Black Hole
Once a giant star dies and a black hole has formed, all its mass is squeezed into a single point. At this point, both space and time stop. Many people think that nothing can escape the intense gravity of black holes. However, only when something (including light) gets within a certain distance from the black hole, will it not be able to escape. Stars and planets at a safe distance will circle around the black hole, much like the motion of the planets around the Sun. The gravitational force on stars and planets orbiting a black hole is the same as when the black hole was a star because gravity depends on how much mass there is--the black hole has the same mass as the star, it's just compressed.Black holes are truly black. Light rays that get too close bend into, and are trapped by the intense gravity of the black hole. Trapped light rays will never escape. Since black holes do not shine, they are difficult to detect.
Neutron Star
Neutron stars are very dense and spin very fast and are typically only 10-15 km in radius. Because neutron stars form from burnt-out stars, they do not glow. The collapse of the star causes the matter to be converted into mostly neutrons, therefore the name neutron star.Some neutron stars emit radio waves that pulse on and off. These stars are called pulsars. Pulsars don't really turn radio waves on and off--it just appears that way to observers on Earth because the star is spinning. What happens is that the radio waves only escape from the North and South magnetic poles of the neutron star. If the spin axis is tilted with respect to the magnetic poles, the escaping radio waves sweep around like the light beam from a lighthouse. Far away on Earth, radio astronomers pick up the radio waves only when the beam sweeps across the Earth.
Supernova
Search the HEASARC site: Supernova One of the most energetic explosive events known is a supernova. These occur at the end of a star's lifetime, when its nuclear fuel is exhausted and it is no longer supported by the release of nuclear energy. If the star is particularly massive, then its core will collapse and in so doing will release a huge amount of energy. This will cause a blast wave that ejects the star's envelope into interstellar space. The result of the collapse may be, in some cases, a rapidly rotating neutron star that can be observed many years later as a radio pulsar.
