek letters to the stars in conjunction with the Latin possessives of the constellation names (all of which now have three-letter abbreviations). Sirius then becomes Alpha of Canis Major, or Alpha Canis Majoris, Alpha CMa. This scheme, still broadly used today, was followed about a century later by one from John Flamsteed, the first Astronomer Royal of England, who was given the task of telescopically measuring the positions of a large number of stars for navigational purposes. His work was lifted by Newton and Halley, who published it early and added numbers from west to east within the constellation of residence. Vega, Alpha Lyrae ("Alpha of Lyra," the Lyre), is also 3 Lyrae. Beyond these names are those from dozens of specialized catalogues.

The Zodiac, Astrology, and Black Holes

A curious mix. But you'll see why, after which we continue the tour. Now go farther down the shining path of the Milky Way to find the scary figure of Scorpius, the Scorpion. Looking just like what it is supposed to be, Scorpius is one of the constellations of the Zodiac. These, like Leo (the Lion), Aries (the Ram), Taurus (the Bull), etc., are the foundations of the pseudoscience of astrology. Sun-sign astrology, wherein your daily fate is determined by the Zodiacal location of the Sun at birth, is but the tip of the pseudo-iceberg. To the believer, the planets (named after the gods) have specific influences upon us that are modified by their positions relative to one another and to the Sun and Moon (which also represented celestial deities). Their power is then sent to the individual through 12 fixed "houses" that encircle the sky, one for love, another for money, and so on. The astrologer calculates where all these fit at the person's birthday and gives out various pronouncements about his or her personality and fate.

Look around the neighborhood. To the north of Scorpius is the sprawling figure of Ophiuchus, who is enwrapped by Serpens, the Serpent, the only constellation to come in two parts, the Head (Serpens Caput to the west) and the tail (Serpens Cauda). Ophiuchus is loosely related to the Lacoön, a figure of the Trojan War whose statue resides in the Vatican. Ophiuchus descends to us through the caduceus, the physician's symbol. His modern boundaries lying across the ecliptic, Ophiuchus is sometimes called the "thirteenth" constellation of the Zodiac, though he really does not belong there. Nevertheless, there are people who consider themselves "Ophiuchans" and get quite upset if you dismiss him.

To the east of Scorpius, within the heart of the Milky Way and within the Zodiac, lies Sagittarius, the centaur Archer, whose outline is best known for its upside-down "Little Milk Dipper." Sagittarius contains a treasury of bright interstellar clouds, "nebulae," of which the "Lagoon" and "Trifid" are the most prominent. Sagittarius's greatest possession, though, is the center of the Galaxy, first seen as a radio source called "Sagittarius A." Later radio observations with continental-sized interferometers found much of the radiation to come from a near-point called Sagittarius A*. Impossible to see in the optical spectrum due to the dimming effects of interstellar dust, infrared observations of stars in orbit around it show Sagittarius A* to weigh in at four million solar masses stuffed into a body smaller than the size of the orbit of Mercury.

Our only conclusion is that Sag A* must be a supermassive black hole, a body so dense that light cannot escape its gravitational grip. Toss a ball in the air. It comes back. But throw it up at 7 miles per second and, though it slows down, it will never return. Squeeze the Earth and the gravity at the surface increases, so you have to throw the ball faster to get it to escape. When the Earth is about the size of a golf ball, the escape speed hits that of light, no radiation can get out, and the Earth becomes a black hole. This is admittedly an unlikely scenario, but real ones of stellar sizes and the monsters at the centers of galaxies (some of which dwarf our own) indeed exist; the evidence is overpowering.

Though the Galaxy's central black hole itself is invisible, its surroundings are very much not. Gravitational interactions among closely orbiting stars can bring one too close, whereupon tidal effects can shred it and then cast it into a hot, brilliant surrounding disk, from which hot gas can pour out in jets in the direction perpendicular to the disk. The central massive black hole of a galaxy is among its most luminous features and can be so energetic as to influence the whole system, indeed can even affect neighboring galaxies.

By odd coincidence, the winter solstice is pretty closely aligned with the center of the galaxy. Because of precession of the Earth's axis, solstice alignment will happen every 13,000 years. This event, which more or less coincides with the ending of the Mayan "long count" calendar, signifies to some the end of the world. Among other things, in 2012 it is supposed to cause the Earth's rotation, or magnetic field, or both, to reverse, buildings to fall, continents to split, and the world to be destroyed. Apparently, this happens periodically. Note that there is no ancient evidence for it, and that we are still here.

Again Continuing On

Now visit autumn's (and the Zodiac's) Taurus the Bull, with its two prominent ragged "open clusters," the Pleiades (Seven Sisters) and the Hyades, which makes the Bull's charging head. In mythology, the Pleiades were the daughters of the god Atlas and mortal Pleione, the Hyades their half sisters. Both are harbingers of cool days to come. Open clusters, which contain from a few to a few thousand stars, are to be compared with the rare great "globular clusters" that jam hundreds of thousands, even millions of stars into much the same space. The best known in the north is the globular in Hercules, a somewhat dim constellation to the northeast of Arcturus. The brightest is Omega Centauri (in Centaurus, the Centaur), which lies well into the southern hemisphere.

Taurus also holds the amazing Crab Nebula. The gaseous expanding remnant of the supernova (exploding star) of the year 1054, it is now some 10 light years across. At its center is a collapsed "neutron star" with a density of a million metric tons per cubic centimeter (a million times that of Sirius's white dwarf). About 20 kilometers across, containing more than a solar mass, with a magnetic field a trillion times that of Earth, it spins 30 times per second and powers the whole nebula. A bit more massive and it would fall into its own black hole. Neutron stars are the collapsed old nuclear-burning cores of stars born with at least 8-10 solar masses, of supergiants like Betelgeuse and Deneb, which are destined to explode as supernovae. (Under the right circumstances, white dwarfs can go off too.)

Andromeda and Family

The last supernovae to be seen in our Galaxy were Tycho's Star of 1572 in Cassiopeia and Kepler's of 1604 in Ophiuchus. Cassiopeia brings us to a whole set of related figures of the Andromeda myth, all nicely on display in autumn skies. Queen of Ethiopia, wife to King Cepheus, Cassiopeia irritated Neptune by bragging that her daughter Andromeda was lovelier than the sea nymphs. As punishment, he ordered the maiden to be chained at the coast to be devoured by Cetus, the Whale or Sea Monster. But along comes Perseus on Pegasus (his Flying Horse), returning home from slaying the dreaded Medusa. With hair of snakes, one look at her would destroy you. He sees the maiden, and what's a hero to do but to save her by showing Cetus the Medusa's head, which turned the monster to stone, following which the couple is happily wed. And they all right there for you to look at.

