Stars are simply very large clouds of hydrogen that have fallen
together by gravity to very much smaller sizes. And what
happens to them
in the end depends on how very large they were at the start. But it's really not so simple as all that. And we're
leaving out all the interesting stuff like why is there gravity at all, why is the
hydrogen made of electricity, and why, when things
fall, do they coast?
These are the really interesting things, but this is not the place for all that.
Long ago J.B.S. Haldane, who gave one of the fascinating
lectures of my youth, wrote a charming little essay On Being the
Size. I hope you've read it. He was into animals and the problem of how the increase in the complexity of their structures
is related to their increase in size. But the problems are similar for stars, so you
can think of this essay as a chapter in his. And
I wrote this stuff up
long ago when I was in a more playful mood. It's the first chapter of "Astronomy for Children under Eighty."
The first consideration
(which we're going to skip for the moment) is whether the star is an early star made of nice clean
hydrogen and helium and spinning slowly in the central bulge of a galaxy, or whether it's a late star,
like our Sun, made of dusty
hydrogen and helium and spinning more rapidly
in the disc. (And we won't ask yet where the dust is from.) But that is what determines
whether the star could have planets like Jupiter and Saturn only (which are really not planets at all), or whether
it could also
have rocky planets like the Earth and Mars. Right now we'll
just consider what happens to young stars in general.
You remember how things fall, harder and harder, faster and faster. Now when the hydrogen
is falling in like that to make a
star, what stops it? Obviously, it
runs into the stuff falling in from the other side. If we had tried to make a star out of super
balls, we would have failed, because the balls would have missed each other and coasted on through to
the other side. But hydrogen
clouds are very spacy and they do collide.
They can't get past the clouds from the other side. So what happens is that the energy of
falling gets scrambled to heat. Now the heat energy can slow down the further collapse but it cannot
be used to return the hydrogen
to its former position. It cannot undo
the collapse. When the catcher behind home plate catches a pitched ball, the heat in the
catcher's glove does not return the ball to the pitcher. We say that the entropy has gone up. The scrambledness of
the energy has
gone up and its useableness has gone down. It's the same
in the birth of a star. The entropy goes up. We say, "Entropy tends to a
maximum." It goes up but doesn't go down. Otherwise galaxies and stars could not be born, and living organisms
could not exist. We
all live in a cascade of increasing entropy by directing
streams of the increase through our forms.
Gravitational energy and the kinetic energy of large moving objects, like planets going around stars,
are completely free of
entropy. But when the kinetic energy of our falling
hydrogen gets scrambled to heat, it gets contaminated with entropy and cannot
prevent the collapse of the star. So as the star gets smaller and hotter and the outer material has fallen as far
in as it can fall,
the star, like our Sun, will be round and shiny.
But it doesn't end there.
The star gets rid of some of its extra energy by shining it away at the surface, and so gradually
the star gets smaller and hotter. Eventually (after several million years), the center of the star gets
hot enough (about ten
million degrees centigrade) to fuse hydrogen to
helium, and we say that the star has joined the "main sequence." (It's not really a
sequence at all. It's just where the stars that are busy fusing hydrogen to helium fall in the Hertzsprung-Russell
just looked like a sequence on the diagram.)
Now when a star converts
hydrogen to helium at its center, an enormous amount of energy is released. Seven tenths of one percent
of the rest energy of the hydrogen is released to kinetic energy and radiation. [What we see as matter
is just rest energy or
potential energy as Einstein pointed out in 1905.
It was suggested much earlier by Swami Vivekananda to Tesla who was a close friend
of Einstein's first wife, Mileva. She was probably responsible for getting it into the appendix of the relativity
Einstein's 1905 quantum mechanics paper they knew that if
something is going away, its energy is reduced. They also knew
relativity paper that its mass is also reduced, and by the same amount. These two ideas are put together in the appendix,
(E = m). But you must remember that what we see as matter is just potential energy
or you'll never get things straight.]
Regardless of what you might have been taught, the energy released by fusion at the center of a star
is not what makes it
hot. The star is heated by gravity. The fusion just
keeps it bloated by releasing energy in th core. And the question now is:
does it keep it bloated for such a very long time? That is because the thermal gradient from the center to the edge of the
star may be only thirty degrees centigrade per mile, and the edge may be half a million
miles away. So it may take about a million
years for the energy released
at the center to leak out to the edge and get away as radiation. That's why the stars take "such an
unconscionably long time adying." That's why they stay so long on the main sequence. And our Sun
is a teenybopper, nearly five
billion years old and only half way through.
