Last week I talked about how the Sun works, and how
it formed in the first place. If you haven't read that post, you
might want to read it now after all. Because this one more or less
follows it, and I'll assume you've read it so I don't annoy people
who did by explaining things twice.
I talked about how the Sun gets its energy from
fusing hydrogen into helium, preventing gravity from collapsing it
further with the energy produced in the core. Yet I didn't touch an
important question: what happens when the hydrogen runs out?
Well, the good news is that that won't happen for a
long time. The Sun has been fusing hydrogen for five billion years,
and is only halfway through its supply. The Sun still has five
billion more years left before its hydrogen runs out. An amount of
time like that can't be imagined at all, really. In that time, the
entirety of life on Earth happened; all of human civilisation could
be repeated 500000 times in it. Think of the oldest person you know;
perhaps a grandparent, or a parent, or just someone else. Imagine how
many seconds their life has lasted so far. Well, take double that
amount, but take years instead of seconds, and you've got five
billion years. And that's the amount of time we'll have to go into
the future to see what happens when the Sun runs out of fuel.
I can't even begin to speculate what Earth looks
like this far into the future, but let's assume it's still there at
least. If humans still have any descendants now, they probably look
nothing like us any more. The first thing you should realise is that
the Sun running out of fuel doesn't mean the Sun now consists of 100%
helium. In fact, most of the Sun still consists of 75% hydrogen and
24% helium. However, all this hydrogen does the Sun no good: the
pressure and temperature simply aren't large enough to overcome the
powerful electric repulsion the atoms have for each other in most of
the Sun. In the only place where it's so hot and so compressed
hydrogen atoms can collide - the core - there is only helium left by
now. You'll recall a hydrogen nucleus consists of one positive
particle named a proton, while a helium nucleus consists of two
protons and two non-charged particles named neutrons, all held
together by a powerful force named the strong nuclear force. Helium's
two protons repel other helium more strongly than hydrogen's single
proton repels other hydrogen, and this means the helium nuclei don't
collide yet.
So what happens? Gravity, for the first time in ten
billion years, can compress the Sun's core further, so it begins to
shrink. The pressure increases, the helium is pressed even more
closely together, and as a result the core's temperature rises. The
core gets smaller and smaller and hotter and hotter, and this
increased heat warms up the layer around the core. While this layer
was of course always millions of degrees hot, it never was quite hot
enough for nuclear fusion, so it still consists mostly of hydrogen.
But as the temperature here increases, hydrogen starts to fuse in
this layer around the core. Crisis averted, you may think.
Unfortunately, it's not averted.
You see, this shell of fusing hydrogen now holds up
the outer layers of the Sun just like the core did, but with an
important difference: the non-fusing core still lies under it, and
it's still contracting. It's not just getting smaller from the
pressure of its own gravity, but also from the energy produced all
around it. So the core keeps shrinking and heating up, and in doing
so it also continues to heat up the shell where fusion now takes
place. When the Sun was fusing hydrogen in its core, it did so at
about fifteen million degrees, but now in the shell around the core,
it could easily be fifty million degrees. At this higher
temperature, the hydrogen atoms bounce around even more rapidly and
powerfully, and therefore they collide more often. Much more often.
The shell fusion burns through its supply of hydrogen way faster than
the core did, and creates far more energy. A thousand times more
energy, possibly even ten thousand times more! The Sun used to be a
385 Yottawatt lightbulb, but now it's becoming a 3850000 Yottawatt
one.
This enormous increase in energy causes the outer
layers of the Sun to be pushed away. The fusion energy pushing from
inside always used to balance out with gravity, but now it's getting
much stronger, making gravity lose temporarily. The Sun's outer
layers expand, and so the Sun grows larger. And as it grows, the
energy produced inside, while much greater than it used to be, is
spread out over a far bigger surface than it used to, and this means
every square kilometre of surface gets less energy than it used to
and cools down. As the Sun grows several times as large as it used to
be, its nearly white light becomes more pronouncedly yellow. As it
keeps expanding further and further, the Sun cools down further,
slowly going orange.
