A star is a body that at some time in its life generates its light and heat by nuclear reactions, specifically by the fusion of hydrogen into helium under conditions of enormous temperature and density. When hydrogen atoms merge to create the next heavier element, helium, mass is lost, the mass (M) converted to energy (E) through Einstein's famous equation E = mc squared, where "c" is the speed of light. The Sun is powered by hydrogen fusion, as are many of the other stars you see at night. The fusion does not take place throughout the star, but only in its deep interior, in its core, where it is hot enough. The temperature at the center of the Sun is 15.6 million degrees Kelvin (K = centigrade degrees above absolute zero, -273 C), and the density is 14 times that of lead. About 40% of the mass of the Sun, occupying about 30% of the radius, is capable of fusing hydrogen. Even under these extreme conditions, the Sun (as well as all other stars) is still a gas throughout. That said, things get more complicated, as nature creates "substars" called "brown dwarfs" that do not have enough mass and therefore internal heat to run full fusion. Even though they do not abide by the formal definition of a "star," they are still referred to as "stars" even if their masses are not much greater than those of planets.
In the second century BC, the Greek astronomer Hipparchus divided the stars into six brightness groups called magnitudes (now apparent visual magnitudes (m or V), first magnitude the brightest, sixth the faintest. The system is still used today, though with a mathematical definition (a star of one magnitude is 2.512... times brighter than the next fainter) that takes the very brightest stars and planets through magnitude zero and into negative numbers. Through the telescope we see much fainter, to near 30th magnitude (4 billion times fainter than the human eye can see alone). Though stars bear some resemblance to the Sun, they appear as points in the sky because they are so far away, the nearest, Alpha Centauri, four light years away. The light year is the distance a ray of light will travel in a year at 300,000 kilometers (186,000 miles) per second, so one light year is about 10 trillion kilometers (63,000 Astronomical Units, where the AU is the average distance between the Earth and the Sun). The stars are so far that distances were not measured until 1846, by means of parallax (viewing the star from opposite sides of the Earth's orbit). The most distant stars the unaided eye can see are over 1000 light years away, which is about the practical limit of parallax measures. The apparent visual magnitude of a star depends on true visual luminosity (in watts) and distance. To compare true visual luminosities, astronomers calculate the absolute visual magnitude (M), the apparent magnitude the star would have were it at a distance of 32.6 light years (10 parsecs, where the parsec is the professional unit of distance, equal to 3.26 light years). The absolute visual magnitude of the Sun is +4.83. Absolute visual magnitudes range from around -10 (a million times more luminous than the Sun) to below +20 (a million times fainter).
We need to set the stars in context. All those you see at night are part of our local collection of stars, all part of our Galaxy. Trillions of other galaxies flock the Universe, ours one of the larger ones. The principal part of our Galaxy (our own with a capitol "G") is in the shape of a flat disk about 100,000 light years across that contains some 200 billion stars. Our Sun is toward the nominal edge, about 25,000 light years from the center, the whole structure at the Sun's distance rotating with a period of 200 million years. The "edge" is not sharp, but just gradually fades away to much greater distances. A large portion of the disk's stars are set within pinwheel-like spiral arms that over millions of years come and go, the stars moving in and out of them as they orbit the Galaxy's center. The arms seem to come off of a central bar. Since we are in the disk, we see the combined light of its billions of stars around our head as the famed "Milky Way," the center of the Galaxy (which contains a supermassive black hole of three million solar masses) located behind the thick star clouds of Sagittarius. The age of the disk is about 10 billion years. Surrounding the disk is a thinly populated rather spherical halo that seems to date to about 14 billion years.
To create the conditions for such "thermonuclear fusion," stars must be massive. The Sun has the mass of 333,000 Earths. Stars can range up to about 100 times the mass of the Sun (at which point nature stops making them) down to around 7.5% that of the Sun, at which point the internal temperature is not high enough to run the full range of nuclear reactions (which requires at least 7 million degrees Kelvin). "Substars" below the 7.5% limit, called "brown dwarfs," do exist in significant numbers however, and down to around 1/80 the solar mass (13 Jupiter-masses) can fuse their natural deuterium (heavy hydrogen, with an extra neutron). The lower limit to brown dwarf (substellar) masses is not known. Masses are measured from double stars and can be calculated from luminosity and temperature using the theories of stellar structure.
Stars are made of the same chemical elements as found in the Earth, though not in the same proportions, the chemical compositions found from the stars' spectra. Most stars are made almost entirely of hydrogen (about 90% by number of atoms) and helium (about 10%), elements that are relatively rare on our planet. About a tenth of a percent is left over, that tenth containing all the other elements found in nature. Of these, oxygen usually dominates, followed by carbon, neon, and nitrogen. Of the metals, iron usually dominates. Nevertheless, there is only one atom of oxygen in the Sun for every 1200 hydrogen atoms and only one of iron for every 32 oxygen atoms. However, within this tenth of a percent, the proportions of the numbers of atoms in the Sun is rather similar to what we find here, in the Earth's crust. Other stars can deviate considerably, depending on their states of aging or upon where they are in the Galaxy. Halo stars, including globular clusters, typically have heavy element contents only a hundredth that found in disk stars, the result of their being older.
