Where I live it was was a
cloudy, overcast day today, with a little rain. Since it's November
and I live at pretty high northern latitude, this meant it was quite
a dark, gloomy, grey day. Yet something you almost never realise, is
that even on a day like this, there is a huge amount of light about.
If it's evening or night when you read this, you may think you're
sitting in a pretty well-lit room.
Well, you're wrong:
unless you've got some kind of light fetish and stocked your room
with big industrial lights, it's likely your room is about a thousand
times darker than the outside is on a cloudy day. Your eyes and brain
do an amazing job of compensating for it; if they didn't, the lit
room would look almost completely dark; or being outside at day would
be like constantly staring into the Sun.
Of course, that's where
all that light is coming from in the first place: the Sun. Compared
to the 60 Watt lightbulb that might very well provide all the light
in your room (as one does in my room), it's a huge light, to
illuminate things a thousand times brighter. But what's more than the
light level is this: it beats that light bulb by a thousand times
while being a hundred and fifty million kilometres away.
An amount of distance like that is completely unimaginable; the
largest thing a human mind can actually picture is probably our own
Earth, yet the distance to the Sun is about twelve thousand times its
size. If you imagine everything a thousand times smaller, a human is
under two millimetres tall and the Earth is the size of a city;
twelve kilometres. A big globe, but quite imaginable. Yet the Sun is
still 150000 km away like that; almost half as far as the Moon really
is.
So
the Sun is a ridiculous distance away, and even at that distance it
easily outperforms a 60 Watt lightbulb. So just how bright a
lightbulb is the Sun? As it turns out, the Sun shines with a
ribonkulous 385 Yottawatts! Yotta- is one of the metric prefixes,
like kilo- and milli-. In fact, it's the very largest of the set.
Like a kilometre is a thousand metres, a Yottawatt is a septillion (a
one with 24 zeroes) watts. Now that's one gigantic light bulb.
Obviously,
one major difference between the Sun and a light bulb is that the Sun
isn't connected to the electricity net, and it's a good thing too, as
its bills would run very high. So it makes its own energy, but how
does it do that? How does the Sun work?
To
answer that, we need to go back in time five billion years, to the
Sun's birth. At this point, there was no Sun yet, just a very large
cloud of very sparse gas. The cloud, like all really large things in
the universe, consisted of 75% hydrogen, 24% helium, and about one
percent heavier atoms: mainly carbon, oxygen, sillicon, and iron. As
I said, the cloud was amazingly sparse; it was far less dense than
any vacuum we can make on Earth, and in fact less dense than Earth's
atmosphere at 400 kilometres height, where the International Space
Station can orbit without any trouble.
Yet
this cloud was also very heavy, as it was big. It was heavier than
the Sun is, in fact, and the Sun's weight makes even its brightness
look small. At some point, the cloud began to get smaller. This
probably happened at first because of a bright star passing by. You
see, when light shines on something, it pushes that thing with a tiny
bit of pressure, like the wind. It's a very weak force, and
completely unnoticable to a human. But the cloud wasn't going
anywhere, and the star probably took about a million years to pass,
so the tiny bit of force built up over aeons of time and compressed
the cloud a bit. It was still way sparser than any vacuum humans can
make, but it was now getting compact enough that its gravity began to
matter.
Again,
however, the forces we're talking about were miniscule, and it
probably took an entire human life's length for the cloud's atoms to
“fall” a single metre closer to the centre of the cloud. But only
the first metre. Because the next metre would've taken significantly
shorter. Gravity doesn't drop things fixed distances: it continually
increases their falling speed. Otherwise, a fall from a kilometre
would be no more deadly than a fall from a metre. So the first metre
takes ages, the second shorter, the third even shorter, etcetera. If
you can afford to wait a few million years, you'd see the entire
cloud get smaller and smaller and smaller.
And
the speed with which the entire cloud contracted increased further:
as it got smaller, everything got closer to the centre, and therefore
gravity's pull got stronger and sped the atoms of the cloud up even
more. As the cloud got smaller, it began to rotate. This may seem
odd, but you can test it out yourself if you have a chair capable of
turning on its axis: just give it a spin while you sit on it with
your arms wide, then retract your arms: you'll instantly start
spinning faster. The same happens to the cloud: as it got thousands
of times smaller, it rotated faster and faster. As this happened, the
outer parts of the cloud started going so fast they stopped falling:
they were now in orbit of the centre of the cloud, and will stay that
way. Eventually eight planets, a couple hundred moons, and millions
of other objects will form from this stuff, but lets look to the
centre of the cloud.
The
gas that managed to get in orbit is only a tiny fraction of the
cloud: more than 99% of the total mass is still contracting, getting
ever smaller. In fact, in the centre of this huge bulge of hydrogen
and helium, pressure is getting higher, and would easily crush a
human being at this point. The atoms in the core are getting pressed
closer together continually, and often get to near collisions with
each other, only their mutual electric repulsion keeping them apart.
