Download Section 14

Stars: The world's second-oldest profession might be that of astronomer; it probably
precedes even that of farmer (which extends roughly about 10,000+ years ago). The stars
have been a source of regularity, relating the passage of years far more reliably then
changes in weather. For instance, the ancient Egyptians used the appearance of the star
Sirius (seemingly the brightest star in the sky, aside from the Sun) to let them know when
the Nile would start flooding, allowing the planting of crops. Astrology developed in
tandem, providing the stars and planets with special significance to mankind.
The number of stars in the sky that we can see on crystal clear nights with just our eyes is
maybe 4000. The number of stars in the universe is estimated to be in the area of
100,000,000,000,000,000 (100 quadrillion) or more.
Birth of Stars: The universe is not a perfect vacuum dotted with stars and planets here
and there. Instead, the universe has gas (about 90% hydrogen, virtually all of the
remainder helium) dispersed about its volume. On average, this dispersion allows for
about 1 hydrogen atom per volume the size of a grape. This rarefaction is significantly
less than any vacuum we have achieved on Earth. However, there are some portions that
have decidedly greater numbers of atoms than that within their volume. It's within these
pockets that stars can come forth and they are sometimes known as "stellar nurseries".
A clump of gas that will become a star typically initially occupies a volume that's on the
order of one light-year x one light-year x one light-year (one light-year is the distance
traveled by light in one year; since light travels about 300,000,000 meters/second, one
light-year is on the order of 6,000,000,000,000 miles). Within this inconceivably huge
volume of gas, there is a battle between mutual gravitational attraction, which wants to
bring this whole mass together, and mutual repulsion that is exactly analogous to what
happens to air molecules when a sound wave passes through--they bounce off each other.
On rare occasions, however, a shock wave passes through the cloud (perhaps these shock
waves are most often caused by supernovas). And on those occasions, that shock wave
will draw atoms together more closely than they would otherwise be. This allows gravity
to gain the upper hand and more and more atoms are drawn into the denser region before
they can bounce away. This process is also assisted by having a rotation set up in the
cloud. This introduces a centripetal force that will tend to draw the atoms in the cloud
together. Thus, a protostar is formed, an object several times heavier than the Sun in
mass and much, much greater in volume.
Over time, gravity will force many of the atoms making up the protostar closer and closer
together. Eventually, the atoms will overcome their mutual electrostatic repulsion and
start undergoing fusion of hydrogen nuclei to helium nuclei. At this point, we now have
a star. Likely, this happens hundreds of times a day, but as this is throughout the entire
universe, the odds of one happening nearby are very small. After this point, the life of
the star becomes a battle between internal thermonuclear fusion, which tries to push the
star apart, and gravity, which tries to pull the star together.
Most stars seem to come in pairs. This is almost certainly as a result of the nature of their
birth, for the clouds from which stars are born are typically much, much larger than even
the gigantic protostars. Thus, several stars are typically forming simultaneously.
Life of Stars: The great bulk of a star's life is a relatively stable equilibrium between
thermonuclear fusion and gravity. In the very center of the star, hydrogen nuclei are
being fused into helium nuclei. The energy from these processes seeps outward from the
core, greatly warming the external portion of the star (which does not undergo
thermonuclear processes itself). The outer portion of a star is alive with convectional
currents and magnetic anomalies like sunspots.
A star can live billions of years this way; the Sun is believed to be approximately halfway
through its estimated 10 billion-year life. However, it's lifetime is determined by its size
at the outset--the larger the star, the shorter its life. However, the larger the star, the
brighter it is. The Sun is relatively small. Very large stars will have lifetimes of "only"
millions of years.
A barometer often used in the study of stellar phenomena is the "Hertzsprung-Russell
Diagram". This allows an astronomer to determine how bright a distant star is and thus
have some indication as to its size and lifespan.
