Seven interesting things about the Moon

(Image credit: Gregory H. Revera)

In the long, cold winter nights that the northern hemisphere get right now, the Moon is often very high in the sky. The full Moon especially is impressively bright and beautiful as it shines almost directly above your head and illuminates the dark winter night. If you looked at it last night, like I did, you probably even saw a very bright “star” close to the Moon (It wasn't actually a star, but Jupiter). It was a wonderful sight, and inspired me to write about the Moon for a bit. Unlike with the Sun, I don't really have a singular story, and will instead just tell several quite interesting things about the Moon.

(Image credit: NASA)
1: The Moon was created when Earth collided with another planet. The planets formed from a disc of gas and debris around the proto-Sun. Little particles would collide with each other and attach to each other, forming pebbles, which would keep colliding to form rocks, and as these series of collisions kept going, the rocks became bigger and their collisions more violent; especially once they became big enough that their gravity began to play a role. In the outer Solar System, this led to four heavy planets being formed, which had gravity strong enough to keep hydrogen and helium, the most common atoms, from escaping, and as a result collected gigantic atmospheres of those two gases. But closer to the Sun, the planets formed were smaller and not heavy enough to get atmospheres of hydrogen and helium, but only of heavier, far rarer gases. These five planets were Mercury, Venus, Earth, Theia, and Mars.
Hold on, Theia? Well, Theia was a planet about the size of Mars; two times smaller and ten times lighter than Earth. It happened to have formed in almost exactly the same orbit as Earth. Normally, Earth's gravity would've caused the two planets to collide and form a single one with them being so close together, but Theia was in exactly the same orbit, orbiting the Sun in one year, just like Earth. As a result, the two planets never came close to each other since they orbited at the same speed.
So why doesn't Theia exist any more then? Well, the Sun's gravity isn't the only thing that affects a planet's orbit. It's the most important one, but not the only one. The planets also affect each other's orbits a little. It's a very small effect, but the little pulls Venus, Mars, and Jupiter gave Theia and Earth were enough to slightly speed one planet up and slow down the other, causing them to approach each other over the course of 50 million years. Finally, they got close enough to be affected by each other's gravity, so they sped towards each other and collided.
The catastrophic energy of that impact was enough to instantly destroy the Earth's entire surface and melt and deform the planet, while Theia was completely pulverised. Within hours, the collision was clouded in a huge sphere of debris as the two molten planets became a single one that glowed almost as hot as the Sun, vaporising the top layer of rock so it added to the debris. Much of the debris rained back to this molten new Earth, and much of it escaped the gravity and began following its own orbit around the Sun, but the rest of the debris stayed in orbit of Earth. And as the Earth slowly cooled down from the collision with Theia, the debris coalesced into bigger and bigger clumps in a small-scale repeat of the origin of the planets. Eventually, these clumps became two large moons, which finally collided to become the single one that still orbits us. (The traces of this second moon can still be seen in the form of the highlands on the far side of the Moon)

(Image made using SpaceEngine)
2: The Moon used to be far closer. It's not quite sure how much closer, but it is thought it might be as much as eight times farther away now than when it was newly formed! Imagine how gigantic the Moon would've been in the sky at such a distance; it would create planet-wide solar eclipses every month! But if it was this close, how come it's now much farther away? The answer is the tides. The tides are caused by the Moon's gravity, and when the Moon was eight times closer, its gravitational influence on Earth would've been 64 times greater. That means the sea didn't rise up six or seven metres at best with flood, as it does now, but about 400 metres!
Imagine a gigantic tsunami like that rising up every single day and washing deep inland. And there's something else: days used to be shorter back then. The Earth rotated in only six hours, so there were days and nights of three hours each. Every day, you'd get this giant tsunami caused by the Moon. All that water in a 400 metre high tidal wave had a lot of mass together. Every time the flood would hit a continent, the Earth would get a pretty big hit in the direction opposite to its rotation by all that water. The Earth is incredibly heavy, of course, far heavier than the massive flood, but all that water crashing into the continents every day eventually slowed its rotation down over millions of years. By the time the dinosaurs walked the Earth, days lasted 22 hours. They're currently 24 hours and still increasing with a few microseconds each year due to the tides.
But there's a natural law saying energy can't simply disappear, and rotation is a form of energy. So all that energy the Earth lost as it rotated slower needed to go somewhere. In fact, it went into the Moon's orbit. Every day, the tides made the Earth rotate a bit slower and made the Moon orbit a little faster, so it moved into a higher orbit, and that's why it's now much farther away than it used to be.

