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)

Here is 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.

for more information

http://cosmology.berkeley.edu/Education/BHfaq.html