So after another five billion years, when all the hydrogen is gone in the middle, what
will keep it bloated? Nothing will keep it bloated. The center will then collapse to the density of about two concrete
trucks, with all that concrete and both drivers, squeezed into
a one pint jar. And the question is, will that be warm or cold? It
be hot, so hot that the hydrogen fusion rate around the condensed helium core will be all out of control.
At present the fusion rate at the center
of the Sun is under the control of a governor. If it fuses too fast, it will bloat the
center and slow down the fusion. And if it fuses too slowly, gravity will compress it and speed it up. But in some
years, when the helium core has collapsed to that awful
density, the fusion rate around it will no longer be able to bloat it and
it off because the density of the core will no longer be determined by temperature. Then, although the rapid fusion around
the core will no longer be able to bloat the core, what it can and will do is to bloat
the rest of the Sun to a red giant. So the Sun
will swell up to a great
red balloon with a diameter equal to the diameter of the Earth's present orbit.
In order to understand what happens next, we have to
talk about what is known to the astronomers as the solar wind. We see
looks like a wind blowing away from the Sun. It wouldn't look like a wind at all if you saw it right next to the Sun. It would
just be the Sun's corona at some two million degrees centigrade. So why, from far
away, does it look like a wind? And why is the
corona so hot? We'll take
the second question first.
The material in the immediate vicinity of the Sun, the Sun's corona, is some three hundred times hotter than the
surface of the
Sun that heats it. Partly it is heated by the continual
turbulence at the surface of the Sun beneath it, and partly by the
changing magnetic fields. But also it is heated by the ultraviolet light that knocks the electrons off the atoms.
(Sometimes half of the twenty-six electrons are stripped off the iron atoms.) Since
the energy released when electrons rejoin the
atoms is much less than
the energy of the ultraviolet radiation that knocks them off, the corona has what we may call a favorable
balance of trade and gets too hot.
It is this hot corona, seen from a distance, that we see as the solar wind. If you
saw it from near the Sun, it would just be
high speed particles moving
every which way. But seen from a great distance, all the particles that reach you would be seen to be
coming from the Sun's direction. That's why you'd see it as a wind.
Now when the Sun swells up to a great red balloon, nearly
filling the Earth's present orbit, and its diameter has increased
two hundred times, how much will the surface area have increased? Since the surface goes up as the square of the
diameter, it will go up by some forty thousand times. And what will happen to the
solar winds? They will be gravely increased,
and the Sun will thus shed
some of its outer garments. Also when the equatorial regions have become so distended that whole atoms
can form there by recapturing their electrons, the energy thus released may puff the surface material
away. We think the Ring
Nebula star has puffed away five times as much
stuff as we have in the Sun. Photographs with very long exposures show two previous
puffs. But only the last puff can be easily seen. It's only about a tenth of a solar mass but is lit by the ultraviolet
the central star which is now exposed and which is seven
times hotter than the Sun.
We now think that the Sun will lose enough mass by puffs and solar winds so that its relaxed gravitational
hold on the planets
will allow the Earth to drift away to about the orbit
of Mars where we'll orbit the Sun as a ball of molten rock and iron. Cheer
things are sure to get worse. There are no permanent habitats.
But we're getting ahead of our story. We've talked only about the outer layers of
the Sun. Meanwhile the Sun's helium core will
have gotten smaller and
hotter as more and more helium is added to it by the rapid fusion around it. Eventually it will get hot
enough explosively to fuse the helium to carbon and oxygen. And you might think that that would release
enough energy to blow up
our great red balloon. But it won't. Not because
the energy is insufficient, but because the gravitational field of the dwarf
is simply too strong and contains the explosion. On the surface of a black dwarf star, with a density comparable to that
of the helium core, you'd weigh as much as one U.S. battleship on the Earth. And think
how flat you'd be. When the last bit of
hydrogen has been puffed away
at the surface, as we see it in the Ring Nebula, it will be the resulting carbon and oxygen dwarf that
will be exposed at the center. It's the ultraviolet light from the central dwarf star in the Ring that
lights up the last puff and
allows us to see it. And the green in the
ring is the oxygen coming off the blue dwarf. The earlier puffs, unlit by the exposed
central dwarf, would not have been easily seen.
