At the center of each galaxy there is a super-massive black hole that is millions to billions of times heavier than our sun. The massive black hole captures nearby stars and drags them into a swirling accretion disk. John Archibald Wheeler invented the name ‘black hole’ because light inside it can never be seen. Before Wheeler, these objects were often referred to as 'frozen stars.

A black hole is a region of space through which nothing can escape, as its gravitational force is so strong. This region is called the event horizon of a black hole. They are objects so dense that even light is unable to escape through it. A black hole is invisible.

The surface of a black hole is known as the event horizon. At the event horizon, the pull of gravity becomes infinitely strong. Nothing can escape the black hole after having fallen past the event horizon. The radius of the event horizon is used to specify the size of a black hole. The radius of a black hole measured in kilometers equals three times the mass of the sun.

To discover a black hole, scientists would have to measure effects that only a black hole could produce.

1. Severe bending of a light beam.
2. Extreme slowing of time.

However, black holes can be detected if they interact with matter outside the event horizon, for example by drawing in gas from an orbiting star. The gas spirals inward, heating up to very high temperatures and emitting large amounts of radiation in the process.

A black hole can form when a huge star runs out of nuclear fuel; the star can no longer support its own weight and is crushed by its own gravitational force. As a result, the core of the star collapses. The mass of the core is three or more solar masses, the core collapses into a singularity in a fraction of a second.

• The German-born physicist Albert Einstein described it in the General Theory Of Relativity, developed in 1916. The theory predicts that when a large amount of mass is present within a small region of space , all paths through space are warped inwards towards the center of the volume, forcing all matter and radiation to fall inward. The gravitational force is strong near a black hole because all the black hole's matter is concentrated at a single point in its center. This point is called the point of singularity.
• In 1974, British physicist Stephen Hawking predicted that energy fluctuations from the vacuum causes the generation of particle-antiparticle pairs near the event horizon of the black hole. One of the particles falls into the black hole while the other escapes, before they have an opportunity to annihilate each other. The net result is that, to someone viewing the black hole, it would appear that a particle had been emitted.