Explain how gravity and fusion have opposite effects on a star?

Answer 1

Fusion is the process in which several hydrogen nuclei, with a single proton each, combine into a single helium nucleus with two protons and two neutrons. During this process, a tiny bit of the mass is "destroyed" and converted into energy, in accordance with Einstein's famous "e=mc^2" equation. Energy released is equivalent to the mass destroyed, times the speed of light squared. The amount of mass is VERY tiny - but the speed of light is a very large number, and multiplied by itself is an ENORMOUS number. So, a very small amount of mass is converted into a very large amount of energy.

That energy of fusion, expressed as heat, is the process that powers nuclear bombs, and the Sun's fusion is equivalent to millions of nuclear bombs going off all at once, every second. That much explosive power would certainly tend to blow anything apart!

Except, it DOES NOT, generally, blow stars apart, because the power of the nuclear fusion is balanced by the force of gravity; the mass of the star generates gravity, which tries to crush everything in the star down to a tiny point. And so a star is in balance; the power of fusion, contained by the power of gravity.

In some cases, when the large star's energy is boosted suddenly, a star CAN blow itself apart. We all this a "nova", and if it's powerful enough, a "super-nova", and it normally happens to very large stars (more than about 3 times the mass of our Sun) at the ends of their lives.