Question: What does the fuel do in stars and the sun and how is the fuel created or how does it get there in the first place? What would happen if these stars did run out of fuel? would we still be able to survive, and for how long?

Dave Jones answered on 17 Jun 2014:

Stars are mainly made of Hydrogen (Hydrogen being the most common element in the Universe, with most of it being formed in the Big Bang). Hydrogen is their fuel, so stars are made of their fuel. Stars burn Hydrogen in a process called nuclear fusion, whereby they smash two Hydrogen atoms together so hard they combine to make one Helium atom. This releases a lot of light and heat which is was we see when the Sun shines, for example. When the star runs out of Hydrogen and is left with only Helium, it can try to fuse that together to make heavier elements like Carbon, Nitrogen and Oxygen but it’s more difficult so it doesn’t work as well. This is what we mean by stars running out of fuel, when they’ve burnt up all their Hydrogen turning it into other elements that don’t burn so easily.

So, the fuel is Hydrogen and was created in the Big Bang. It got into the stars because it was in the clouds of gas that the stars formed from, i.e. it didn’t get put there it was always there. When they run out of fuel it can go two different ways depending on how massive they are. Really massive stars (ones that are about 8x the mass of the Sun or even more massive) explode as supernovae, while the smaller ones just sort of fade away like a camp fire that has gone out. The stars are still there it’s just that they aren’t burning anymore, so they just cool down really, really slowly over billions of years.

When other stars die, it doesn’t really matter to us, but when the Sun dies we will be in serious trouble (luckily we have about another 6,000,000,000 years before that happens, phew!). Just before it runs out of fuel it will swell up and become so big that it will actually swallow the Earth. After that it’s outer layers will sort of drift off into space forming a cloud of ionised gas called a planetary nebula (the kind of thing I’m interested in, there is a picture on my profile page), leaving just the core behind where the Sun once was. We call this core a white dwarf and it’s not really a star, just the hot middle bit that has been left to cool down for the rest of eternity. With all that happening, life on Earth would definitely not be able to survive without fleeing to another planet or even another Solar System, so we’d better get working on those rocket ships, only 6,000,000,000 years to go!