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 .
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.
High mass stars and
supergiants
Supernovae
Frequency and
candidates
Supernova
remnants
Neutron stars
and pulsars
Black
holes
Double
stars
Masses from double
stars
Multiple
stars
Formation of
double stars
Eclipsing double
stars
Evolution of
wide double stars
Evolution of
close double stars
Contamination
Novae
X-rays, neutron stars,
and black holes
White dwarf
supernovae
Open
Clusters
Associations
Globular
Clusters
Cluster
evolution and blue stragglers
Stars and cosmic
recycling
Other
galaxies
Stars are a fundamental component of other galaxies, other
collections of stars that work and act at least something like
our own Galaxy. Were there enough time and
astronomers, nearly a trillion galaxies could
be counted extending to billions of light years away. There are
many different
kinds and a number of different classification schemes.
Among the larger galaxies there are two broad groups,
spiral (disk) galaxies like ours (class "S") and
elliptical galaxies (class "E") that have no disks. E
galaxies are more like the halo of our own galaxy, but are much
more thickly populated. Each kind has its own subgroups. Spiral
subgroups are based on whether or not there is a central
bar from which the arms emanate ("SB") and on the degrees at
which the arms open outward. Elliptical subgroups depend on the
flattening of the elliptical shapes. Spirals are mostly larger
systems like ours, whereas ellipticals can range from small dwarfs
to
giant ellipticals much bigger and more massive than ours.
Set against these are small
irregulars with no well defined shapes. The dusty gases
of an interstellar medium fill the inner disks of
most spiral galaxies and constitute good portions of the masses
of irregular galaxies, whereas they are largely absent from
ellipticals and small spheroidals, which then also lack star
formation. There being no massive blue stars, ellipticals take
on reddish colors.
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