Perseus, like Cassiopeia and Cepheus in the Milky Way, is known for its Double Cluster and for Algol, the Demon Star (the Medusa herself), in which two orbiting stars eclipse to cut the light in half every 2.9 days. Andromeda is best known for the Andromeda Nebula. A bit more than two million light years away, it's the nearest galaxy comparable to ours and the farthest thing you can easily see with the naked eye. Cetus is famed for Mira ("the wonderful"), a "long-period variable star" that can be a bright part of its constellation for a month or two and then completely disappears from view in a nearly year-long cycle. Pegasus has the first star with a known orbiting planet and a beautiful globular cluster, while Cepheus contains a variable star, Delta Cephei, that holds the key to the distances to far-off galaxies. A standard light, members of its family were used to make the first measure of the Andromeda galaxy's distance. It and the rest of its kind are indispensable to modern cosmology. Cepheus also has among the largest stars known, "Herschel's Garnet Star" (Mu Cephei) and VV Cephei, another eclipser whose bigger member approaches the size of Saturn's orbit.

The Moderns

The modern (seventeenth and eighteenth century) constellations are a mixed bag. The northern ones feature sets of stars faint enough that they were ignored by the ancients, falling into the "amorphotoi," the unformed. Presumably they were a canvas upon which the gods could paint more stories. Examples are Camelopardalis (the Giraffe), Lynx, Monoceros (the Unicorn), Fornax (the Furnace), etc. Many also are the fallen, those that did not succeed and that include such gems as Officina Typographica (the Printing Office) and Bufo (the Toad). What is fascinating about the moderns compared with the ancient figures is how both reflect the life and technology of the times. What would we have today? Celestial running shoes, computers, and cell phones?

The deep southern hemisphere is different, as there are several bright (as well as faint) patterns that could not be seen and recorded by the ancients and that include the eponymous Phoenix, Triangulum Australe (the Southern Triangle), and dim Octans (the Octant), which holds the south celestial pole. The best known is a hybrid, a configuration known since ancient times but re- worked as Crux, the Southern Cross and that was taken from the feet of Centaurus. At the southernmost reach of the Milky Way (the northernmost in Cassiopeia and Cepheus) and with two stars of first magnitude, Crux is a spectacular figure, especially when combined with first magnitude Alpha and Beta Centauri to the east of it, the configuration presenting an unforgettable sight. Just four light years away, Alpha Cen (Rigil Kentaurus, the Centaur's Foot) has already been cited as the closest star to the Earth. The actual nearest, however, and not by much, is a dim telescopic companion to Alpha not unreasonably called "Proxima Cen." Alpha itself is double, its brightest member a near solar clone. Is anybody there? Leaving the tour, we now approach the stars themselves.

V. BIRTHS OF STARS AND PLANETS

Where did all these stars come from? Are there new ones still coming along? Why are there so many different kinds? Part of the key lies in the Milky Way, that great band of stars made of the disk of our Galaxy of which we are a part. It's incredibly structured with complexes of dark splotches, which are interstellar clouds filled with obscuring dust. Among the best known is the "Coalsack" tucked into the southeast corner of the Southern Cross. Some clouds in the southern hemisphere are so dark and thick that the Incas made dark constellations from them, the Coalsack their "Yutu," a partridge-like bird. Through the telescope, dark clouds, small and large, fill the Milky Way, the Horsehead Nebula in Orion a fine example. The individual clouds gang together within the central plane of the Milky Way, making the white band look like a sandwich with a dark filling.

The clouds are made of gas and tiny particles of dust, the latter blocking distant stars. The blend of gas and dust has pretty much the solar composition of 90 percent hydrogen, 10 percent helium, with a tiny fraction of everything else. Where it is thinly spread, the interstellar gas, which fills the Milky Way, is heated to thousands of degrees by energetic light from hot stars, causing the hydrogen to glow. If a cloud is thick enough, though, the dust acts like a shield that sends the cloud's internal temperature plummeting, in the most extreme cases to near absolute zero. The cold temperatures allow the hydrogen atoms to combine into hydrogen molecules (made of paired hydrogen atoms), the darkest of the obscured blobs thereby called "molecular clouds." Then follows a complex chemistry (that includes "cosmic rays," high-speed particles from exploding stars) that builds a huge variety of chemicals that include water, ammonia, alcohols, acetic acid, uric acid, and substances too fragile to exist on Earth. Nobody knows how complex the chemistry might get. It seems at least possible that we are seeing the seeds of life beginning to form, even if they are (rather, were) not directly responsible for us here on Earth.

Everywhere within and around the interstellar clouds we find evidence for star formation. Heat a gas and it expands, chill it and it contracts. The clouds are shredded by collisions among themselves and rocked by shock waves from nearby exploding stars. A shock wave is formed when a disturbing particle moves faster than the natural speed of sound. Examples are the bow wave off a speeding boat and a sonic boom from a supersonic aircraft. The "boom" is not the "breaking" of the sound barrier, but is a continuous over-pressure in the air that follows along after the craft. (And yes, as long as there is a transmitting medium, there is "sound" in space, even if it could not be heard by a human.)

The result is that the molecular clouds are lumpy, filled with denser blobs. If one gets dense enough, it can contract under its own gravity to form a star, or if subdivided into bloblets, even collections of stars that include doubles, multiples, and clusters. But the cloud is rotating (why would it not?) and as a result we have to consider the conservation of angular momentum that was encountered in Kepler's second law of planetary motion. As a singular blob of gas and dust collapses, it should rotate faster, enough to tear itself apart. Various means slow the rotation. Chief among them seems to be the magnetic field of the rotating Galaxy, which grabs onto the blob thanks to ions from penetrating cosmic rays. But the blob still cannot be brought to a halt. As its core contracts and thus heats inside, some of it is flung out into a disk of dusty gas from which emerge opposing perpendicular jets that help carry away even more angular momentum. When the core's dense center heats up enough inside, into the millions of degrees, it hits the point where it can sustain thermonuclear fusion, which halts the contraction, allowing a new star to be born. The vast majority of stars are of low mass, far less than that of the Sun, perhaps all the way down to what we might call big planets. The greater the mass, the fewer blobs there are until much above 100 solar masses, no stars are made at all.