At about this point, Mercury meets its fiery maker
as the Sun's surface expands beyond its orbit. The rocky little world
melts as the Sun's surface gets closer, and finally gets engulfed by
it. Yet it survives as a single piece of molten rock, still orbiting
inside the Sun, for a surprisingly long time: as the Sun expanded,
its mass didn't increase, so its outer layers have become incredibly
sparse: it's the same amount of Sun, just spread out over a far
larger space. The Sun's sparse outer layers still erode Mercury and
slow down its orbit so it falls deeper into the Sun, making it reach
warmer and denser layers where it will truly be destroyed, but this
will take a long time.
And the Sun keeps expanding: Venus soon shares
Mercury's fate as it too gets engulfed by the ever-huger star, which
is now a fiery red. So what about Earth? Well, eventually the Sun's
expansion stops, and this will be a bit outside Earth's current
orbit. So that seems to be the end of the world, but there's a 'but':
as the Sun's outer layers grow, they become very tenuous and get very
far away from the centre of its gravity. Combined with their heat,
the Sun is leaking lots of gas now, and getting lighter in the
process. The Sun getting lighter causes its planets to move into
higher orbits, and this might just be enough to save Earth. As
knowledge stands now, there seems to be about a fifty-fifty chance of
Earth following Mercury and Venus in or it orbiting just above the
Sun's surface. I'll assume it survives for the rest of this post.
Not that that will actually save it: the Sun
increased in brightness by at least a thousand times, so while the
Sun's surface is only half as hot as it is now, its proximity and
size still roast the Earth. The oceans boiled away, the atmosphere
heated up so much the planet's gravity couldn't hold it down any
more, and eventually the surface itself, as well as the Moon's,
melts. Seen from this molten world, the Sun would fill almost the
entire sky; a gigantic red ball of fire. Its surface wouldn't be
nearly as bright as it is now, though; you could probably look
straight into it while squinting, and even see darker spots on it
with the naked eye. However, its enormously increased size in the sky
still makes it far brighter than it ever was, even if you can now
look into it.
Even aside from its colour and dimmer surface, the
Sun wouldn't look much like it used to. The outer layers now contain
gigantic convection cells which take material from as deep as the
hydrogen-fusing shell and take it all the way to the surface, while
gas at the surface gets submerged and taken into the deep. These
gigantic convection cells and the Sun's gravity's tenuous hold on its
distant surface make the Sun bubble and bulge like a boiling pot of
water, even distorting its shape: the Sun no longer is a sphere, but
an odd, more-or-less-round bulgy thing. The Sun is now a red giant.
(Image made using SpaceEngine)
The core, meanwhile, has reached a pressure so
incredibly high it simply can't get any smaller. The helium is
squeezed so incredibly tightly together it really can't get any
closer without “breaking” the particles. In stars several times
heavier than the Sun, this will actually happen and cause extremely
strange things to happen. But the Sun's mass isn't big enough to pack
the helium any tighter. You might think the helium will begin fusing
too at some point if it's this close together. And you're right: it
does. Two helium nuclei collide on occasion, and form a nucleus with
four protons and four neutrons named Beryllium-8. There's just one
problem: Beryllium-8 is a very unstable nucleus. The strong nuclear
force just doesn't seem to have a good grip on it, and within a
fraction of a second, it falls apart into two helium nuclei again. So
the helium-fusion isn't going anywhere.
In the shell around the core, hydrogen fusion
happens at an incredible rate. The Sun's days of slow and stable
fusion are over; the hydrogen in the shell around the core gets
squandered a thousand times faster than the core hydrogen was. Within
only a few million years, the hydrogen here is gone too, and gravity
once again has free play. The entire Sun's mass once again rests on
the core, which is already as small as it could be, and now
surrounded by a layer of new helium. This causes the core to heat up
even further.
When the core reaches a hundred million degrees,
something happens. The helium, which has occasionally been fusing
with other helium to form beryllium-8, reaches a point where fusion
is so common that it becomes possible for the beryllium-8 formed in
the fusion of two helium nuclei to be hit by another helium nucleus
before it decays. The third helium nucleus adds to the nucleus so
that it has six protons and six neutrons; it's become carbon. And
carbon is quite stable: you should know, as you mainly consist of the
stuff. Sometimes, the carbon gets hit by a fourth helium nucleus,
fusing to form oxygen. The fusion of three helium nuclei to carbon –
or four to oxygen - creates a great deal of energy, and in the
strange conditions that now rule in the Sun's core, this energy
immediately sets off more helium fusion, which causes more helium
fusion in a chain reaction that makes the Sun burn through a fifth of
its helium in a single moment called the helium-flash.
