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In: Planetary Science
The nucleus of an atom is governed by two forces: electromagnetism, which makes the positively charged protons want to fly away from each other; and, the strong nuclear force, which wants to hold protons and neutrons together. The strong force is…strong…but has a very short range, while the electromagnetic force does not have a limit to its range (although its strength drops off quickly with the square of the distance).
To create nuclei, then, you have to apply enough force to overcome the electromagnetic repulsion between protons to get them close enough for the strong force to grab them and bring them together. If you do that, energy is released (as long as the atom you make is lighter than iron – you *lose* energy this way if the atom is heavier than iron and gain energy if the atom splits instead). Forcing protons together this way takes a *lot* of force. In stars, this force comes from the gravity of all of its mass crushing inwards.
The mechanism for fusion is a little more complicated, involving quantum mechanics. Even the strong force isn’t strong enough to stick two protons together – neutrons are needed, since they have no electric charge and don’t repel the protons. Protons and electrons can sort of fuse into neutrons, and then neutrons can get stuck to protons, and *then* those heavier hydrogen nuclei made of a proton and one or two neutrons all together will fuse into helium. Buuuut that’s the more complicated version that isn’t super important right now. The important thing to know is that there’s a lot of stuff going on inside the Sun due to its gravity crushing all the particles together.
Energy is released as hydrogen (one proton and maybe a couple neutrons) fuses into helium (two protons and at least one, but probably at least two neutrons). That energy comes out as heat and photons. This energy pushes the atoms apart, holding them up against the crush of gravity. As fusion increases, more energy is created, which forces the atoms apart. That slows fusion down. When fusion slows down, there’s less energy and less pressure holding the atoms apart, so gravity crushes back inward. This cycle holds the Sun in equilibrium – gravity is balanced against the energy created by fusion. Stars *do* go through periods of increased or decreased activity, but overall it’s balanced.
The Sun actually puts out very little energy *per mass*. *You* create more energy per mass than the Sun. But, of course, the Sun has a *lot* of mass – 99.9% of the mass of the solar system is just in the Sun. Even a little bit of energy per mass is a *lot* of energy.
Despite being *really big*, the Sun has a finite amount of hydrogen to fuse into helium. Helium is heavier and more dense than hydrogen, so it sinks into the core and the Sun shrinks. This occurs over *billions* of years. Eventually, there won’t be enough hydrogen in the core for the Sun to fuse anymore. When this happens, there will still be enough energy and gravity to fuse hydrogen in huge, circulating currents of hydrogen and helium. With so much mass in the core, the energy from this fusion will cause the Sun to swell into a gas giant, much bigger than the Sun currently is. Eventually, the Sun will run out of hydrogen to fuse in these circulating currents, too, and fusion will stop. The Sun will contract into a white dwarf, which is only stays hot because of the heat left over from its lifetime. When *that* runs out, the Sun will become a black dwarf – a cold lump of mostly helium.
Stars with more mass have enough gravity to crush helium and cause it to fuse into heavier elements. Stars that are very massive will continue fusing heavier and heavier elements until they start fusing iron. As stated, fusing iron *takes* energy, rather than producing energy. This will cause the star to rapidly collapse, in a matter of minutes and with the outer layers of the star reaching relativistic speeds before impacting against the core. This sudden, massive impact creates a *nova*, and fuses the iron core into even more massive elements, which explode out into a nebula, which will eventually collapse from gravity into new stars and planets. The core remains as a neutron star. Even *more* massive stars don’t stop at neutron stars, collapsing into black holes.
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