The matter in the surrounding disk begins to accumulate from tiny dust grains into bigger particles, then into rocks, then larger "planetesimals," then into planetary cores. Say the new creation is our Sun. Out near where Jupiter is now, it's cold, and the new bodies can accumulate light hydrogen, water, and other volatile stuff. In the inner system, where developing sunlight keeps things too warm to accumulate light stuff, we get stuck with the planetary cores, which become our Earth and the other terrestrials. The heat of formation, especially with larger bodies, partially melts the new planets (even what will eventually be the asteroids), which sends heavy iron downward to form the metallic planetary cores and allows light would-be rock to float to the outside. The debris from a final collision with a competing planet created our Moon. As the new planets cool and accumulate the last debris, they become heavily cratered, and if nothing wipes the craters away, they are still there for our perusal (as on the Moon, Mercury, parts of Mars, various asteroids, and outer satellites). Recycling much of its crust into the mantle, helped by erosive processes, Earth got rid of the evidence long ago, as did Venus.

It all sounds so logical and simple. Other stars might have planetary systems that look a lot like ours. Curiously, as we will see, those we have found (and there are a lot of them) commonly do not resemble "home." Nobody really knows why. Or, in the big question, why life turned up here and not on Mars (or so we think) or anywhere else. Recall here also the roles played by exploding stars, which aid in cloud chemistry, blob formation within the molecular clouds, and blob rotation. Stellar life, our life, thus emerges from stellar death.

VI. KINDS OF STARS AND THE IMPORTANCE OF SPECTRA

As we saw in the tour, there are lots of different kinds of stars. As for bugs and elephants, the first step is classification, a seeming dreaded word implying extreme boredom. To the contrary! Through classification, we open up stellar natures.

We had little clue about stellar kinds, just colors, until we could look at their spectra, which (like that of the Sun) consist a colored background upon which are superimposed dark "lines," spectral gaps, caused by absorption by hydrogen atoms, those of various metals, and so on. On the back of a number of false starts (and one that was really good), E. C. Pickering of the Harvard College Observatory got the stellar idea of classifying stars on the basis of the hydrogen lines, "A" for the strongest, "B" for weaker, and so on down to "Q," the criteria subjective but defined by particular stars.

From among the dozens of female observatory assistants, three stood out: Annie Cannon, Antonia Maury, and Williamina Fleming. The trio largely took over the classification task. They and Pickering dropped some classes as erroneous or duplicated and un- needed, then re-arranged them to account for continuity among other absorptions (in particular what eventually turned out to be those of helium). The result is the fundamental, classical, alphabet of stellar astronomy into which most stars fall, OBAFGKM, which Cannon also decimalized. And, lo! the spectral sequence correlates with star color, O stars blue, B and A white, FGK progressively more yellow then yellow-orange, M stars reddish. Low temperature stars can emit only lower energy photons, and are red to the eye. As temperature increases, higher energy photons come out and the apparent color of a radiating body changes, going through those of the rainbow, becoming progressively orange, yellow, white (subbing for green), and then blue. It was thus brilliantly clear that the spectral sequence is also a temperature sequence that runs from hot and blue O near 50,000 Kelvin through white A stars like Vega near 10,000 K, past the class G2 Sun at 6000 K, on into class M in the 3000 K range. The first thing that any astronomer wants to know about a star is its class.

The wild changes that appear in the spectra along the sequence are not caused by differing chemical compositions, but by increasing ionization and efficiencies of absorption as one climbs the temperature ladder. While hydrogen lines make a strong showing, O stars are known for their ionized helium, B stars for their neutral helium, while from class A on down we see a diminishment of metal ionization, atomic excitation, and weakened hydrogen absorptions caused by a lowering of the efficiency of line production. Class M is highlighted by molecules, particularly titanium oxide, which are broken up at higher temperatures by collisions among ever-faster-moving particles. All can be calculated using atomic rules, from which chemical abundances can be found, and lo again!, most are similar to those found in the Sun (with some wild variations on the side). Here are the classes and their characteristics (new classes L and T discussed later).

THE SPECTRAL SEQUENCE
Class Spectrum Color Temperature
O ionized and neutral helium, weakened hydrogen bluish 31,500-49,000 K
B neutral helium, stronger hydrogen blue-white 10,000-31,500 K
A strong hydrogen, ionized metals white 7500-10,000 K
F weaker hydrogen, ionized metals yellowish white 6000-7500 K
G still weaker hydrogen, ionized and neutral metals yellowish 5300-6000 K
K weak hydrogen, neutral metals orange 3800-5300 K
M little or no hydrogen, neutral metals, molecules reddish 2100-3800 K
L no hydrogen, metallic hydrides, alkali metals red-infrared 1200-2100 K
T methane bands infrared under 1200 K

The Big Tool: the HR Diagram

The magnitude of a star as we see it, the "apparent magnitude," depends on the star's actual luminosity in the visual band (in watts) and on the distance. Place a star 10 times farther away and it looks 100 times fainter. By the early nineteenth century we had enough distances through parallax (the tiny shifts in position caused by the orbiting Earth) to allow us to calculate actual luminosities. As a substitute for wattage, astronomers use "absolute magnitude," the apparent magnitude the star would have at a distance of 32.6 light years (the odd number the distance at which the Astronomical Unit would appear one second of arc, 1/3600 degree, across). The Sun, at apparent magnitude -27, appears brilliant, but only because it's close. Put it at the standard distance and it shrinks to fifth magnitude, and would be invisible in a moonlit sky.

We now have two parameters, temperature through class and luminosity through absolute magnitude. Why not see how they correlate? That's what Henry Norris Russell of Princeton and Denmark's Ejnar Hertzsprung did. The brightness of a heated dense body like a star depends on the fourth power of its temperature. Double T and 16 times more energy (2**4) comes tumbling out. We thus would expect hot stars to be the brighter. And in the "Hertzsprung-Russell (HR) diagram," that is what we find. Perverse as always, the HR diagram plots absolute magnitude (with numbers getting smaller as we go up to brighter stars) against OBAFGKM, along which temperature declines to the right.

In one of the great surprises of the 20th century, there is a branch on the diagram in which stars become brighter the COOLER they get. The only way this could be is that such stars must be bigger, that is, have more radiating surface area. So Russell divided stars into two fundamental kinds that reflect sizes, "giants" and "dwarfs." As big as it is, the Sun is a class G2 dwarf, Arcturus a K1 giant. While so-called dwarfs can be large (10 or more times that of the Sun) at the upper end, giants are much bigger, 10 to 100 or so times the solar dimension.

