Black holes are extremely compact space objects that were once massive stars, which collapsed inward due to the force of their own gravity.
Consequently, black holes are very dense. If it were not for the effect that black holes have on the objects around them, we would be unable to detect them. A black hole has a powerful gravitational field, which traps everything that goes near it. Scientists now theorize that some galaxies have huge black holes in their centers, which release tremendous amounts of energy that powers the spectacular energetic events that go on within the galaxy.
The fuel for the black hole, scientists believe, may be the trapped gas, stars, and dust that are pulled into the hole. Gas that is pulled into a black hole swirls down into the hole much like a whirlpool. By using a spectroscope, the Hubble Space Telescope has the ability to clock the speed of this gas as it swirls around the entrance to the hole. The speed with which the gas swirls is considered the black hole's signature. By knowing the speed of the gas, the mass of the black hole can be calculated.
A black hole in the center of the M87 galaxy in the constellation Virgo, which is 50 million light-years away, has been calculated to have a mass equal to that of 3 billion Suns! An even more effective way of studying black holes is through the use of X-ray observations. X-rays have the capacity to penetrate through gas and dust much better than optical light. With the data delivered to us by X-ray observations and the Hubble Space Telescope, scientists now believe that the presence of black holes explains a lot of the powerful cosmological events, which occur in the Universe.