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Stellar Evolution
STELLAR EVOLUTION
(C) Copyright 1991 Max Pandaemonium
Our galaxy's spiral arms are literally filled with dust and gas.
Occasionally, a cloud of this gas begins collapsing (for reasons not
completely understood) under its own gravity, and a star begins to
form. Due to conservation of angular momentum, this cloud forms into
the shape of a rotating disk, with the inner regions rotating faster
than the outer ones. After a certain time, the center of this
"accretion disk" begins to heat up from the pressure of the gas
particles, and, one day, thermonuclear reactions start in its core.
Two hydrogen atoms are squeezed together with such pressure and heat
that they fuse into one atom of helium, thus liberating considerable
energy [footnote 1]. This energy is what drives the stars. When this
happens, the object is now a protostar -- the first stage of a star's
life.
The thermonuclear reactions in the core of a protostar cause
pressure -- the star wants to fly apart. However, gravity keeps it
from flinging its guts into space. This constant interplay between
radiation pressure and gravitation dictates the course of a star's
life -- and, eventually, the star's end.
The protostar's accretion disk sometimes buds off into small
vortices of condensing matter, away from the tumult of the stellar
furnace. These protoplanets eventually become the other bodies
orbiting in the system [footnote 2].
The protostar, when it finally "turns on," pushes the gas and dust
in its vicinity away. This clears the system out, and leaves the
protoplanets orbiting around their young sun [footnote 3].
After a few million years the system begins to mature. The
protostar, which might have been wildly variable -- changing in size
and luminosity -- begins to settle down. The protoplanets begin
forming their more recognizable cousins, the planets, asteroids,
comets, and other interplanetary bodies. The star has now become a
main-sequence star, the most populous of all stars in the galaxy
[footnote 4].
The course of a star's evolution is dictated almost entirely by
its initial mass. The lower the mass of a star, the longer it will
live, and the less spectacularly it will die. The higher the mass of
the star, the shorter its life, but the more violent the death. Red
dwarfs, the coolest stars, are very small, but can live for hundreds
of billions of years -- while blue main-sequence stars, some of the
hottest, can burn themselves out in only a hundred million years or
so. Our Sun, a yellow main-sequence dwarf, somewhere in between, will
live ten billion years, from birth to death.
When a star begins to age, it wanders off the main-sequence. It
begins to run out of hydrogen in its core, and the star starts to
quiver. It is now a subgiant. It begins swells, and starts becoming
unstable. The reactions in the core are beginning to change, and so
the balance between expansion and gravitation becomes upset. At this
point in a its life, the star can again become wildly unstable and
begin to pulsate, change brightness, or even change color.
After a time, the hydrogen in the core runs out almost completely,
and it collapses. New reactions, such as helium-to-carbon, begin to
take place in the core. These reactions liberate more energy than do
the hydrogen-to-helium reactions, and so the star expands. When a
star roughly the mass of our Sun or below comes to this stage, it is a
red giant. Its radius has increased many times -- so much so that our
Sun, when it eventually reaches this stage in five billion years or
so, will engulf the inner planets, out to Mars.
As red giants remain in this stage, their cores, as they run out
of their new fuel sources, switch to others. Helium-to-carbon,
carbon-to-oxygen, oxygen-to-silicon, and so on [footnote 5]. New
elements are formed in the cores of stars all the way up to iron. But
iron is the limit to nuclear reactions in stars -- iron is too heavy
to be fused into higher elements with a gain of energy. The giant has
reached its evolutionary dead-end. When a giant reaches the end of
its life, and the reactions proceeding in its core are delivering a
very large amount of energy indeed, it begins to shed its outer
layers, since gravity is not strong enough to hold onto them anymore.
These layers are flung into space, leaving a fiery core. Such an
object is called a planetary nebula [footnote 6].
