If you take normal matter — something made of protons, neutrons and electrons — and compress it as far as it will go, something incredible happens. At high enough temperatures and densities, something requiring a tremendous amount of mass hundreds of thousands of times as great as planet Earth, nuclear fusion occurs, giving rise to a living star. Burn through all the hydrogen, though, and your star’s core will be made of helium, which will collapse further and heat up to even higher temperatures and densities. Reach a critical temperature and helium will be begin burning, forming carbon. After some time, you’ll run out of helium, too, where your now-carbon core begins to contract, heating up and becoming more dense. At this stage, one of two critical things can occur.
Either your star isn’t massive enough to ignite carbon, in which case it will gently blow off its outer layers and form a white dwarf at the center: a degenerate mass of atoms that’s maybe the mass of the Sun but only the physical size of Earth. This sounds like an incredible state of matter, but it’s still relatively sparse, at “only” a few hundred thousand times the density of our planet. The atoms themselves are sufficient to prevent gravitational collapse from taking things further.
Published by Frobes. See more at: http://www.forbes.com/sites/startswithabang/2016/02/03/the-cosmic-gift-of-neutron-stars-a-live-blog-event/#75cfd6b32884