The Death of Stars: All stars start their death throes in the same manner. During their
long period of equilibrium between gravitational contraction and the explosive outburst
of fusion processes, stars are using hydrogen as fuel to create helium. Eventually, their
amount of hydrogen falls low and the fusion processes slow. This allows gravity to gain
the upper hand and the star shrinks. This, though, raises the temperature in the core of
the star. When the temperature gets high enough, the fusion processes start again. This
time, however, helium is fused to create lithium, beryllium and other elements. Since the
core of the star is hotter, the outer shell of the star expands. This expansion is actually
great enough that although the star is emitting more energy, it's through a greater surface
area and the star's surface gets a little cooler--hence, the star gets a little redder.
Eventually, the helium runs low, though, and gravity gains the upper hand--the star
shrinks. But when it shrinks enough, the temperature in the core is raised yet further and
fusion processes again starts up, using the "ashes" of the previous fusion cycle. The star
swells further and further, becoming a red supergiant. This will someday happen to the
Sun, but don't buy insurance just yet. There's still about 5 billion years to go.
Depending upon the mass of the star, this process could keep going until fusion processes
are producing iron. What happens after that depends upon the star. Small stars will
never reach the point where they're producing iron--the gravitational collapse of such
relatively little mass is insufficient to heat the interior to that point. Thus, the star
collapses down to the point where the atoms are essentially rubbing up against each
other, much more close than in even the densest terrestrial material. This is called a
"white dwarf" and is what will happen to the Sun after it leaves its red giant phase. A star
can remain in the white dwarf phase for billions of years; eventually, the star will burn
out, ultimately becoming a black dwarf.
If the white dwarf is part of a system of two or more stars, it can lead to an interesting
effect. As the non-white dwarf star produces energy, it is also emitting mass. This mass
can accumulate on the surface of the white dwarf. If enough mass accumulates, it can
lead to fusion processes within it. The onset of these fusion processes, though, is very
swift--the whole thing violently explodes. This is called a "nova" in the textbook but is
more properly called a "Type I Supernova". The explosion causes the white dwarf to
briefly shine about as brightly as any star can. This process can repeat itself many times
for the same binary system.
A more cataclysmic fate awaits stars whose mass exceeds the Sun's by a factor of 1.4 or
more. In those stars, it turns out that there is enough mass to allow the creation of iron in
fusion processes. Iron, though, is a very special element. It represents the most optimum
balance between two basic forces in nature, the electromagnetic force and the strong
nuclear force. Without getting into details, the more protons and neutrons there are in an
atomic nucleus, the greater the presence of the strong force, which wants to bring these
particles together. However, the more protons there are in a nucleus, the greater the
repulsion due to the electromagnetic forces. Therefore, nuclei smaller than iron don't
have as much of the strong nuclear force binding it together, but nuclei larger than iron
have a greater repulsive electromagnetic presence. In short, fusion processes lose energy
when they try to fuse iron into heavier nuclei. Thus, fusion is no longer an option when
the gravitational collapse continues. In fact, there is no option. In sufficiently massive
stars, when a great deal of iron has accumulated in the core, the fusion processes shut
themselves off almost like a light switch. This temporarily removes any obstruction from
gravity's relentless desire to contract the star. In literally seconds, the star collapses.
However, the core collapses much more quickly than the outer portion. The core
effectively bounces, releasing a tremendous amount of energy that passes through the
outer portion of the star. This huge amount of energy is sufficient to trigger fusion
processes in that entire portion--it's as if the whole star is now undergoing fusion instead
of just the core. The unimaginably gigantic explosion that results is called a "supernova".
It is during this process that elements heavier than iron are created--there's so much
energy running around that it can happen.
Supernovae are not common events since so few of all stars have sufficient mass. A
supernova "only" 100,000 light-years away occurred in January, 1987. It's the closest
supernova to us in the last 400 years. In fact, prior to the supernova in 1987, all of our
knowledge about this phenomenon had been gathered from observing supernovae in
other galaxies.
Within the category of stars that undergo a supernova are two possible ultimate demises.