But that is hardly all. Across the top of the HR diagram are sprinkled a few immensely brighter coolish stars that must be bigger than the giants, the already-introduced "supergiants." Of Solar System proportions, the largest (like VV Cephei mentioned above) approach the orbit of Saturn in size. Then way down on the diagram, we see another fainter sequence that dims with temperature. Given their high temperatures, their dimness can be explained only by small sizes, about that of Earth. The first ones found, Sirius B and Procyon B, are white. Since the term "dwarf" had already been used for main sequence stars, these were called "white dwarfs." Because they actually range across the whole temperature sequence, there are blue white dwarfs and red white dwarfs. Sirius A is a dwarf that is white, Sirius B a white dwarf. And they are not the same thing! Astro- nomenclature, historically derived, is strange indeed, but is so ingrained from more than a century of use that it is pretty much unchangeable.

On the HR diagram, the dwarfs fall into a continuous band, the "main sequence," that runs from dim cool red dwarfs at lower right on the diagram to hot B and O dwarfs at upper left. Studies of orbiting double stars, in which we find masses from Kepler's Laws, show that the main sequence is a mass sequence that runs from just under ten percent the solar mass among the faintest red dwarfs to more than 100 times solar in class O, none of the former visible to the naked eye, the latter visible across vast stretches of the Universe. By analogy with the Sun, dwarfs are also all fusing hydrogen to helium in their cores. Though the higher mass stars have cores that run off a different set of reactions (the "carbon cycle," which uses the element as a nuclear catalyst), the result is the same: four protons are turned into an atom of helium. The giants, supergiants, and white dwarfs (plus kinds yet to be named) must be doing something else. The red dwarfs are hugely numerous, their creation from interstellar matter preferred. Of the stars on the main sequence (classes O through M), 70 percent are dim red dwarfs. On the other hand, the hot, massive blue O and B dwarfs are rare, which is a GOOD THING as they are destined to explode and you would not want one to be very close. Fortunately, their rarity makes it unlikely that there would be one in our immediate neighborhood. And there isn't.

The descending main sequence stops at 0.08 solar mass because the core temperature gets so low (as a result of less gravitational compression) that the nuclear reactions that transform hydrogen into helium cease. But, since one thing has nothing to do with another, why would nature stop condensing bodies at that same mass? For decades, astronomers struggled to find substars, "brown dwarfs," that are too cool inside to run the reaction chain. They would also be so cool outside that they could radiate little but infrared radiation. Suddenly, toward the end of the twentieth century the technology got good enough to find them, and there they were all over the place. There were so many that two new classes had to be added, L and T. Cooler than M (2400 to 1400 Kelvin), about half the L dwarfs are also brown dwarfs, while T, which goes to less than 1000 K and might overlap with planets, are all brown dwarfs. The final sequence then reads OBAFGKMLT. The discoveries just increase nature's propensity toward dim stars.

VII. STAR LIVES

Stars have fixed fuel supplies and cannot last forever. Eventually, the fuel runs out. Can the various non-dwarfs be explained by aging effects? Both observation and theory show us that main sequence dwarfs are remarkably stable affairs, which is demonstrated for us not only by the long term steadiness of the Sun, but by the main sequence's very existence. As hydrogen is converted to helium and the number of atoms per unit volume (never mind their kinds) shrinks, the core must contract to maintain pressure, which also jacks up the temperature and that in turn makes the reactions run faster. The balance is almost perfect, allowing main sequence stars to achieve great stability.

But there is that "almost" to deal with, as the compensation is not quite exact. Calculations show that when the Sun was born 4.5 billion years ago it was 30 percent dimmer than it is now. The Earth should have been cold. Yet life came along very quickly, the anomaly called the "faint young Sun paradox." What could have kept the Earth warm enough? Two possibilities come to mind. The solar wind drags the Sun's magnetic field out with it. Still attached to the Sun, the field lines act like ropes that slow the rotation. Spinning much faster in the early days, solar activity and its potential heating effects could have been much greater. Alternatively, the carbon dioxide level could have been higher, raising the greenhouse effect and consequently the terrestrial temperature. Or for that matter, both, or something else. We just don't know.

Coming back to the present, the Sun, now nearly five billion years old, has another five billion years of core hydrogen- fusion, and thus main sequence life, left to it. But again the compensation will not be perfect. The once-fainter Sun is destined to slowly brighten, probably making life impossible a billion years hence. Such change has nothing to do with global warming, as it is far too slow.

The Hydrogen Runs Out

And when it does, the fun starts, at least if you are watching from another planetary system. When the fuel supply is gone and energy production stops, the core loses its support. It then has to contract, which makes it heat, just the opposite of what you might expect. The heat spreads into the surrounding hydrogen- rich envelope, which causes H-fusion in a shell around the now- dead helium core. Calculations show that the Sun, or any sunlike dwarf of a few solar masses, then also takes a sudden turn to the right on the HR diagram, toward cooler surface temperatures, but at the same luminosity, which requires that it also swell to higher radius. First behaving as an intermediate "subgiant," the cooling star next begins to brighten into a real "red giant" to a thousand times the current solar luminosity. The core, still surrounded by its hydrogen-fusing shell, shrinks to near the size of Earth while the outer photosphere expands to the sizes of the orbits of the inner planets. Mercury is doomed.

You can extract energy by fusing lighter atoms together up to number 26, iron, and by fissioning heavier atoms down to it. Iron is the bottom of the energy well, which is why there is so much of it. To fuse further than iron REQUIRES energy. And it takes successively higher temperatures to go up the nuclear ladder to get there. When the core temperature hits 100 million degrees, the helium atoms suddenly start to fuse together. But the fusion product of two He-4 atoms is very unstable beryllium- 8, which almost immediately falls back to He-4. To get energy takes a near-simultaneous collision of three He-4 atoms into ordinary carbon-12. (That is why there is so little lithium, beryllium, and boron, elements 3, 4, and 5, in the Universe. They get skipped over, number 2 helium going directly to number 6 carbon.) Another hit with a helium nucleus makes oxygen. With a new core energy source (surrounded by a shell fusing hydrogen into helium), the star stabilizes. The core expands, the outer envelope shrinks, the luminosity declines some, and the star (and our Sun) becomes one of the huge number of cool class K orange giants in the sky. Without them, the constellations would be severely altered and if you took out the class A dwarfs would pretty much disappear altogether.

