Question: @Ciarán, what powers the sun?

Ciarán O'Brien answered on 10 Nov 2014:

Oddly enough, the sun powers the sun!

The sun, like all other stars, is an absolutely gigantic ball of (mostly) hydrogen gas. A huge cloud of gas, in deep space, starts to come together due to the tiny gravitational pull from all the other hydrogen atoms. It’s a tiny tiny pull, but there’s no other object nearby so it’s still the strongest.

As the cloud of hydrogen pulls tighter together under its own gravity, the pressure inside the cloud increases. Normally the pressure would cause the gas to expand until the pressure was gone, but there ‘s just so much hydrogen that the pull of gravity is stronger than the pressure. As the pressure increases, the temperature of the cloud goes up. And up. And up. Seriously, it gets very, very hot.

Eventually, the gravity of the cloud has condensed it into a tight ball (a really, really massive ball tens of thousands of miles across), and the pressure is so great that the temperature is measured in hundreds of thousands of degrees Celsius, if not millions. And eventually, the combination of enormous pressure and massively high temperature foces the hydrogen at the centre of the cloud (where the pressure and temperature is greatest) to get squished together, where the hydrogen atoms fuse together to become helium. The energy released by the generation of helium pushes more hydrogen atoms together to fuse into helium, and a chain reaction begins, and BAM! Your hot dense cloud of gas ignites and becomes a colossal nuclear fusion generator called a star.

The sun is bright and hot because it’s constantly turning its hydrogen into helium with nuclear fusion. It’s the same reaction that powered the bomb that destroyed Nagasaki in World War 2, only the sun is so mind-bogglingly huge that there are MILLIONS of hydrogen bombs worth of nuclear fusion going off EVERY SECOND, and it’s been doing this for billions of years, and will keep doing it for billions more.

Astrophysicists can actually tell how old a star is by figuring out how much of it is made of helium, which shows how much hydrogen has been used up to create it.