Eventually, when the core runs out of fuel and has converted the
majority of its mass into heavy elements, such as iron, it collapses
-- there is insufficient outward pressure from thermonuclear reactions
with which to keep gravity at bay. It collapses until electron
pressure -- the electrical repulsive force between electrons, halts
the collapse. Such an object is very dense, and is called a white
dwarf (or degenerate star). It is the mass of the Sun compressed into
a sphere the size of the earth [footnote 7]. Eventually, the white
dwarf cools, venting its energy into space, until it no longer shines.
This dead star is called a brown dwarf, or an exhausted star. Such a
fate eventually awaits our Sun.
More massive stars, however, do not have such a kindly death.
When a star is quite a bit more massive than the Sun, then the
powerful reactions in its core turn it into a supergiant -- some
supergiants have radii corresponding to Jupiter's, Saturn's, or even
out to Neptune's orbit. Supergiant stars are extremely luminous, and
have tremendous surface areas. When a supergiant runs out of its
fuel (again, iron is the limit), then it begins to quake. Supergiant
stars, occasionally, undergo tremendous change. Their cores, with no
fuel left to sustain reactions, collapse violently. This collapse
triggers a tremendous explosion, called a supernova. Supernovae are
so brilliant that they can outshine an entire _galaxy_ [footnote 8].
The outer layers of the star are thrown off violently, and the
remaining core has two fates.
If the star was only moderately more massive than our Sun, even
electron pressure in its core is not strong enough to halt the
collapse. The core continues to collapse, plunging in on itself,
until the protons and electrons in its core are forced together so
violently that they are transformed into neutrons. Thus, when almost
all of the core of the star is transformed into neutrons, the collapse
is halted by this new pressure. Such an object is called a neutron
star. It is the mass of several Suns compressed into the size of the
center a city -- only 10 kilometers in diameter or so. Neutron stars
are made up of stuff called neutronium, which is the densest matter
known [footnote 9]. The neutron star, still ringing from its
formation, slowly cools, and dies [footnote 10].
However, if a star is quite a bit more massive than our Sun, even
the pressure from the neutrons squeezed together in its core is not
enough to stop the contraction. Such an object falls in on itself,
until the escape velocity of its surface exceeds that of light itself.
The object disappears from view in our universe, and is now a black
hole, or collapsar. Black holes are so dense that not even light can
escape -- hence the reason they are called black.
New study has shown that even black holes can die. A black hole
emits radiation like any hot body. As it does, it loses mass, and
eventually it dies in a puff of photons and exotic particles [footnote
11].
It is interesting to note that the early universe, according to
cosmologists, was composed of entirely hydrogen and helium. All of
the other elements -- the carbon in our bodies, the oxygen in our
atmosphere, the silicon in the rocks -- were formed in the process of
evolution of the stars. The elements up to iron were formed in the
cores of massive stars, and all of the other elements -- those higher
than iron -- were formed in supernovae [footnote 12]. The supernova
explosion is so powerful that it forces atoms together to fuse into
very high elements indeed -- making silver, gold, uranium, and all of
the other heavy elements.
We are all undeniably tied to the history of the universe.
----------
Footnote 1. Actually, the energy created by nuclear fusion in the
Sun's core is in the form of high-frequency gamma rays. Why then do
we see visible light from the surface? Because the Sun is so dense,
light cannot make the journey directly from the core to the surface,
only about 700 000 kilometers. It follows what physicists call a
"random walk," where a photon of light moves only a few centimeters
and gets absorbed by an atom; then it is reradiated by the atom in a
random direction and absorbed shortly thereafter again. In the
process it loses a little energy (sometimes it is reradiated as two or
more photons). Thus it takes an individual photon of light a long
time to get to the surface after its formation. How long? It has
been estimated that it takes -- on the average -- one million years.
Interestingly enough, another subatomic particle is created in the
nuclear fires inside the Sun's core -- the neutrino. The neutrino
travels at or near the speed of light, but, unlike photons, it does
interact strongly with matter. It makes the trip from the Sun's core
in three seconds. In fact, a neutrino has a roughly 50-50 chance of
being stopped by several _light-years_ of solid lead!