Stars are supported, kept from shrinking under their own gravity, by energy generated by internal fusion of light atoms into heavier ones. Fusion of hydrogen into helium can take place only under the extreme conditions of temperature and density found in a star's deep core. The "proton-proton chain" operates in ordinary stars (those that have not yet begun the death process) with masses more or less like that of the Sun and under (while higher mass stars do it by the carbon cycle. It begins when two protons (bare hydrogen atoms) ram together strongly enough to overcome the mutual repulsion caused by their positive electric charges and get close enough to stick together under the "strong force" (which operates only over a very short range). One of the protons ejects its positive charge in the form of a "positron," a positive electron that hits a normal electron to generate energy in the form of gamma rays. The conversion creates a deuterium (heavy hydrogen) nucleus as well as a tiny particle called a "neutrino." Detection of neutrinos on Earth allow us to "see" directly into the solar center. The fusion of the deuterium with another proton produces a light form of helium (with two protons and one neutron), while the fusion of two light helium atoms into a normal helium atom with two protons and two neutrons (with the ejection of two protons) completes the process, each reaction generating heat and light as a result of a slight loss of mass.
Higher mass stars (with masses greater than about 1.5 times that of the Sun) fuse hydrogen into helium via the "carbon cycle," which works only under high-temperature conditions, but is then more efficient than the proton-proton chain. It begins when a normal carbon atom (C-12, with 6 protons and 6 neutrons) picks up a proton to make radioactive nitrogen-13, one of whose protons ejects a positron (positive electron) to make stable carbon-13 (with the additional ejection of a neutrino). Carbon- 13 plus a proton makes normal nitrogen-14, while an additional proton collision makes oxygen-15, which (like N-13) decays into nitrogen-15. The N-15 collects another proton, and then falls apart into the original carbon-12 and a helium nucleus. Each event produces some energy either itself or through the collisions of positrons and electrons.
The space between the stars is filled with dusty gas. Thick dust clouds can even be seen with the naked eye within the Milky Way blocking the light of distant stars and providing much of the Milky Way's structure. Interstellar matter is compressed by the Galaxy's winding spiral arms. The clouds can be further compressed through collisions or by blast waves from exploding high-mass stars ( supernovae). Lumps of matter therefore form within the interstellar clouds. If their gravity is great enough, they can condense into one or more stars. Contraction causes more rapid spin, which creates a disk around the birthing star, from which it can draw matter. Further condensation within the disk can create planets (or even stellar companions). The contraction of forming stars raises the internal temperature, finally to the point of ignition of hydrogen fusion. Gravity would like to make the star as small as possible, but the fusion reactions stabilize it and keep it from contracting any further. The whole life story of a star from here on out is told by the battle between gravity and nuclear fusion, first one, then the other getting the upper hand. New high mass stars commonly light up their surroundings to produce diffuse nebulae like the Orion Nebula.
For decades, astronomers predicted the existence of substars we now call "brown dwarfs," stars too small and light (of insufficient mass, less than 0.073 solar masses) to run the full nuclear fusion process (from ordinary hydrogen to helium). After all, the star formation process should "know" nothing of the conditions under which nuclear reactions should turn on. Brown dwarfs (which are still called "stars") turned out to be so cool that only new infrared technologies could find them. We now know they are very common, so common that new classes, L and T (cooler than M) had to be made for them. Between 0.073 solar masses (78 Jupiter-masses) and 13 Jupiter-masses, brown dwarfs do fuse their natural deuterium (heavy hydrogen, with an extra neutron) to helium. Below 13 Jupiters, fusion stops altogether. As noted above, the lower end of brown dwarf masses is not known. They quite likely overlap the masses of planets. Planets are by current definition made from the "bottom up," accumulated from dust in disks surrounding new stars, while stars (including brown dwarfs) are made from the "top down," by direct condensation from interstellar gases. But here even the definitions become confused and might overlap as well.
As a new star condenses from a gaseous lump in interstellar space, it spins faster, the outer parts of the contracting cloud spinning out into a dusty disk. The dust particles, in orbit about the new star, accumulate, building themselves into planets. Here at home, the planets that formed close to the Sun (Mercury through Mars) were in an environment too hot to incorporate much water or light atoms like hydrogen, so they are made of heavy stuff like iron, silicon, and oxygen. In the outer System, the planets contain huge amounts of hydrogen and helium and could grow large, their satellites made largely of water ice. Other stars should grow planets too, planets that could be quite different from our own and that are now being discovered.