If the star has a mass less than three times the Sun's, the core that remains will have
collapsed down so much that even the electrons making up the constituent atoms will be
forced into their respective nuclei. The whole star will almost be like a giant, uncharged
nucleus. This is called a "neutron star". The only reason a neutron star doesn't collapse
further is that the neutrons are basically shoulder-to-shoulder with each other. Such stars
have been observed, so this isn't speculation. If you look at the constellation of Orion,
the middle star in the short sword handing down from his waist is not a star--it's a cloud
of dust. It is what remains of a supernova that was observed on Earth in 1054. At the
center of the cloud, observable in a wavelength of light that we can't see with our eyes, is
the neutron star that was left behind. This neutron star, and many others, spin around
very rapidly. They also have very concentrated magnetic fields. Because of these two
effects, it's almost as if there's a searchlight rotating around and around with incredible
regularity. In 1967, when these objects were first discovered, this regularity was so
incredible that it was initially thought the object could be a beacon left by an
extraterrestrial civilization--the object found was initially denoted as "LGM-1", where the
LGM stands for "Little Green Men".
Black Holes: For stars with a mass 3 or more times greater than the Sun's, even the
rubbing together of neutrons isn't enough to slow down the gravitational collapse. In
fact, nothing is. The star collapses down to a point-size particle, i.e., zero length, zero
width and zero height; this object is called a "black hole". However, it still leaves behind
an effect, that being its gravitational field. From far away, the black hole's gravitational
field is just like that of any other star or other massive object. But the source of its
gravitational field is concentrated at a single point, unlike any other object. Thus, when
you get really close, you get significant effects. For instance, suppose the Sun were
collapsed to a black hole and you were 3.1 km away with your feet pointing down
towards the surface. Thus, your feet are about 2999 m away from the black hole while
your head is about 3001 m away. The gravitational force at your feet is so much greater
than the gravitational force at your head that you would easily be pulled apart like a
cheap rubber band.
The gravitational pull of a black hole is so strong that not even light can escape. That
light would even be influenced by gravity was a concept that wasn't even suggested until
Albert Einstein published his General Theory of Relativity in 1916. However, in 1919
scientists went to a region in Africa that would be subject to a solar eclipse. There, they
were able to see that stars whose light was passing close to the Sun had that light
deflected by the Sun by exactly the amount Einstein had predicted. The Sun, though,
obviously won't prevent light from escaping. It takes a greater gravitational pull to do
that. And this is why it is impossible to directly observe a black hole. We can only
deduce its existence from indirect effects.
If we sent a probe towards a black hole, what would happen to it? Assuming it survives,
we would find that the frequency at which it is emitting its signal is getting longer and
longer, i.e., what might have originally been a transmission rate of 50,000 bits/second is,
to us, continuously decreasing as the probe gets closer and closer. In addition, the probe
even appears to be slowing down. This is not because anything is happening to the
probe, though, it's because of something happening to the light waves being emitted or
reflected by the probe. Eventually, the probe will appear to come to a dead stop, finally
vanishing as the last bit of light left it. In fact, though, the probe has passed through the
black hole's "event horizon", the point at which no signals can escape a black hole. The
event horizon's distance from the center of the black hole depends upon the mass that was
used in the creation of the black hole in the first place. For a typical, new black hole, this
event horizon will have a radius of about 10 km. Anything closer means that it has been
sucked into the black hole, forever gone.
What is it like inside a black hole? We can use our theories to speculate. For instance, it
has not been ruled out that black holes act as conduits to other parts of the universe or
even to entirely different universes. But a basic theory seems to uphold a bizarre
intuition. Consider what life is like outside a black hole--theoretically, there are no
constraints on our motion in space, but we are constrained to move in a very particular
direction in time. The reverse might possibly be true in a black hole, i.e., you are
constrained to move in a particular direction in space (toward the exact center of the
black hole, of course), but you are as free to move about in time in a black hole as you are
free to move about in space outside a black hole.