The amount of time it takes to make a giant out of a dwarf depends on mass, the main sequence also a lifetime sequence. Though they have far more fuel than the Sun, higher mass stars also have higher internal temperatures and thus use their fuel at such a frantic pace that they live remarkably shorter main sequence lives, at the top but a few million years. Down below the Sun, while stars get feebler, they also stretch out their lives. At about 80 percent the solar mass, the lifetime becomes greater than that of the Galaxy. No star with a lesser mass, which includes all in types K and M (and L), has ever died. Their long lives just make low mass stars all the more numerous (nature also loving to make them), while the short lives of high mass stars contribute mightily to their rarity.

Then the Helium Runs Out

Eventually, the helium fuel in the core must run out, all of it turned into carbon and oxygen. Reverting to its earlier behavior, the core again contracts and the star brightens even more than it did before as helium fusion moves out into a surrounding shell (which in turn is surrounded by a hydrogen fusing shell, the two turning on and off in sequence). The stellar radius again vastly increases, in the case of the Sun wiping out Venus and at least approaching Earth. This can't go on forever of course, else the sky would be filled with brilliant dying stars. The end, and perhaps the salvation of Earth, is achieved through winds. Though it hardly seems that way to us, our Sun pumps out a really weakling wind. As the giants get ever larger, their surface gravities go down, allowing for stronger winds. The cause changes too. The solar wind is a creature of the Sun's magnetism. In giant stars, the push of radiation takes over. "Second-timers" with dead carbon cores become so luminous that radiation acting on condensing dust grains (that then couple to the gas) helps blow winds that can reach 10 billion times more strong than what we experience out of the Sun. At the same time, the stars can become highly unstable and pulsate, varying wildly over periods of months and years. The prime example is the star Mira, which has a "now you see it, now you don't" relation with its constellation Cetus. The winds, promoted by the pulsations, are SO strong that they remove most of the entire stellar envelope, leaving not much more than the old nuclear-burning core. Thus the future evolving Sun may never grow enough in size to reach us.

At this point, if the star has the right mass, convection currents might sweep some of the internal carbon upward, and the star dramatically alters its chemical composition, becoming a deep red "carbon star" of the sort known since the mid nineteenth century. At various other points of evolution, freshly made nitrogen and helium can be dredged up too. The dust in the star's wind is silicate-rich for ordinary advanced giants, carbon-rich for carbon stars. Here is the origin of much of the dust in the Milky Way, the same dust that helps make molecular clouds and form stars, stellar death again aiding in stellar birth.

Of crucial importance, elements heavier than iron can capture free neutrons, which allows manufacture of a variety of yet heavier elements that can also get dredged up to the stellar surface. Proof that "slow neutron capture" operates in these advanced giants is the observation of the element technetium. All of its isotopes are so unstable that there is none on Earth. Technetium's very existence shows that it is being made in the stars even as we watch. Nearly all our zirconium and good fractions of other heavy elements were made by this particular process before the Sun was born. Thus begins one of the great themes in astronomy, that other than hydrogen and helium (which came out of the Big Bang), our chemical elements were made in the stars and distributed into the cosmos through winds (and as we will see, more importantly through stellar explosions) and incorporated into later generations.

The End is Near

As the stellar wind reveals the near-dead core, the surface temperature of the star goes up, while at the same time the wind blows ever faster, which shovels the escaping mass in the wind into a dense, dusty shell. When the inner star hits close to 30,000 Kelvin, its growing ultraviolet light begins to ionize the expanding shell, giving rise to among the most lovely of celestial sights, a "planetary nebula." A misnomer, a planetary nebula has nothing to do with planets; the nebulae got the generic name from their discoverer, none other than William Herschel, who meant it to mean "disk-like."

A typical nebula appears as a complex disk with multiple shells and arcs, the structure reflecting the complexity of the wind that made it. At the center is the glowing ember of the star that pumped it out. The ancient stellar core can reach a surface temperature of well over 100,000 Kelvin, making the central stars of planetary nebulae among the hottest known. Back yard favorites for beginning amateurs are the Ring Nebula in Lyra and the Dumbbell Nebula in the modern constellation Vulpecula. Thousands are known. Their spectra, which consist not of absorptions but of emissions from a free, low density gas, show that a good fraction are enriched in nitrogen, helium, carbon, and other elements. The nebulae are carrying away the elements made in the stars back when they were advanced red giants and Mira-type variables. We are seeing the chemical enrichment of interstellar space. We carry within us the by-products of stars that lived and died before the Sun was born.

At some point that depends on the core's (and the original star's) mass, it gives up, stops heating, and begins to cool and dim. At the same time the surrounding nebula is expanding to meld with the cosmic interstellar gases, leaving the core behind. Now shrinking, the core becomes denser and denser as it turns into a white dwarf, the surrounding nebula disappearing altogether into the cosmic gloom. This process cannot go on forever either, else the white dwarfs would shrink to nothing and, instead of the myriads that surround us, we would see hardly an of them at all.

Degeneracy

Photons behave both as waves and as particles. So do electrons and other subatomic particles. The wave character means you can't really pin electrons (or for that matter, any other particles) down. Instead of "orbiting" their nuclei, electrons more surround it in wavelike fashion. The high internal temperatures of the white dwarfs cause the atoms to become highly ionized, which frees the electrons into a gas of their own. As the density approaches a ton per cubic centimeter, the wave character of the electrons causes them to interfere with one another to the point where they can get no closer together, and are said to be "degenerate." Collectively halting the contraction, they permanently stabilize the white dwarf, which has no future but to cool forever. The cooling time is so very long, however, that every white dwarf ever made is still visible. Such will be the fate of the Sun, which will turn into a white dwarf with just over half its current mass.

The most important aspect of the white dwarf state was worked out in the early twentieth century by a young Indian astrophysicist, Subramanyan Chandrasekhar, while on his way to England to work with Sir Arthur Eddington, one of the great figures of science. If you increase the mass of a white dwarf, the internal temperature goes up. The higher the temperature, the faster the electrons move. Inevitably, they approach the speed of light, and the rules switch over to Einstein's relativity. Chandra found that when a white dwarf hits a critical mass of 1.4 times that of the Sun, the degenerate electrons can no longer support the little star, and it has to collapse. White dwarf masses are closely tied to the mass of the original star. The Chandrasekhar limit is breached by stars with birth masses greater than 8 to 10 Suns. Beyond that stars can no longer make white dwarfs. Instead they make something else, something far more spectacular.

