Neutron Stars and Black Holes
describe the nature of neutron stars and black holes
Neutron Stars
When a massive star collapses the protons in it capture electrons and become neutrons. We are left with an incredibly dense ball of neutrons spinning rapidly. A neutron star is incredibly dense, made entirely of nuclear matter. Neutron stars often spin emitting jets of matter and radiation into space. The first pulsars detected were thought possibly to be signals from an alien intelligence until it was figured out that they were spinning neutron stars emitting radio waves.
Black Holes
Before any evidence of black holes existed they were known to be a theoretical possibility. After a supernova explosion there remains an incredibly dense neutron star. If the mass of the Sun from which it originated was great enough then the neutron star could be a black hole. The gravitational field produced by a black hole is so great that even light cannot escape, hence the name. (its escape velocity is greater than 3 x 108 m/s) Any object getting too close to a black hole would be sucked in and "spaghettified", i.e. stretched into spaghetti by huge gravitational forces. There is thought to be a large black hole at the centre of our, and many other, galaxies. Like a whirlpool our galaxies stars rotate around it, any that get too close being sucked in and crushed.
Evidence for black holes
When charged particles move in a circle they produce electromagnetic waves. Scientists discovered sources of X rays in distant galaxies. These X rays could be produced by charged particles spinning around a black hole.
Here is an artists impression (from NASA site) and an image of a galaxy taken by the HST.
By measuring the speed of rotation of the galaxy and its size they they have calculated the mass at the centre and found it great enough to be a black hole. Observations of the stars around the centre of our own galaxy suggest an incredibly dense object. A super massive black hole. It is now thought that there is one of these at the centre of every galaxy.
The third picture above shows an affect called gravitational lensing. Light can be bent by very strong gravitational fields. The light from distant galaxies appears to have been bent, possibly by a very dense but invisible object.
for more information
http://cosmology.berkeley.edu/Education/BHfaq.html