There are many kinds and classes of stars. Those that are actively fusing hydrogen into helium in the middle, that is, in their cores (either through the proton-proton chain or the carbon cycle), are called "main sequence" stars. (For historical reasons, main sequence stars are also commonly referred to as "dwarfs"). The main sequence is the first stage following birth. In general, main sequence stars have chemical compositions similar to that of the Sun. The higher the mass of the main sequence star, the greater its diameter and the higher its surface temperature. Dimensions range from about 10% the size of the Sun (which is 1.5 million kilometers -- 109 Earths -- across) to just over ten times solar, and surface temperatures from under 2000 degrees Kelvin to about 49,000 K (the Sun's surface is at 5780 K). Around the beginning of the 20th century, astronomers divided the stars (of all kinds including giants, supergiants, and others) into seven basic lettered groups that they later learned were related to their surface temperatures, which for the main sequence are: O (above 31,500 K), B (10,000 - 31,500 K), A (7500 - 10,000 K), F(6000 -7500 K), G(5300 - 6000 K), K(3800 - 5300 K), and M (2100 - 3800 K). A century later, two more classes were added to account for faint red stars turned up by new technologies: class L (1200 - 2100 K) and T (below 1200 K), the whole set now OBAFGKMLT. Class L is a mixture of dwarfs and brown dwarf substars, while class T consists entirely of brown dwarfs. The Sun is a G star. The system is decimalized, making the Sun class G2. Examples of main sequence stars are Acrux, Vega, Sirius, Porrima, Chara, Alpha Centauri A and B, and Proxima Centauri. The classes are actually derived from the stars' spectra. The stellar astronomer's greatest tool is the HR diagram, a plot of absolute visual magnitude against spectral class, in which we can see nearly all of the stages of stellar life and death. On it, the main sequence is a band that runs from the highest-mass hydrogen-fusing stars at the upper left to the lowest masses at the lower right.
Since the color of a heated body depends on temperature, the different classes take on different, though subtle, colors, from slightly reddish or orange for class M to orange-yellow for K, through yellow-white to bluish for classes B and O. Star colors can be noted rather easily even with the unaided eye, especially when those close together (a in double star pairings) contrast against each other. Stars of classes L and T, none of which are visible to the naked eye, range from red through deep red to "infrared" (these optically invisible under any circumstances). Carbon stars like R Leporis, whose blue spectra have been removed by line absorption, are also deep red. Most of these are advanced giants. Color can be expressed numerically by the difference in magnitudes measured at different wavelengths. The observed color of a star compared to the color expected from the spectral class allows the calculation of the dimming of the starlight by interstellar dust.
Main sequence (dwarf) stars have only a certain amount of internal fuel available within their hot cores. When the hydrogen fuel has all turned to helium, the stars begin to die and to produce a number of other different kinds: lower mass stars become giants, while those of higher mass (above roughly 8 or 9 solar masses) into supergiants. Giants then die as white dwarfs, while supergiants explode as supernovae. The whole process is commonly known as stellar evolution. Because higher mass stars use their hydrogen fuel much more quickly than lower mass stars, those of higher mass live shorter lives. The Sun has a 10 billion year main sequence lifetime (of which half is gone). The most massive stars live only a couple million years, the least massive for trillions, so long that no star with a mass less than about 0.8 solar masses has ever died in the history of the Galaxy. From theory, we calculate that such a 0.8 solar mass star should live for about 12-13 billion years. The Galaxy should be about as old as its oldest stars, and is thus 12-13 billion years old, in accord with the 13.7 billion year age of the Universe found from its expansion rate.
Begin with stars more or less like the Sun, those with masses
from about 0.8 times that of the Sun to about 5 times the solar
mass. When the fuel in a solar-type star's core runs out, the
helium core contracts under the effect of gravity and heats up.
Hydrogen fusion then expands into a shell around the old
burnt-out core, and so much energy is produced that the star
brightens and expands by many times over, the expansion cooling
the surface, turning the star into a class M red giant.
When the core temperature hits around 100 million degrees Kelvin,
the helium is hot enough to fuse into carbon (through the near-
simultaneous collision of three helium atoms) and even a bit
further, into oxygen. This new power source stops the core's
contraction and the star stabilizes for a time, dimming and
heating somewhat at the surface. We commonly see these helium-
fusing stars as yellow-orange type K giants. Good examples of giant stars are Aldebaran and Arcturus. Such stars can have diameters
tens of times that of the Sun. The giant and subsequent stages
up to the actual death of the star (the end of nuclear fusion)
takes roughly 10 or 20 percent of the main sequence lifetime.
From about 5 solar masses to 9 or so, helium fusing stars have
higher temperatures and may appear as class F and G giants and
even supergiants.
Giants can also be defined strictly by their spectra. On the HR diagram, the giants run roughly from the
middle toward the upper right (higher luminosity), where they are
fusing helium, are about to do so, or have already done so.
Class A and B giants are only somewhat cooler than dwarfs of the
same absolute visual brightness and are not yet fusing helium.
Among G and K giants, because of their lower gas densities,
temperatures are up to a few hundred degrees cooler than they are
for main sequence dwarfs of the same class.
Between the dwarf and giant stages, stars appear as subgiants. Like giants, dwarfs, and supergiants, they can be defined by their spectra and position on the HR diagram. In the context of stellar evolution, they are stars that have just given up core hydrogen fusion or are about to do so and, with helium cores, are making the transition to becoming true giants.