VIII. SUPERGIANTS AND SUPERNOVAE

Which is of course a part of "Star Lives," but one so important that it merits particular attention. Here we treat those special stars that explode. While dangerous if too close to us, exploders ("supernovae") are crucial to star formation and to the creation of the chemical elements. There are two kinds.

Core Collapse

Stars born above 8-10 solar masses, those of class O and the hottest of B, are so massive, and get so hot inside, that they can run the fusion chain farther. When the hydrogen runs out, they swell with dead helium cores to become supergiants, which (as for giants) stabilize by the fusion in the core of helium into carbon and oxygen. When the helium runs out, the C/O core is no longer stuck there to become a white dwarf, but can now contract until it gets hot enough to fuse to a mix of neon, magnesium, and oxygen. Which eventually fuses to a mix of silicon and sulfur, which goes at last to iron. While all this is going on, each earlier fusion stage moves outward into a shell, the inner part of the star layering itself like a celestial onion.

Iron can't fuse with itself or anything else to produce energy, as the nuclear energy is all extracted. When the iron core contracts there are no further fusion stages left to stop it, and it undergoes a catastrophic collapse. The density becomes so great that the iron atoms break down and all the freed protons and electrons are jammed together into neutrons. The collapse is abruptly halted when these and all the other neutrons interact as waves, like the electrons of the white dwarf. At a radius of 10 kilometers or so, they too turn degenerate. The stop is so violent that it sends a monster shock wave (aided perhaps by a flood of neutrinos and who knows what else, the theory "incomplete," that is, it does not work properly) throughout the rest of the star. The tremendous heat, in the billions of degrees, causes an expanding nuclear holocaust in the old core's shells and envelope that tries to drive everything back to iron. The expansion and cooling, though, do not let things get that far. The result, along with the rapid capture of neutrons, is the creation of all the elements of the periodic table beyond hydrogen, including a tenth of a solar mass of new iron and the really heavy stuff like uranium. The optical "supernova" is so spectacular that it is visible across much of the Universe. (And that is but a hundredth of the total energy output.) Most of the stuff of which Earth and we are made was created in supernova explosions long before the Sun was born 4.5 billion years ago.

The great supernova of the year 1054 that produced the Crab Nebula in Taurus was one of these as was the one that appeared in the Large Magellanic Cloud (a small companion galaxy to ours) in 1987. Though more than 150,000 light years away, it was easily visible to the naked eye. We also caught 11 neutrinos from it, just about the expected number, giving us a "look" inside the self-destroying star. For a few moments the Earth was flooded by more neutrinos than we get from the Sun!

As the old core collapses to become a "neutron star" the size of Manhattan, its magnetic field gets compressed to gigantic proportions, a trillion times that of Earth, and conservation of angular momentum makes it spin ever faster, to many times per second. The field is tilted to the rotation axis. Radiation beams sent out along the field axis then wobble madly in space and, if the Earth is in the way, we get a blast, the neutron star now also called a "pulsar." Thousands are known, most with longer rotation periods as they radiate away their energy. The expanding debris, rich with freshly-made elements, then expands outward as a "supernova remnant" mostly made visible by the shocked impact the explosion has on the surrounding interstellar gases, the result producing some of the grandest images to be seen in space.

Just like there is a mass limit to white dwarfs, so there is one to neutron stars, roughly three times that of the Sun. Beyond that limit, the degenerate neutrons can no longer provide support. A neutron star can then only collapse forever as it turns into a black hole from which light cannot escape. We know such black holes exist because there is a handful in double star systems, wherein a tidally distorted companion feeds matter into a disk around the black hole. The disk gets so hot it radiates X-rays. The best known of these is Cygnus X-1, in which the companion is a supergiant that first has no business emitting X-rays and second is being shifted in an orbit by a massive, otherwise invisible, body.

Collapse to black holes has been related to gigantic explosions from deep space that produce bursts of gamma rays first detected by a defense satellite in the 1960s that was designed to check for Soviet nuclear tests. Though rare within a galaxy, there are so many galaxies (billions) that we see maybe one of these gamma ray bursts (a "GRB") per day. The burst is beamed out through the rotation axis of the collapsing star and if we are in the way we see a high energy flash. As the gamma rays hit the surrounding matter lost previously by the star, they create an optical afterglow, the brightest of which might be visible to the naked eye even though billions of light years away.

The best candidate in the Galaxy for a GRB is the southern star Eta Carinae, which weighs in at a birth mass of around 100 solar masses. In the 1840s, the star, now just modestly visible to the naked eye, underwent a gigantic eruption that made it the second brightest star in the sky after Sirius and produced an expanding dusty cloud that virtually buried the erupter itself. Though 6000 or so light years away, if Eta Car went supernova, its possible gamma ray burst could be strong enough to damage Earth. Fortunately, the rotation axis seems to point somewhere else. Yet such an event may have happened in the past to produce one of the several mass extinctions that Earth has experienced. Even "ordinary" supernovae could do harm to the Earth's fragile atmosphere if within say 30 light years.

Novae and White Dwarf Supernovae

Given that there are "supernovae," where are the "novae" from which the term is derived? They are common, even plentiful, several recorded every year, one hitting first magnitude every generation or so. Ordinary novae are produced in close double systems in which an ordinary dwarf, usually one of low mass, feeds matter to a dense white dwarf. When the fresh hydrogen layer is compressed and hot enough, it undergoes a nuclear explosion that rips off the new surface layer and brightens the star enormously. Usually, the system returns to normal as it waits to produce another blast thousands of years hence. However, if the white dwarf is already fairly massive and can be pushed over Chandrasekhar's limit of 1.4 Suns, it has no choice but to collapse and flame out in a gigantic nuclear bomb, annihilating itself. The result is a "Type Ia" supernova that is brighter than the usual core-collapse (Type II) version and are exceeded only rarely by those that collapse into black holes. They make even more iron, about triple what the core-collapsers offer. The Ia's have the remarkable property of being very uniform and as such make wonderful luminous standards by which we can judge the properties of the distant Universe. From them we have found that the expansion rate is increasing as a result of a "dark energy" that dominates the mass-energy content of space.

The bottom line is that the two kinds of supernovae plus the evolution of ordinary giant stars have made us what we are today. While the Earth may seem puny and insignificant, it is really rather special, made of the distillate of billions of years of the nuclear debris of ageing stars, as are we ourselves.