Footnote 2. Scientists have found evidence of planetary systems in
the process of forming. A star quite close to us, Beta Hydri, was
noticed to have a fuzziness to it -- a disk of gas and dust
surrounding the star. This is probably the accretion disk of matter
of a forming solar system.
Another "disk star," as they are sometimes called, is designated
MWC 349 and is in the constellation Cygnus. Unlike Beta Hydri,
however, this one is very far away -- around 8000 light-years. It is
a huge supergiant star with a large disk of dust and gas around it.
Unfortunately, since MWC 349 is burning itself up so fast, it will
probably go supernova before a life has a chance to form within its
newly-forming solar system.
Footnote 3. Some astrophysicists also speculate that this is what
causes other systems to be formed. The wavefront of gas and dust,
caused by the photon pressure of the new star, crashes into other,
more uniform fields of gas, causing another collapse and another star
to be born.
Footnote 4. There is a division of population within main-sequence
stars as well. Cool, low-mass main-sequence stars are much more
populous than hot, high-mass ones. The most common star in the
universe appears to be a red main-sequence dwarf -- there are large
number of them in our immediate vicinity. Our Sun, a yellow main-
sequence dwarf, once thought to be a common, ordinary star, has gotten
some of its prestige back: yellow dwarfs aren't as common as we
thought them to be.
Footnote 5. The reactions are more complicated than simple fusion.
For instance, nitrogen fuses into silicon, and some of the silicon
decomposes into magnesium. Then silicon and magnesium fuse to form
iron.
Footnote 6. Actually, the term _planetary nebula_ is a misnomer. In
1785, when William Herschel looked through his eyepiece at the first
discovered planetary nebula, he named it such because of its
similarity in appearance to the blue-green disk of the distant gas
planets, such as Uranus or Neptune. From the text, however, we can
see that planetary nebulae really have little to do with planetary
systems.
Footnote 7. Some white dwarfs, at certain ages, temperatures and
densities, could be composed of crystalline carbon (the chief element
left over from the nuclear furnace inside its parent star). Without a
doubt they would be the largest diamonds in the Universe -- about the
size of the Earth.
Footnote 8. Supernovae, because of their great ferocity, are
relatively rare events. There have only been seven supernovae in our
Galaxy in all of recorded human history. Since we can see supernovae
in other galaxies (the supernovae occasionally outshine the galaxy
that they are contained in), however, we have more of an opportunity
to study them.
Footnote 9. Neutronium is, literally, the density of the nucleus of
an atom: the neutrons are so close in the neutron star that they are
touching, so you can think of the entire body as one gigantic atom.
Footnote 10. When a neutron star collapses, it speeds up, due to
conservation of angular momentum (the same reason a spinning skater
speeds up when she pulls in her arms). Also, the magnetic fields of
the neutron stars become much more intense; radio waves are sometimes
emitted along these field lines. If by chance the radio waves sweep
past our planet, then we detect a periodic pulsation of radio energy,
and the neutron star is called a pulsar. When pulsars were first
discovered, they were thought to be beacons of extraterrestrial origin
-- in fact, astronomers temporarily designated the first pulsar as
LGM-1 (Little Green Men One).
Footnote 11. This black hole radiation, or Hawking radiation (named
after the physicist who suggested it, Stephen W. Hawking), is a direct
consequence of quantum physics. According to quantum physics,
particle-antiparticle pairs (or virtual particle pairs) are being
created all the time. They appear and then annihilate each other,
returning to equilibrium. Hawking radiation involves one of these
virtual particle pairs being created near the black hole's event
horizon. The particle with negative energy is swallowed by the black
hole, and the other escapes. Negative energy means a reduction in
energy, so the black hole loses mass. The particle that escaped
appeared to have been emitted, and so we call it "radiation," even
though it is somewhat misleading.
Footnote 12. The vast amounts of gold, uranium, and other elements
with high atomic numbers on the Earth give you some idea of the size
of titanic explosions that we call supernovae.
-)(-
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