When more massive stars (2 to 8 times that of the Sun) pass through mid-temperatures either on their way to fusing helium or during various stages of helium fusion, they can become unstable and pulsate in size, temperature, and luminosity. The first of these discovered, Delta Cephei, gave the name "Cepheid" variable to the group. Cepheids, usually classed as F and G supergiants (though not as massive as true supergiants), vary by a couple to a few magnitudes over periods of one to 100 days. A strict relation between absolute magnitude and pulsation period allows us to determine their distances (period gives absolute brightness, and comparison to apparent brightness yields distance.) Cepheids are the major keys to learning distances to other galaxies. The brightest Cepheid in the sky is Polaris , though the variations are too small to be seen by eye. Cepheids occupy the upper range of the HR diagram's "instability strip."
When the helium in the core has turned to carbon and oxygen, the core shrinks again, and the helium begins to fuse to carbon and oxygen in a shell around the old core, this shell surrounded by another one fusing hydrogen into helium, the two turning on and off in sequence. The star now brightens again, expands even more, and becomes cooler and even redder than before. As the star brightens it becomes unstable and begins to pulsate, the pulsations making it vary, or change in brightness. The star become so huge, near or greater than the orbit of the Earth, that the pulsations can take a year or more. The first of these found, Mira in Cetus, changes from second or third magnitude to tenth, becoming quite invisible to the naked eye. Such stars are now called "long-period variables" (LPVs) or "Mira variables." Thousands, all cool class M giants, are known. On the HR diagram, such advanced giants are at the cool end of the "giant branch," the Miras occupying the coolest and brightest portion. In astronomical jargon, such stars are called asymptotic giant branch stars (or AGB stars) because of the appearance of their distribution on the HR diagram.
The gases of red giants can circulate upward to the tops of the stars, carrying the by-products of nuclear fusion with them. Oxygen is normally more abundant than carbon. If conditions are right, the surfaces of some stars can change their chemical compositions, some becoming very rich in the carbon that was made below by helium fusion, resulting in the reversal of the normal ratio. Mira variables and other old red giants thus divide into oxygen-rich stars like Mira itself and carbon stars such as 19 Piscium and R Leporis Raised up along with the carbon are elements such as zirconium and many others that have been made in a huge variety of nuclear reactions that go on at the same time as helium fusion. Other stars' surfaces are enriched in helium and nitrogen.
Such huge giant stars have low gravities and lose mass through powerful winds that blow from their surfaces. Some of the gas condenses into molecules and dust. There may be so much that the star can be buried in it and become invisible to the eye, the glow of the heated dust seen only by its infrared (heat) radiation. Oxygen-rich giant stars make silicate dust, while carbon stars make carbon-dust similar to graphite and soot. Most of the dust that inhabits interstellar space began this way, though since inception it has been highly modified in the freezer of interstellar space. These stars therefore play a powerful role in later star formation. The winds are so strong during the giant stage of a star's life that it can lose half or more of its mass back into space, whittling itself down to little more than the parts that underwent nuclear fusion.
As a giant star loses almost all of its remaining outer hydrogen envelope, it comes close to revealing its intensely hot core. A fast wind from the core first compresses the inner edge of the old expanding wind. High-energy radiation from the hot core then lights up this inner compressed portion, which is now many times the size of the whole Solar System. These illuminated clouds, which can be quite beautiful, were discovered by William Herschel around 1790, who termed them"planetary nebulae" for their disk-like appearances (they have nothing else to do with planets). The best known is the Ring Nebula in Lyra. Their complex appearances depend to a degree on how matter is lost from the giant stars that make them. Expanding at rates of tens of kilometers per second, they last no more than a few tens of thousands of years. From their emission spectra we can analyze their chemical compositions, and find that many are enriched in the by-products of prior nuclear fusion in the parent advanced giant stars.
As the planetary nebula dissipates into the gases of interstellar space, it leaves behind the spent, old core that now includes the dead nuclear fusing shells. These stars, made of carbon and oxygen and compressed under their own gravity, have shrunk to about the size of Earth. The first ones found (Sirius-B, Procyon-B, and 40 Eridani B) were fairly hot and white, so the class acquired the name "white dwarf" to discriminate it from the main sequence of stars (which were originally called "dwarfs" to distinguish them from the giants). Though small, white dwarfs still contain near the mass of the Sun, giving them astonishing average densities of a metric ton per cubic centimeter. The tremendous outward pressure provided by tightly packed "degenerate electrons" (which behave like waves that keep them from getting closer) prevents gravity from shrinking white dwarfs any further. White dwarfs are therefore also called degenerate stars. These small stars, the remains of stars that began their lives between 0.8 and 9 or so solar masses, no longer have any source of energy generation and are destined only to cool. The cooling time is so long, however, that all white dwarfs ever created are still visible, though the oldest are becoming cool, dim, and reddish. (There is no such thing as an invisible, cold "black dwarf.") The age of the Galaxy calculated (with the aid of theory) from the oldest white dwarfs roughly agrees with that derived from the coolest (lowest mass) evolved main sequence star. On the HR diagram, they fall in a line rather parallel to, but far fainter than, the main sequence dwarfs. Masses of white dwarfs are tied to the original stellar birth masses and range from about half a solar mass (for a birth mass roughly solar) to a limit of 1.4 times that of the Sun for a birth mass of 8 or 9 solar. Beyond 1.4 solar masses, the degenerate electrons can no longer provide support, and the core must collapse, the star exploding as a supernova. Overflow of the limit by mass accreted from a close binary companion can also produce collapse and a different kind of supernova.