IX. OTHER STARS, OTHER PLANETS

Which brings us right back home. We are the stuff of stars. And there is no reason to believe that we are alone. Should not other stars undergoing formation also distill planetary systems? We speculated on the subject for ages, but it has taken our own times to come up with the technologies to find them. And they are everywhere.

To examine them we return to what appears at first to be the scorned "ninth planet," Pluto. It is now treated not as the tail end of the planets, but as the lead body of the vast Kuiper Belt of leftovers that were probably just too thinly spaced to coalesce into what we would call a true "planet," which is as much a matter of semantics as anything else. Running between about 30 and 55 Astronomical Units, we find the tip of what lies atop billions of small bodies, from Pluto and its few mates on down to grains of dust from constant collisions that make an outer disk around the Sun. Asteroid collisions and comet destruction by the Sun add more dust on the inside.

If there are planetary systems surrounding other stars, they too should have dusty disks. The first turned up in the 1980s from satellite observation of the familiar bright star Vega, which is surrounded by an infrared radiating disk of small grains heated by starlight to modest temperatures. Should there not be planets buried someplace within? The same sort of disk appears around bright Fomalhaut (Alpha Piscis Austrini, in Piscis Austrinus, the Southern Fish, which lies south of Aquarius). More riveting, in a dazzling image, an edge-on disk some ten times the extent of ours is seen surrounding the fairly faint southern star Beta Pictoris (in modern Pictor, the Easel).

The first actual planet to be found orbits 51 Pegasi. The gravity between two objects is always mutual. As the Earth orbits the center of mass of the Earth-Moon system, the Sun orbits the center of mass of the Solar System. Considering just Jupiter, the Sun has an orbit about their mutual center of mass of about its own radius. Planets can therefore be detected through the motions they induce on their parent stars via the "Doppler effect." As you forge into a stream of waves, be they ocean, sound, or light, you hit the individual waves more often and the wavelength seems smaller. And vice versa if you run along the direction of the wave motion. The effect in sound is to change the pitch of a body moving relative to you. Given its huge velocity, the effect smaller for light, but can still easily be detected by the shift it induces in the barcode lines, which are moved toward the red if the radiating body is receding, to the violet if approaching. It matters not whether it is the source that is moving, you, or both. The degree of shift gives the relative velocity.

The barcode from the modest sunlike star 51 Peg shifted so as to announce a planet in orbit. From the velocity data and deductions about the nature of the star, we can extract the orbital radius, period, and (since we don't know the orbital tilt) a lower limit to the planetary mass. And what a surprise! We thought all planetary systems would be like our own, with inner terrestrials and outer jovians. And here was at least half a Jupiter mass orbiting in just four days only 0.05 AU away from its parent star. A HOT Jupiter.

The Doppler technique is rivaled by planetary transits. If the planet's orbital plane lies more of less along the line of sight, the planet will cut in front of the star once each orbital revolution and cause a slight dip in the star's brightness, from which we get the orbital period, the planet's diameter, and the orbital tilt, which with Doppler measures yield the actual mass. The "Kepler mission" was designed to observe transits on a grand scale. The Earth-orbiting Kepler satellite continuously monitors about 150,000 sunlike stars toward western Cygnus, a significant fraction of which are bound to produce transits. The real prize, though, is direct detection, actually SEEING the planet, which is tough to do in the glare of the star. But in a small number of cases, there they are. The bright star Fomalhaut holds one inside its dusty disk, while a star next door to 51 Pegasi has at least four planets, all seen to be moving about the star in the middle.

As the technologies and time base improved we discovered bodies the mass of Neptune, then superearths, multiple planet systems, planets around and within double-star systems (including a "hot earth" in a 3.2 day orbit about the lesser component of the nearest star, Alpha Centauri B, a class K dwarf), a waterworld with a steamy atmosphere, all in a bewildering array. More than 800 planets have been found to date and there are over two thousand candidates thanks to the Kepler mission. Each planet, or system of planets, seems to be unique to its own star, its setup depending on the original circumstellar disk's history, on its origin, development, and dissipation into planets.

A lot can be explained by youthful planetary movement caused by interaction with the residual debris disk and by mutual gravitational perturbations among the planets. Say a giant planet forms orbiting another star way out where it was cold, as were our Jupiter and Saturn. Friction with a dense residual gaseous disk could have brought it inward. If the disk dissipated just before the planet spiralled to its death, we are left with a "hot Jupiter." Many are the stars that probably devoured their planets. In our own system, simulations suggest that the young Jupiter moved inward some, while Uranus and Neptune moved outward, Neptune then responsible for shoveling up the icy bodies of the Kuiper Belt. Many too might be the stars whose planets gravitationally eject one another. The Galaxy may be filled with more vagabond planets wandering silently and coldly among the stars than there are stars themselves. We might have lost one ourselves.

X. LIFE

The bigger question is what might be ON those planets? The search for life is a difficult task, especially since we have only begun to explore our own planetary system for signs of it. And have so far found nothing but us. But there are clues we can look for. We can forget the giant jupiters as either too hot or too hostile. So we concentrate on the more earthlike bodies. The only way we can now know is through their spectra, which makes things devilishly difficult with our current technologies. But for example, if we detect the infrared signature of methane, we can at least have our suspicions, as methane, constantly destroyed by light from the central sun, must be renewed, and what better way for it than through life processes?

But bacteria, grass, and cows don't talk. The real search is for INTELLIGENT life. This quest has nothing to do with UFOs, "Unidentified Flying Objects." For decades, people have seen funny lights in the sky, or thought they had. Since nobody knew what they actually were, why not identify them as being driven by "space aliens?" Make sense? UFOs are a wildly mixed bag. Venus tops the list. So bright that it can throw shadows and be seen in the daytime sky, it follows along with you as you walk or drive. People have called the police on it, for that matter the police have even chased it. Then there are Jupiter, Mars, meteors (comet fluff that burns up upon hitting Earth's atmosphere), odd aircraft, hoaxes, conspiracy theories, and delusions. Nobody has ever encountered an alien, and nobody has any real proof they exist. Amateur and professional astronomers, who make a business out of watching the night sky, never see UFOs.

