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 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. 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 km
(63,000 times the distance between the Earth and the Sun). The
stars are so far that distances were not measured until 1846,
through 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.
A star is a body that at some time in its life has or will
generate its light and heat by nuclear reactions, 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 this "hydrogen burning"
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,
its core, where it is hot enough. The temperature at the center of
the Sun, for example, 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. "Substars" below the 8% limit,
called "brown dwarfs," do exist however.
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. Clouds can be 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 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 of stars. Those that are actively fusing
hydrogen into helium in the middle, that is, in their cores, are
called "main sequence" stars. The main sequence is the first stage
following birth. The higher the mass of the main sequence star,
the greater its diameter and the higher the 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 G2. Examples of naked-eye main
sequence stars are Vega, Altair, and Sirius.
These 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.
Main sequence stars only have 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 (our home collection of 200 billion
stars), which is about 15 billion years old.
When the fuel in a 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 "M
ira 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 and reversing the normal ratio. Mira variables and other old red giants thus divide into oxygen-rich stars and carbon stars. Along with the carbon comes other elements that have been made in a huge variety of nuclear reactions that go on at the same time as helium fusion.
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. Most of the dust that inhabits interstellar space began this way, though it has since inception been highly modified. These stars therefore play a powerful role in 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. The core lights up the inner compressed portion of the fleeing matter, 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 star. Expanding at a rate 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 them from main sequence 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, no longer having any source of
energy generation, 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 a "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 the course of evolution becomes very different. High mass
stars are already large and luminous. As the dead helium core
contracts, 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 fuse to a mixture
of neon, magnesium and oxygen. Hydrogen and helium fusion had
already moved outward into nested shells around the core, and now
it is carbon-fusion's turn. When carbon fusion dies out, the neon-
magnesium-oxygen mixture fuses into a mix of silicon and sulfur,
each fusion stage taking a shorter period of time.
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
iron fuse 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" visible from
huge distances away.
There are other 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. 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,
their 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 could probably damage the Earth.
Fortunately, no candidate is nearly that close
As the debris of a supernova clears, we see an gaseous expanding
shell around the old star, the "supernova remnant," that is now
rich in the by-products of nuclear reactions. The most famous is
the Crab Nebula in Taurus, the remains of the great supernova of
1054, which was well observed by Chinese astronomers. 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. 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 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 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 sight. No longer 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 it is greater, then even its 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 escape its surface, and the
star disappears forever into a collapsing "black hole."
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; 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 fast 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 each orbit, 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, and to find the stars' diameters, temperatures, and even shapes in the cases that their mutual gravity 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 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. 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.
If the white dwarf and main sequence remnant of a close double is 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 dwarfs 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.
The term "supernova" is derived from "nova" in that the supernova is vastly brighter, no matter that the mechanism of 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 dwarfs 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 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 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 size 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
end products send newly made chemical elements into the
interstellar stew, out of which new stars are made, some perhaps to
be orbited by "earths" that are made from the deaths of earlier
generations of stars, our Sun someday to make its own contribution,
however modest, to generations yet unborn.