As they start to die, higher mass stars (those with masses over about 9 or 10 times that of the Sun) initially develop the same way as giants, but then their course of evolution becomes very different. High mass stars are already large and luminous. As their dead helium cores contract, heating and firing to fuse the helium to carbon and oxygen, the stars expand to approach the sizes of the orbits of the outer planets, becoming distended red "supergiants." Excellent examples are first magnitude Betelgeuse in Orion and Antares in Scorpius. Supergiants are so massive, in spite of great mass loss through huge winds, that nuclear fusion can proceed farther than it can in ordinary giants. When the helium runs out, the carbon and oxygen mixture compresses and heats, causing it to fuse to a mixture of neon, magnesium and oxygen. Hydrogen and helium fusion had already moved outward into nested shells around the core. When carbon fusion dies out in the core, leaving a mix of neon, magnesium, and oxygen, it too moves outward into a shell. The neon-magnesium-oxygen mixture now in the core then heats and fuses into a mix of silicon and sulfur, each fusion stage taking a shorter period of time. During the course of their evolution, red supergiants can also contract some and heat to make blue supergiants. The great mass-loss suffered by supergiants can strip some of them of their outer envelopes to the point that we see huge surface enrichments of helium, nitrogen, and carbon that have been made by nuclear fusion. Look for them scattered across the top of the HR diagram.
Finally, the silicon and sulfur fuse to iron, an element that is incapable of energy-generating fusion reactions. Gravity now wins the war that has been going on for the star's lifetime, and since the iron refuses to support itself, the core catastrophically collapses. The iron breaks down into its component particles, protons, neutrons, and electrons (the constituents of atoms), and the whole mass gets compressed into a tight ball of neutrons only a few tens of kilometers across. The collapse produces a shocking blast wave that rips through the surrounding nuclear fusing shells and the remaining outer envelope, and rips the rest of the star apart. On Earth we see the star explode in a grand " supernova," an event so powerful it is easily visible even in another galaxy a huge distance away. The part of the star that is exploded outward is so hot that nuclear reactions produce all the chemical elements, including a tenth of a solar mass of iron, which then blend with the gasses of interstellar space, out of which new stars are formed. A supernova can also be caused by the collapse of a white dwarf in a binary system.
There are ways of making supernovae other than through core collapse. Nevertheless, supernovae are still rare, taking place in our Galaxy only two or three times a century. Most are hidden from us by the vast clouds of dust that birth the stars. On Earth we observe about five supernovae per millennium, and have not seen one since Kepler's Star of 1604 (probably created in the collapse of a white dwarf, as described later). The great supernovae of 1006 , 1054 (the "Chinese Guest Star"), and 1572 (Tycho's Star) were visible in daylight. Our knowledge of supernovae comes almost entirely from observing them in other galaxies, the best of these exploding in 1987 (SN 1987a) in the Large Magellanic Cloud, a companion to our Galaxy some 165,000 light years away. But keep your eye on Betelgeuse or Antares, which are quite good candidates for core collapse. An even better candidate is the southern hemisphere's Eta Carinae, which underwent a huge eruption in the 19th century and produced a surrounding nebula, a vast cloud of dusty gas. The star should go off within the next million years or so. At their current distances, the explosions of such stars would rival the brightness of a crescent Moon. The blast is so powerful that it if occurred within 30 or so light years, it would probably damage the Earth. Fortunately, no candidate is nearly that close (though such nearby events have almost certainly happened in the past).
As the debris of a supernova clears, we see a gaseous expanding shell around the old star, the "supernova remnant," which consists of the debris of the explosion that is rich in the by- products of myriad nuclear reactions mixed in with local interstellar matter that is compressed by the mighty blast. Supernova remnants are readily identifiable by their X-rays and radio radiation. We believe all the iron in the Universe has come from such (and related) explosions. Indeed, between ordinary giants, planetary nebulae, and supernovae, all the elements other than hydrogen and helium (and some lithium) were created in or by stars. The most famous supernova remnant is the Crab Nebula in Taurus, the remains of the great supernova of 1054, which was well observed by Chinese astronomers. Tens of thousands of years after a supernova event, we may still see the blast waves sweeping through the gases of interstellar space, compressing and heating them and perhaps making new stars.
