Despite my ignorance, I can't resist trying to explain what happens
in the presence of really strong gravitational fields. A typical
scenario has a large sun (at least 10 times as big as ours, usually; relax!)
cooling off until the gravitational attraction is strong enough to supply
the energy of confinement necessary to overcome the uncertainty
principle that normally prevents electrons from being confined inside
protons. Then the reaction 
(a sort of inverse neutron beta-decay) begins to convert hydrogen atoms
to neutrons, emitting neutrinos as they go. The neutrons further enhance
the gravitational energy density until there is a sudden chain reaction
producing a supernova (the most violent explosion known) that blows
off the exterior of the star (which is now rich in heavy elements because
of all the neutrons being generated)
and leaves behind a neutron star - basically a giant atomic nucleus
that doesn't fission because gravity holds it together.
Neutron stars are generally spinning very rapidly and have enormous magnetic fields "locked in" to their spin, so that the fields sweep up nearby charged particles and turn them into a beacon emitting electromagnetic radiation synchronized with the spinning star. Such beacons are "seen" on Earth as regularly pulsing radio sources or " pulsars," many of which are now known. Most nebulae (the remnants of supernovae) contain neutron stars at their cores.
The phenomenology of neutron stars is itself a huge and fascinating subject about which I know too little. Let's both go look them up and read more about them!