These near collisions give them energy and make them move rapidly and
erratically, bouncing around like vigintillions of tiny bouncing
balls. And it just so happens a high temperature is nothing more than
atoms bouncing around: when it's extremely cold, atoms are sluggish
and do little, and when it's hot they bounce around. So, the
temperature rises. It rises a lot, in fact, and soon it gets so warm
in the core of the proto-Sun it glows red.
Ever
since it was just a cloud, the proto-Sun has done nothing but get
smaller all the time, but now that is reaching its end: the heat's
energy pushes back the atoms further outside and slows down the
proto-Sun's collapse. It gets smaller more slowly, but it doesn't end
yet: because gravity never wears off and always keeps pulling, but
the heat leaks away out of the surface of the proto-Sun in the form
of lots of infrared radiation and some red light and so pushes less.
The proto-Sun loses heat from this, and so gravity wins bit by little
bit. The proto-Sun gets smaller and denser, although more slowly than
before, and its temperature keeps increasing. The red light it sheds
gets brighter and brighter, and slowly becomes very bright and
orange, then yellow.
At
this point, the core is a raging inferno. The temperature is about a
million degrees, and the pressure is so high no simile will suffice
to describe it. Since temperature is the speed atoms bounce around
with, you can imagine the atoms in the core are now going completely
bazonkers. They're pressed tightly together, yet bouncing around like
they're each attached to rockets. But despite all this bouncing in
close quarters, they never actually touch. They get close, sure, but
the electric force pushing them away from each other is still large
enough to keep them apart.
Until
this point, when the core is about a million degrees hot. As you
recall, the proto-Sun consists almost entirely from hydrogen and
helium. The nuclei of these atoms are the two smallest and lightest
nuclei in the universe. Every nucleus consists of a certain number of
protons, positive particles, and neutrons, non-charged particles. These
are bound together by an odd force called the strong nuclear force.
It works only over very small distances, but is strong enough to
overpower the electric force that pushes the protons apart. But only
over small distances: over distances larger than the ones inside the
nucleus, the electric force is the only one that matters. The nucleus
of hydrogen is very simple: it consists of a single proton, while a
helium atom's nucleus consists of four particles: two protons and two
neutrons.
At
some point, hydrogen atoms begin to collide. As soon as they do, they
get within range of the strong nuclear force. It instantly binds the
two protons tightly together. Meanwhile, another force called the
weak nuclear force changes one of the protons into a neutron and a
negative particle called an electron that is shot away (don't ask me
how that works). So now, we have a proton and a neutron sticking
together in a nucleus called deuterium.
What
happens next is that the deuterium hits another hydrogen nucleus. The
strong nuclear force, which is amazingly strong as its name
indicates, again says “Gotcha!” as soon as the proton that makes
up the hydrogen nucleus gets in its range, but this time the weak
nuclear force doesn't do anything. So we have a nucleus with two
protons and one neutron, which is known as Helium-3.
While
my narration may make it sound like this is an isolated event, it's
happening all over the place. Lots of helium-3 is getting formed. So
two helium-3 nuclei can collide with each other. You'd think this
would result in a nucleus with six particles, but in the collision
two protons actually get blasted away. The remaining two protons and
two neutrons form a single nucleus, however. If this sounds familiar,
that's because it's a helium nucleus!
So
since those two Helium-3 nuclei were each originally formed from
three hydrogen nuclei, what has essentially happened is that six
hydrogen nuclei became one helium nucleus and two hydrogen nuclei.
But we forget something: during each of these nuclear reactions,
energy was released too. A humongous amount of energy, in fact. Well,
a humongous amount compared to the size of the atoms. But there are a
lot of atoms in the proto-Sun's core, and therefore a lot of this
nuclear fusion happens. It starts slowly, but once it gets going, the
core heats up to fifteen million degrees, and emits so much energy
gravity can't make it any smaller any more. And with this, the Sun is
born. The nuclear fusion's energy can go on for as long as the Sun
has hydrogen to fuse, which is about ten billion years. During this
time, gravity is powerless to make the Sun collapse any further.
However,
it's not this same energy that the Sun eventually emits as light and
heat. The energy from the Sun's core gets about halfway to the
surface before it stops at the underside of a layer called the
convective zone. The convective zone constantly has currents flowing
through it that very slowly take extremely hot gas from its lower
parts and take that to the cooler upper part, and they also take
cooler gas from its upper parts and submerge it until it finally
reaches the blazing underside of the layer. It takes about ten
thousand years for the superhot gas from the bottom of the convective
zone to reach the upper part.
When
it finally gets there, it warms up the outer layer of the Sun, called
the photosphere, to about 6000 degrees. This is warm enough for it to
glow a bright, nearly white, yellow. And this glow due to the heat is
that 385 Yottawatts of energy we saw earlier. It's enough to heat up
a planet a hundred and fifty million kilometres further away, and
even enough to cause blindness if you stare into it long enough from
that distance.
Of
course, there's far more to tell about the Sun, but I really think
I've gone on long enough for now. I hope you enjoyed reading about
our 385 Yottawatt lightbulb.
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