(Image credit: NASA)
3: The Moon also rotates. Earth obviously rotates once a day, causing day and night. Yet the Moon always has the same side turned to the Earth. The reason one side of the Moon is always facing us is because it rotates once every 27 days; precisely the same time it takes to orbit the Earth. What an incredible coincidence, both its orbit and its rotation take 27 days! What are the odds?
Of course, it's not really a coincidence. This is another thing that's caused by the tides. Remember the tides being 64 times stronger when the Moon was newly formed? Well, just imagine how strong they were on the Moon itself. Earth is 80 times heavier than the Moon, so the tides on the Moon were about five thousand times stronger than they currently are on Earth! Of course, the Moon has no oceans for these tides to cause ebb and flood, but water isn't the only thing affected by the tides. Even the solid rock a planet consists of it affected by it. It's a far smaller effect, and as good as immeasurable on Earth, but when you make that effect 5000 times stronger, it becomes quite powerful. The Moon's surface would've risen up and down every day due to Earth's gravity, probably causing big earthquakes (well, moonquakes) and heavy volcanism (in fact, this might be what caused the dark “seas” of solidified lava on the Moon to form). While it's not quite as intuitive as the idea of a tidal wave smashing into the continents slowing the Earth down, this pulling on the Moon's surface also caused its rotation to slow down due to friction. And since the tidal effects on the Moon were 5000 times stronger, the Moon's days rapidly became longer and longer. All the way until it rotated exactly once for every orbit it made, always facing the Earth with one side.

(Clause Joseph Vernet – Seaport by Moonlight. Work in the public domain)
4: Moonlight is amazingly bright. If you've only ever been outside on a night with full Moon in a place with street lights, you may not have noticed this, but the Moon reflects a lot of light from the Sun. The full Moon gives off enough light to read by and easily find your way around with. It also means you see far fewer stars on night when the Moon is out: the dimmer stars just can't compete with the far brighter Moon. Moonlight casts clear shadows on the ground, and when it's partially clouded it often illuminates one side of the clouds but not the other, creating a very beautiful effect.

(Image credit: Claude Schneider)
5: You can see Earthlight on the Moon. Given how bright moonlight is, it stands to reason the Earth shines even brighter on the Moon. The Earth is four times the size of the Moon, so it has a sixteen times bigger surface to reflect light with. Added to that is that the Earth's surface simply reflects more light than the Moon: the Moon's surface consists of dark grey rock, while the Earth has lots of bright white clouds that reflect far more light. Oddly enough, the oceans are actually quite dark and don't reflect much light; the difference lies mostly in the clouds. All this means Earthlight on the Moon can get up to forty times brighter than Moonlight on Earth. This is actually bright enough to see the Moon illuminated by it from Earth with the naked eye (though binoculars or a telescope make it easier).
Full Earth gives off forty times more light than full Moon, so you'd think full Earth is the best time to observe Earthlight. Unfortunately, when it's full Earth, it's new Moon which means the Moon is very close to the Sun in the sky and invisible. The best time to look for Earthlight is when the Moon is a crescent: the Earth is still close to full seen from the Moon then, but the Moon is visible at night near sunrise or sunset. By the time it's half Moon, Earthlight is getting weaker (since it's also half Earth then) and the Sun's daylight on the Moon makes it harder to see the earthlight on the Moon's night side anyway. It's still visible with binoculars or a telescope at half Moon, but usually not with the naked eye.

(Image credit: NASA)
6: The Moon is the largest moon in the Solar System compared to its planet. It's not the actual largest (It's a big one, but three moons of Jupiter and one of Saturn are bigger), but all other moons are absolutely tiny in comparison to their planet. Mars has two moons the size of mountains. Jupiter, Saturn, Uranus, and Neptune also have big moons, just like Earth, but since these planets are all far larger than Earth, their moons are still tiny compared to them. Our Moon, on the other hand, is only four times smaller than Earth, and eighty times lighter. This is so close some people don't even consider it a moon, but instead consider Earth and Moon two planets that orbit each other (a double-planet). The Moon is certainly the size of a small planet: Mercury, the smallest planet, is only 1.5 times larger (and actually looks a bit like the Moon). The reason the Moon is so big compared to the Earth is probably the unique way it was formed: the other moons in our Solar System probably weren't formed by a planetary collision.

(Image credit: NASA and Buzz Aldrin)
7: Twelve humans have walked on the Moon. Their names are Neil Armstrong, Buzz Aldrin, Pete Conrad, Alan Bean, Alan Shepard, Edgar Mitchell, David Scott, James Irwin, John Young, Charles Duke, Eugene Cernan, and Harrison Schmitt. All of them did so between 1969 and 1972. However, India, China, and Russia all have plans for a manned Moon landing in the early 2020s.
During the six Apollo missions where a manned Moon landing happened, a bunch of stuff was left on the Moon. There's six American flags (which are by now probably just pieces of white cloth due to the Sun's radiation bleaching them), a lot of footprints (there's very little erosion on a world without air or water), six landing modules, six crashed ascent modules, and the three Moon buggies the Apollo 15, 16, and 17 crews used to ride around on the Moon and make bigger trips than they could walking. These are the locations where the Apollo missions landed; they're all visible with the naked eye on the Moon:

(Image credit: NASA)
By the way, if you're in the southern hemisphere you're usually seeing the Moon upside-down compared to this picture.

Well, I guess that's enough about the Moon for now. I hope you found it interesting.


What happens when the Sun runs out of fuel?

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.


How does the Sun work?

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.