In the second century BC, the Greek astronomer Hipparchus divided
the stars into six brightness groups called magnitudes, first
magnitude the brightest, sixth the faintest. The system is still
used today, though with a mathematical definition (a star of one
magnitude is 2.5 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 per second, so one light year is about 10
trillion kilometers (63,000 times the 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.
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 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. 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. 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 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 15 billion years.
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 million degrees Kelvin (K =
centigrade degrees above absolute zero, -273 C), and the density
is 10 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 is still a gas
throughout.
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 8% 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 8% limit,
called "brown dwarfs," do exist however.
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. Other stars can deviate considerably, depending on their states of aging or upon where they are in the Galaxy.
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. Lumps of matter therefore form
within the interstellar clouds. If their gravity is great
enough, they can condense into one or more stars. 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.
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 of 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, 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 5% the size of the Sun (which is 1.5 million
kilometers -- 109 Earths -- across) to about ten times solar, and
surface temperatures from about 3000 degrees Kelvin to about
50,000 K (the Sun's surface is at 5800 K). Around the beginning
of the 20th century, astronomers divided the stars into seven
basic lettered groups that they later learned were related to
surface temperature, O (above 30,000 K), B (9500 - 30,000 K), A
(7000 - 9500 K), F(6000 -7000 K), G(5200 - 6000 K), K(3900 - 5200
K), and M (below 3900 K). The Sun is a G star. The system is
decimalized, making the Sun class G2. Examples of naked-eye main
sequence stars are Vega, Altair, and Sirius. The classes are actually derived
from the stars' spectra.
Since the color of a heated body depends on temperature, the
different classes take on different, though subtle, colors, from
slightly reddish for class M to orange 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 contrast against each other.
Main sequence 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. 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 million years, the least massive for trillions,
so long that no star with a mass less than 0.8 solar masses has
ever died in the history of the Galaxy. From theory, we
calculate that such a star should live for about 15 billion
years. The Galaxy should be about as old as its oldest stars,
and is thus about 15 billion years old.
Begin the with stars more or less like the Sun, those with masses from about 0.8 times that of the Sun to about 10 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 temporarily brightens and expands by many times over, the expansion cooling the surface, turning the star into a class M "red giant." When the temperature hits around 100 million degrees Kelvin, the helium is hot enough to fuse into carbon 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 type K giants. Good examples are Aldebaran and Arcturus. Such stars 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% of the main sequence lifetime.
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" or "Mi
ra variables." Thousands, all cool class M giants, are
known.
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 and "carbon stars." 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). 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.
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, compressed under
their gravity, have shrunk to only about the size of Earth. The
first ones found 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 "ordinary 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 exerted under the great density
prevents gravity from shrinking them any further. White dwarfs,
the remains of stars that began their lives between 0.8 and 10
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."
Higher mass stars, those with masses over about 10 times that of
the Sun, develop the same way as giants as they start to die, 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.
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.
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), which was so
bright that it was visible in daylight. Our knowledge of
supernovae comes almost entirely from observing them in other
galaxies, the best of these exploding in 1987 in the Large
Magellanic Cloud, a companion to our Galaxy some 170,000 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 should go 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
As the debris of a supernova clears, we see a gaseous expanding
shell around the old star, the "supernova remnant," the debris
rich in the by-products of myriad nuclear reactions. 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 were
created in 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 the explosion we can still see the
mighty blast waves sweeping through the gases of interstellar
space, compressing 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.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.
Most of stars you see at night have companions, a great many obviously double 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.
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. Even more complicated multiples exist. The theory easily explains why doubles are so common.
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," 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 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 become compressed, fuse to helium, and then explode violently as the helium fuses to carbon, the result 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 create a vast amount of iron and other elements. Kepler's supernova of 1604, the last seen in this Galaxy, was probably of this kind.
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.