To find out if intelligent life really exists, we need communication that reaches out over the light years. We make the somewhat rash assumption that another civilization will use radio signals, as do we. The first to look for them was Frank Drake at the National Radio Astronomy Observatory in West Virginia. Picking a couple nearby sunlike stars, he found nothing. His work has morphed into various guises, chief of which is an institution dedicated to the "Search for Extraterrestrial Intelligence," now the "SETI Institute," which has the largest detection system in the world, one that uses a specialized radio telescope. After looking at countless stars, there is still nothing. (Yes, "looking." Radio astronomers do not "listen" to the sky. They look for natural, or in this case artificial, radio radiation, not signals superimposed on it, as in AM or FM radio.)

But what are the odds of finding anything? They are expressed through the "Drake Equation," which in summary form says that the number of communicating civilizations in our Galaxy is the product of the:



We can measure, or make guesses of, a couple numbers, but most are quite nebulous. We have no idea for example of the number of planets that can develop life, let alone intelligent life, let alone those with communicating possibilities. In our planetary system we have one planet out of eight with life. We don't know about Mars yet, but it is certainly not jumping out at us. Since the planet was once so Earthlike, why should it not have developed life? Its lack augers ill for believers. But the small possibilities are offset by the sheer number of stars, 200- 400 billion in our Galaxy alone. Let alone all the other galaxies, which number in the hundreds of billions.

We've been announcing ourselves for a mere century, not much given all the time our species has walked the Earth. But if anybody is out there watching, we surely have told of our presence. We have been broadcasting powerful radio signals into the cosmos since the 1920s. There is now a bubble of radio radiation expanding around us almost 200 light years across. Our planet is "radio bright," and anyone seeking us will find us. Imagine their view of us by watching our television shows.

If there is anyone else. Or anyone else who cares to look.

Origins

Though there are significant exceptions, an intriguing (if small) possibility derives from the discovery that stars with planets tend to be rich in metals, on the average a bit more than the Sun. (In the astronomers simple universe, there are but three chemical elements, hydrogen, helium, and everything else, the latter all lumped together as "metals.") To follow this lead we have to go back to the beginning of time.

To say our Galaxy is "one of many" is more than understated. When we look outside of our own, into the vast distances, the number of other galaxies seems uncountable. We estimate that given enough time, at our current technology level, we could count close to a trillion of them. Many are bunched into gravitationally bound clusters, with these assembled even into sets of clusters. Most profoundly, in all but a few of local exceptions (the Andromeda galaxy, which is part of our local cluster), the barcode spectrum shows that they are moving away from us with velocities directly proportional to their distances. The Universe is thus expanding. It's not expanding into anything, as it is already everything, it's just getting bigger on the large scale between clusters of galaxies. (Our Galaxy is not expanding, as its own gravity overwhelms the large scale effect). It's really a matter that involves Einstein's four-dimensional spacetime, which in principle can take on various qualities and shapes, from being bent outward and infinite to finite and closing in on itself. Running time backwards brings the galaxies closer together, and quickly reveals that at some time in the distant past they (rather, the matter of which they are composed) must have started out in a hot dense lump that quickly expanded in a "Big Bang." The evidence, which includes a steady radio noise that pervades space called the "Cosmic Microwave Background" from the initial fireball, is overwhelming.

We can look out to any distant galaxy, measure the velocity away from us, and then figure out how long it took to get there, which gives us an "age" to the Universe. Since there is a one-to-one relation between speed and distance, all galaxies give the same answer. But first we must determine the shape of spacetime, how it is curved and distorted by gravity, or even IF it is curved. That brings in two new concepts. Since the 1930s, it has been plain that the gravitational forces within and between galaxies is far greater than can be accounted for by stars and interstellar matter alone. Even factoring in the huge amounts of hot gas that lie between galaxies ("visible" through its X-ray radiation), we do not come close. Astronomers have had no choice but to invoke a mysterious form of matter that does not radiate, which is called simply "dark matter." On the large scale there is more than five times as much dark matter as normal everyday matter. While we have no firm idea of what it is, the leading candidate involves exotic massive but unresponsive particles that have yet to be found in the lab.

Then in the last gasp of the twentieth century, we found that the expansion of the Universe was not quite constant. We expected the expansion from the Big Bang to slow down under the dragging force of gravity (factoring in both dark and bright matter). Looking for the deceleration, two research groups found instead that the expansion is, of all things, accelerating. There must then be some mysterious force, a "dark energy," that pushes outward to make the galaxies scream away from each other at an ever- faster pace. We calculate the energy required, convert it to mass via E=Mc**2, and find that it constitutes three-fourths of the mass-energy of the Universe. Accounting for normal matter, dark matter, and dark energy, cosmologists find the Universe not to be curved in spacetime at all, allowing us to find an age since the Big Bang of not quite 14 billion years. Which fits very nicely with our measures of the ages of the oldest stars.

The Big Bang, though, seems to have produced nothing much but hydrogen and helium. As the initial "fireball" cooled, some of its energy condensed into protons (the first hydrogen atoms), neutrons, electrons, and other simple particles. Within the first three minutes, some of the resulting protons and neutrons could fuse permanently into helium. But unlike a star, whose core temporarily heats with time, the gas of the Big Bang was rapidly cooling, and after the helium was formed, it got too cool to make much of anything else except a bit of lithium.

The first heavy elements must have been forged from the first exploding stars. It's unclear which came first, the massive exploders or the original condensations of matter that would make the first galaxies, though the evidence falls on the first scenario. Whatever the case, these primitive assemblies lacked heavy elements. It took billions of years worth of stellar and galactic evolution, of giant stars and supernovae, to build the Galactic metal content to that of our Sun as it was born 4.5 billion years ago.

Could it be then that we are just the first ones, the first civilization, to come along, the first since we crossed some critical metal-abundance line? It's an intriguing thought that as flawed as humanity is we may be leading the way. (Though here are on thin ice. There are lots of lower metal stars with planets, three with VERY low metals. Nothing is ever simple.) The Universe thus may be teeming with life or we may be the lonely only ones around.

Possibly, the technology is not yet ready to finding life elsewhere. We encountered the same thing in our search for brown dwarfs and then for planets themselves. Numerous discovery papers were written based on marginal data. When the telescopes and detectors were up to the job, they seemingly fell from the sky as rain. Will life elsewhere be next? We can but wait it out.

We do know one thing, however, that we are the children not just of Earth, or even the stars, but of the entire Universe. It took all of it to make us. Far from being "insignificant" motes on a random planet, we are not only the progeny of the whole affair but can look outward to understand at least a little bit of the grandeur of what nature has given us. And if the past is an instructor, there is far more, many more surprises, yet to come.

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