The helium-flash is incredibly energetic, and makes
the core expand, yet the Sun is so huge and distended by now it's
barely noticeable by the time the flash reaches the surface. But
after the helium-flash, the Sun's core continues fusing helium in the
core at a slower pace. With fusion once again taking place in the
core, the Sun's energy production lowers, and gravity contracts the
red giant it has become. It looks like the Sun might be returning to
its old days: it shrinks, becomes hotter, and since gravity has a
stronger hold on the smaller surface, the convection cells stop
making the Sun look like a bulgy bubbling mess. It once again becomes
smooth and round and yellowish. The Sun doesn't shrink down all the
way to its old size, but for a while, it has entered a second youth.
But this second youth doesn't last as long as the
first. Not only does the Sun still burn much brighter than it used
to, squandering its resources rapidly, but helium fusion also
produces far less energy than hydrogen fusion, and therefore happens
quicker to produce the same energy. The Sun's second youth lasts
about fifty million years before trouble arises once again as all the
helium in the core has fused to carbon and oxygen. The core contracts
to its absolute limit once again, this time causing helium to fuse in
the shell around it. But the helium-fusion in the shell around the
core produces so much heat that in another shell around the first
shell, hydrogen is also fusing to helium now. The Sun's inside is a
bit like an onion now, with all these layers, and its heat quickly
makes it grow to a red giant again. But this time, its growth doesn't
make it reach a stable endsize: the Sun keeps growing and shrinking
alternately. That's because the helium fusion is very sensitive to
temperature, and in the shell where helium fuses to carbon and
oxygen, the temperature varies. This causes the Sun's energy output
to fluctuate wildly, and with it, the Sun's outer layers contract and
expand rapidly.
Every time the Sun expands, it loses a lot of gas.
The outer layers are just too far away from the core; there is very
little gravity working on them at this distance. So the hot gas
escapes from the Sun's gravity, expanding and cooling down like a
smoke ring. Every few weeks or months, the Sun expands and contracts
again, and every time it blows a bit of its own outer layers away. A
heavier star will eventually begin to fuse carbon and oxygen to neon,
magnesium, sulphur, and silicon; and then fuse silicon and sulphur to
iron and create all the other light elements in the periodic table
before exploding in an explosion brighter than an entire galaxy, in
which heavier elements like gold and uranium are also formed. But our
Sun isn't heavy enough to reach the 600 million degrees needed to
fuse carbon or oxygen, and slowly blows its outer layers away
instead, creating a beautiful nebula around our Solar System.
(Image credit: NASA and ESA)
As the Sun loses its mass, the fusion in the core
slows down, with gravity pulling less hard. The core cools down as
the pressure decreases. Over the course of millions of years, fusion
eventually stops entirely, as all that's left of the Sun is the core:
an incredibly dense thing the size of the Earth consisting mainly of
carbon and oxygen. It glows a fierce white-bluish from its heat, but
it's so tiny that the Earth now cools down deep below freezing, its
Sun becoming a single bright point of white light in the sky. The Sun
has become a white dwarf, and there's nothing left for it to do but
to slowly cool down over billions of years. The white dwarf slowly
becomes cooler and fainter, its white light eventually fading to
yellow. Then orange, and then it only shines a very faint red light.
Eventually that last light dims too, and all that's left is a cold,
dark dense object called a black dwarf. The Sun is dead.
But the universe is still young. Stars continue to
be formed, and the nebula that was once the Sun's outer layers - a
very sparse cloud of mainly hydrogen and helium, with a bit of the
carbon and oxygen fused by the Sun - mixes with other similar clouds,
and becomes part of new suns, and their solar systems. The silicon
and iron Earth consists of, the carbon in our bodies, the nitrogen
and oxygen in our air, were all once created inside a star. Only
hydrogen and helium were formed in the Big Bang; all the heavier
elements come from stars. Carl Sagan used to say: “We're made of
star-stuff.” And the Sun's atoms too will become part of worlds of
star-stuff in the far future.
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