At the center of the expanding cloud is a lone neutron star spinning many times per second, with a mass greater than the Sun, a diameter the size of a small town, and an amazing density of 100 million tons per cubic centimeter. As white dwarfs are supported by "degenerate electrons," neutron stars are supported by degenerate neutrons. The magnetic fields of such collapsed stars are magnified along with the density to strengths millions of millions of times that of Earth. The magnetism is so strong that radiation is beamed out the magnetic axis. The axis is tilted relative to the rotation axis (like that of the Earth), and wobbles around as the little star spins, the beamed energy spraying into space. From a distance, the star looks like a lighthouse: if the Earth is in the way, we get a blast of radiation, and from here see the neutron star as a "pulsar. Young pulsars emit from low-energy radio waves through high- energy X-rays and gamma rays. As the pulsar ages, it slows, and finally emits only radio waves, which is the case for most of the 600 or so pulsars known. When the rotation period is about 4 seconds there is insufficient energy for the pulsar to be seen at all, and it disappears from view. Not fusing anything, the neutron star is held up forever against gravity by pressure exerted its own extreme density.
The collapsing star of a supernova will turn into a neutron star only if its mass is less than about two or three times that of the Sun. If the mass is greater, then even the star's huge density cannot hold gravity back, and instead of a neutron star the supernova creates a "star" that nothing can support against gravity, and the body contracts forever. At a small enough radius, the gravitational force becomes so great that light can not escape, and the star disappears forever into a collapsing "black hole." What we refer to as the black hole is actually a kind of "surface" at which the velocity required for escape equals light-speed. What goes on inside is unknown. The center of our Galaxy, 26,000 light years away, contains a supermassive black hole called Sagittarius A* that carries some three million solar masses.
Most of stars you see at night have companions, with a great many obviously double ("binary") even through a modest telescope. The components of some double stars are nearly equal in mass and brightness. More commonly, one dominates the other, sometimes to the point where a little companion is not really visible at all, and detectable only with the most sophisticated techniques. At the lowest end, we have stars with low-mass brown dwarfs for companions. The stars of some doubles are so far apart that they take thousands of years to orbit; others are so close that they revolve around each other in only days or even hours. Gravitational theory allows us to measure the masses of the stars from the orbits' characters; indeed such measurements are the only way in which we can find stellar masses. Examples of visually-seen double stars are Alpha Centauri, Acrux, Almach, Albireo, and Mizar.
Double stars are vitally important in the measure of stellar masses, which are derived from Kepler's Laws as generalized by Isaac Newton. Pretend a lesser star goes around a stationary more massive one, as seen for example for Alpha Centauri, Castor, or Algieba. The first law states that the orbit must be a conic section (circle, ellipse, parabola, or hyperbola), here specifically an ellipse with the more massive member at one focus, the second law that the orbiting star speeds up in a known way as it gets closer to its mate, slows down as it gets farther away. The crucial third law states that the square of the orbital period in years equals the cube of the average distance between the stars divided by the sum of the masses (in solar masses), which can then be found. In reality, the two stars orbit a common center that lies on a line between them positioned from each in inverse proportion to the mass ratio. The sum of masses along with the location of the center of mass (and thus the mass ratio) then gives the individual masses, which can be used to test theory. The mass of the Sun is found using the orbit of the Earth (whose mass is inconsequential).
Stars can also bond into more complicated multiples. There are two kinds, stable "hierarchical" systems and unstable "trapezium" systems. In the first, a distant star orbits an inner double (which it senses gravitationally as one) to make a triple (as in the Zeta Cancri system), or two doubles may orbit each other as a quadruple, of which Epsilon Lyrae (THE famed "Double- Double") is the prime example. In more complex systems, a star or even another double can orbit an inner triple or double-double to make a quintuple or sextuple system (like Mizar-Alcor or Castor). The structures of the orbits will depend on relative masses. In the second kind of multiple, named after the Trapezium (Theta-1 Orionis) in Orion's Sword, the member stars are all rather mixed together, which allows close encounters to eject stars until some kind of stability is achieved. Trapezium systems must all therefore be young.
Formation is still contended. The oldest idea involves simple fission. When a new star condenses from the interstellar gases, it spins faster. If the contracting blob is spinning rapidly enough, it can separate or otherwise develop into a pair or stars rather than a single star. Each of these contracting components can further separate into a double, producing a "double-double" star, the most famous of which is fourth magnitude Epsilon Lyrae. This idea is now widely discounted. More likely scenarios involve capture within a dense stellar environment, fragmentation of the collapsing birthcloud, and condensation of a companion from a the circumstellar disk that surrounds a new-born star. Formation of multiples is even less understood, especially of stars with distant fragilely-bound members of the sort we find in the Alpha Centauri system.
If the two stars of a pair are fairly close together, and if the plane of the orbit is close to the line of sight, each star can get in the way of the other every orbital turn, and we see a pair of eclipses, one of which is usually of much greater visibility than the other. Eclipsing systems are very important in stellar astronomy, and are used to help determine masses, to find the stars' diameters, temperatures, and even to assess shapes in the cases that the stars' mutual gravities distort each other. Eclipsing doubles are quite common, the most famous second magnitude Algol in Perseus.
