We have all, at some time or another heard of black holes, but what exactly is a black hole and why is it ‘black’? As I described in my post about star types, most stars grow slowly into massive red giants as they run out of hydrogen. Most then collapse heating up in their final years. This collapse is halted quickly however as most stars don’t have enough mass to create a gravitational pull strong enough to overcome electron degeneracy pressure thus preventing further collapse. In simple terms this is the force that prevents electrons sticking to the protons in the nucleus of the atom. The star can’t release any more energy and it slowly loses its outer layers to space leaving a cooling white dwarf the size of Earth. More massive stars (those with more than 10 solar masses) continue gravitational collapse past electron degeneracy. This means that in the final moments of a massive star’s life it actually fuses electrons and protons together to form neutrons. This process releases mas...