Question: how do stars survive for thousands of years?

Mark Kennedy answered on 8 Nov 2016:

Michel and Roisin have covered this pretty well, but I’ll throw some numbers at you anyway!

As Roisin said, stars burn hydrogen and helium to stay alive. The Sun is called a main sequence star because it burns hydrogen, and all stars in the Universe start off burning hydrogen.

When I say burning hydrogen, I actually mean fusing hydrogen. Under very extreme conditions (high density, high temperature, high pressure) you can force 2 protons to overcome their electrostatic repulsion and fuse together to form an atom with 2 protons in its core (helium). This results in the release of energy (which is what comes out of the Sun and comes us warm). So, if we know how much hydrogen is in the Sun, and how often 2 hydrogen atoms are fused together in the Suns core, we can estimate how long it lives for.

First things first: fusion only occurs in the Suns core. The outer layers are too cool for fusion to occur.

So, in the core, 3.6e38 (that’s 360,000,000,000,000,000,000,000,000,000,000,000,000) protons are fused together every second! But a proton is really small – it only weighs 1.67e-27 kg! So that means 6.01e11 (601 billion kg) of hydrogen is fused in the core of the Sun every second. The core of the Sun has a total of 5e29 kg.

So that means (from my very simple calculations above) that the Sun should continue burning hydrogen for a total of 8.3e17 seconds, which is the same as 26 billion years! In reality, the Sun us 4.6 billion years old, and has another 4.6 billion to go. So I’ve really simplified the physics for my calculation, as the reality is the Sun will go from hydrogen burning to helium burning long before the last of the hydrogen is used up in its core.

I hope those numbers help give you context for why stars last for so long!