In a double star system in which the two have significantly different masses (by far the most common), the higher mass star will use its internal hydrogen fuel the fastest and become a giant first. We then see a red giant, or maybe a helium-fusing, orange class K giant coupled with a main sequence star, also very common. Eventually, the giant produces its planetary nebula and dies as a white dwarf. Good examples of such systems are Sirius and Procyon, each of which are orbited by the tiny dead stars. For each of these systems, and for many others, the white dwarf is by far the LESS massive of the pair, proving that stars really do lose a great deal of their mass back into interstellar space.
If the two stars of a double are close together, they can interact. When the more massive becomes a giant, its surface significantly approaches that of the other star. The lower-mass main sequence star can then raise tides in the giant, distorting it. If the two are close enough, matter can flow from the giant to the main sequence star. Good examples that display such behavior are Algol and Sheliak. In more extreme cases, the lost matter can encompass both stars, creating a "common envelope." Friction will then bring the stars even closer together, making the process go yet faster. The stirring of the lost mass can create unusually distorted planetary nebulae. At the end, the white dwarf created from the giant finds itself very close to the remaining main sequence star. In high mass double stars, the higher-mass component can explode and produce a nearby neutron star or even a black hole companion.
Some giant stars have the masses and internal constructions that allow them to bring by-products of deep nuclear fusion to the stars' surfaces, in the most extreme examples creating carbon stars. Mass lost from one of these enriched giants to a close companion can contaminate the companion with the giant's newly-formed chemical elements. When the giant becomes a white dwarf we are left with a seemingly single star (main sequence or evolved giant) with an odd chemical composition. Only with determined observation can we tell that a dim white dwarf is present. Among the most prominent examples are "barium stars" (Alphard an example), giants that have very strong absorptions -- and great overabundances -- of the heavy element barium among several others. All seem to be companions of what were once mightier stars that had become carbon stars and that are now reduced to white dwarfs.
If the white dwarf and main sequence remnant of a close double are close enough, the white dwarf can raise tides in the main sequence star, and mass will flow the other way, from the main sequence star to the white dwarf. Theory and observation both show that the flowing matter first enters a disk around the white dwarf from which it falls onto the white dwarf's surface. Instabilities in the disk can make such a star "flicker" over periods of days and weeks, even producing sudden outbursts of light. The star that became the white dwarf had lost almost all of its hydrogen envelope during its own evolution. When enough fresh hydrogen from the main sequence star has fallen onto the white dwarf, it can, in the nuclear sense, ignite, fusing suddenly and explosively to helium. The surface of the white dwarf blasts into space, the star becoming temporarily vastly brighter. On Earth we see a "new" star or " nova" (meaning "new in Latin) erupt into the nighttime sky, not a new star at all but an old one undergoing eruption. Novae are common, 25 or so going off in the Galaxy every year, once a generation one close enough to reach first magnitude. Nova Cygni in 1975 rivalled Deneb, giving the celestial Swan two tails.
In a massive double star system, the more massive of the pair may develop an iron core and explode as a supernova, becoming either a neutron star or a black hole. Either of these stellar remains in turn may raise tides in the more-normal companion, causing matter to flow into a disk around the collapsed body, from which it falls into an immense gravitational field. Matter in the disk is so hot it can radiate X-rays. From the motion of the normal star, we can calculate information on the mass of the collapsed one. If the mass of the dark orbiting companion is below a two-to-three solar mass limit, as in X Persei, it is a neutron star. But if the mass is great enough, we can infer the existence of an orbiting black hole, the best actual proof we have. Fresh hydrogen falling from the disk onto a neutron star can produce great variability, become compressed and fuse to helium, and then explode violently as the helium fuses to carbon. The result is an X-ray burst similar in nature to a nova.
The term "supernova" is derived from "nova" in that the supernova is vastly brighter, no matter that the mechanism of the core collapse of a supergiant is completely different from the mechanism of nova production. White dwarfs, however, can also produce supernovae. No white dwarf can exceed a mass of 1.4 times that of the Sun, a limit discovered in the 1930s by Subramanyan Chandrasekhar when he applied relativity theory to the gases in white dwarfs. If the limit is exceeded, even the white dwarf's enormous pressure cannot hold gravity back and the white dwarf must collapse into a neutron star or a black hole or perhaps even annihilate itself. There are two alternative theories for such an event. A massive white dwarf may accept enough mass from a close main sequence companion and be pushed over the edge before a nova eruption can take place. The white dwarf then collapses, creating a supernova that is grander even than one produced by the collapse of a supergiant's iron core. The main sequence star of a double that contains a white dwarf can also evolve through the giant stage to become a white dwarf, creating a DOUBLE white dwarf system. If the two have been drawn close enough together by interaction during a common envelope phase, they can spiral together by the radiation of gravitational waves predicted by relativity theory. The white dwarfs then merge, again producing a spectacular supernova. In either case, the collapse and resulting explosion makes nuclear reactions that again create all the chemical elements and even more iron than in the type of supernova produced by the collapse of the iron core of a massive star. Kepler's supernova of 1604, the last seen in this Galaxy, was probably of this kind.