It is now thought that as the Sun loses mass, puffed away at the surface, and its
gravitational hold on us is relaxed, that the
Earth will drift away to
about the orbit of Mars as a ball of molten rock. Cheer up, things are sure to get worse. There are no
Little stars like our Sun end up as white dwarfs cooling off to black, made of carbon and oxygen squeezed
far beyond the
density of diamonds. So next time you hear "Twinkle
twinkle little star", remember! Only little stars go to diamond.
Bigger stars get hotter because they fall together harder, and they end up in a different
way. Because of the repulsion of
their electrical charges, it is very
difficult to push two protons together hard enough to get them to fuse to helium. The
temperature must rise to some ten million degrees. But alpha particles (helium nuclei) have two charges each, so
it is very
much harder to push them together by threes and fours
to make carbon and oxygen. So that's as far as smaller stars can go,
carbon and oxygen nuclei have six and eight charges each, and their repulsion is fierce. But the centers of stars much
heavier than our Sun are hot enough to fuse the heavier elements all the way to iron.
And all those nuclear reactions release
energy and delay the collapse
of the core. But once the core is iron, any further nuclear reactions absorb energy and simply
hasten the collapse.
What the physicists call the free-fall time (the time it takes a cloud or a star to collapse by gravity
if nothing stands
in its way) depends only on its density. It doesn't
depend on its size, and at the present density of the Sun (nearly one and a half
times the density of water) it's three quarters of an hour. But at the density of an iron core star, it's only three
quarters of a
second. In three quarters of a second, the iron core collapses
to a neutron star with the density of a hundred thousand U.S.
carriers all covered with airplanes and sailors on parade squeezed into a one-pint mayonnaise jar.
Now the energy released in this collapse
is much more than you could think. But try! The energy of an atomic bomb, just the
energy that is released when it blows up, weighs one-thirtieth of an ounce, or one gram. The energy of the Saint
weighed one pound. The energy of the explosion that
blew Crater Lake was forty-two pounds. It blew thirty-five cubic miles of rock
to powder and put them in the stratosphere at eighty thousand feet. That was forty-two pounds. The energy which the
each second is four and one-half million tons. It's been
doing it for some five billion years, and will continue for another five
years. But when one of these iron core stars collapses, in three quarters of one second it releases a hundred times as much
energy as the Sun will release in its ten billion years. I knew you couldn't handle
it, but I wanted you to try.
If you had your hand on a piece of a neutron star as big as a large grapefruit or a small cantelope, your hand would
five hundred pounds and you would not be able to pull away.
It would scrape your bloody fingers off on the stones as it fell
thr Earth faster than a super ball falls through the air.
What happens to a star in the end depends on how big it was at the start. Jupiter
and Saturn are too small to get any smaller.
The Sun is more than a thousand
times bigger than Jupiter, but it will go down to the size of the Earth, at some hundred thousand
pounds per pint. Stars big enough to go down to neutron stars go down to some ten or twelve miles in
diameter. It is thought that
stars whose collapsed centers are more than
three times as massive as the Sun collapse to what are called black holes. If they fall
to less than three miles in diameter, gravity won't let light escape.
"Twinkle, twinkle little star" we have, but
who will sing of bigger stars which make the heavy elements of which the Earth is
made? Up to iron, the elements are made within the stars. But heavier elements like gold are made in the explosions
the collapse of the cores. And the interesting thing is that
the stars that make the heavy elements deliver them abroad, all
the galaxies, so that late stars like our Sun can have planets like the Earth and Mars. But I didn't tell you the
difference between the early stars in the central bulge and the late stars in the
disk of the galaxy.
When a great cloud of hydrogen (admixed with some helium) collapses to form a galaxy, most of the cloud is blown
away by the
stellar winds of the early stars. And the dusts (the heavy
elements) of the early stellar explosions are blown away with it
I call the hovering layer (it's now called the halo). Into this hovering layer goes most of the angular momentum (the spin)
of the early stars, carried away by their stellar winds. It is the material in this
hovering layer that gradually flattens into the
disc because of the overall
spin of the cloud. That is why Jupiter and Saturn, and our Sun when it was young, had so much spin. And
because the disc is dusty, disc stars like our Sun can have rocky planets like the Earth and Mars. And
because we're in the disc, we
go round and round the galaxy in an almost