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.