Stars have a strong tendency to be born in groups, in whole clusters. If they are bound together tightly enough by their own gravity, they can survive for millions, even billions, of years, even for the lifetime of the Galaxy. There are two kinds, open clusters and globular clusters. Open clusters, the sparser but by far the more numerous of the two, are found in the disk of the Galaxy, and therefore lie largely in the plane of the Milky Way. Many of the closer ones, such as the Pleiades and Hyades, are easily visible to the naked eye. Some are angularly so large that they make constellations of their own, or at least significant parts of them. Thousands of open clusters dot the Galaxy's disk. Though their sizes vary greatly, they typically contain a few hundred loosely arranged stars packed within a diameter 10 or so light years across. And though bound together by their own gravity, most open clusters gradually break up as a result of random encounters among stars that speed members to the escape velocity, and because of stretching by tides raised by the Galaxy. Open clusters thus tend to be young, under a billion years of age (indeed, some are just born), though in the far reaches of the outer Galaxy they can survive for far more than a billion years.
Somewhat related to open clusters, stellar associations (commonly called OB associations) are large, loosely organized, stellar groupings that lie within the Milky Way and are defined by their young, blue O and B stars. Gravitationally unbound, OB associations are expanding systems, their stars moving away from individual common centers that may have core open clusters. Though they contain stars with a full range of masses, associations are best recognized through their most massive, luminous, and hottest members, which cannot get far away from their birthplaces before they expire, rendering ageing associations essentially invisible. OB associations are mostly named for their constellations of residence, for example Orion OB1, Perseus OB2, Upper Scorpius, and so on. Several constellations, in particular Orion and Scorpius, owe their prominence and sparkle to being made largely of OB associations. Having massive stars, associations are prime sources of supernovae.
While there are thousands of open clusters in the Galaxy, there are but 150 or so known globular clusters. Distinct from open clusters, their home is a huge spheroidal halo that surrounds the Galaxy's disk. And while open clusters are sparse, loose, and comparatively young, globulars are compact, closely spherical, and can contain over a million stars packed into a volume only a hundred or so light years across. Their compactness gives them (at least those that have survived) long lifetimes. With ages of 11 to 12 billion years, formed when the metal content of the Galaxy was much less than it is today (the increase the result of stellar evolution), they are among the oldest things known and among the first things to be created after the Big Bang, the event that formed our Universe.
Clusters of both kinds are profoundly important in establishing the distance scale in astronomy and in testing and guiding the theories of stellar evolution -- the ageing process. Clusters are born with an intact array of stars that occupy the entire main sequence (dwarf sequence) of the HR diagram, in which the higher the mass, the greater the luminosity. Since high mass stars die first, a cluster evolves by losing its dwarf sequence from the top down. Application of evolutionary theory can then tell us the age of any given cluster by the most luminous and hottest dwarfs still left. The Double Cluster's hot class O stars tell that it is young. The most luminous stars of the somewhat older Pleiades are of class B, while the still older Hyades has lost even these. Open clusters range in age from just born to nearly 10 billion years, which gives the age of the Galaxy's disk. By contrast, globular clusters have lost the entire upper main sequence (dwarf) population down to stars somewhat below a solar mass, which gives them ages 11 to 12 billion years or so, nearly the age of the Galaxy itself. In most dense globular clusters, and even in some open clusters, some stars with masses higher than the main sequence cutoff linger, refusing to evolve. Since these stars are also bluer in color than the majority of evolving dwarfs, they are called blue stragglers. Blue stragglers are believed to be caused by stellar mergers within the dense cluster environments, either by direct collision or by the mergers of close double stars, which increases their masses beyond the cutoff and thus seems to hold back their evolution.
Stars can range in size, depending on mass and age, from only a few kilometers across to the diameter of the orbit of perhaps Saturn. They can range in temperature from near "cold" at only 2000 K for an extreme red giant through far over 100,000 K for the star inside a planetary nebula to over a million K for a neutron star. All the stars you see in the sky will eventually expire, some soon, some not for aeons. Lower mass stars create planetary nebulae and white dwarfs, while higher mass stars make supernovae that result in neutron stars or black holes. Double stars add spice to the product, making novae and a different kind of supernova. All these endings send newly made chemical elements into the interstellar stew, out of which new stars are made. As a result, the heavy element content of the Galaxy increases with time. Ancient main sequence stars, the subdwarfs, and their giant star progeny have a low abundance of metals, whereas younger stars like the Sun have higher metal contents, allowing us to track the oldest and youngest stars and to determine the age of the Galaxy. New stars therefore contain the by-products of the old, our Earth a distillate of earlier generations. Our Sun will someday make its own contribution, however modest it may be, to generations of stars and planets yet unborn.
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