Black Holes
A body when crunched down into smaller volume causes the gravitational attraction on it to increase. Consequently, the escape velocity on it goes on increasing. Escape velocity is the speed with which an object has to travel to break free from the gravitational pull of an entity, for Earth, its 11 km per second (7 miles per second). As the escape velocity increases, things have to be thrown with greater force and eventually there comes a point when light traveling at 186 thousand miles a second is beaten by the gravity. At this point, it is safe to say nothing can escape the gravity as nothing travels faster than the speed of light. This entity is referred to as a black hole. We see things because light reflects off them and enters our cornea, since no light escapes black holes they obviously can�t be seen. However, there are some pretty sturdy reasons to support their existence.
Formation
When a massive star runs out of fuel it explodes into a supernova. The leftover of the outburst condenses to a form extremely dense entity known as a neutron star. The existences of neutron stars have been detected using radio telescopes. If the size of the neutron star is too large, the gravitational force overpowers the pressure gradients and the collapse is made unstoppable. The neutron star goes on shrinking and finally transforms into a black hole. There is a specific mass limit for a neutron star to be able to make the transformation, it�s about the twice the mass of our sun. So its expected quite a few neutron stars have this mass and thereby we can expect to find quite a few black holes. A supernova happens in our galaxy once in 300 years, also there are around 500 neutron stars identified in the neighboring galaxies.
This further supports the fact that there should be some black holes in our vicinity. Over the years black holes grows by gorging on everything around them. Just before matter enters the black hole, it gets accelerated so much that it heats up to point where it starts emitting X-rays. This phenomenon allows for better detection of blackholes, NASA�s Chandra X-ray telescope allows it to spot very distant emissions and thereby detect black holes. The first stellar black hole, Cygnus X-1 was spotted in this manner. In 2001, the observation of Cygnus 1 provided first glimpses of material actually being sucked into the black hole.
Gravity Alone?
A longstanding puzzle was how does a black hole manage to consume matter around it? Matter should actually be orbiting them just like the planets orbiting the suns, the angular momentum of the dust particles should allow it coast around the black holes forever. Astronomers knew that gravity alone couldn�t do it on its own. Recently the solution was found in the form of magnetic fields. A team of scientists from the University of Michigan studied a binary system of seven solar mass black holes. They noticed a wind of glowing particles being blown out from the gaseous discs, and a magnetic field was powering the charged wind. It is believed that the glowing breeze is used to zap the spin energy of the disc, once its prey is weakened; the black hole slurps it up.