Black holes were theorized more than 200 years ago, and later were predicted by Einstein's theory of general relativity. The discovery of active galaxies forced astronomers to think that monstrous black holes really do exist and are the "engines" at the heart of these fireworks. The gushers of light and other radiation from such objects could not be explained by starlight alone.

By definition, a black hole is very hard to find. It is a celestial object that squeezes a lot of material into a very small space. The resulting gravitational pull is so intense that anything passing nearby, even light, is trapped forever.

Like a ghost in a mystery story, a black hole's presence must be inferred by the effects on its surroundings. Its powerful gravity will influence the motion of neighboring stars. The closer the stars are to the black hole, the faster they should be moving, just as orbiting planets move faster the closer they are to the Sun. If no black hole is present, the speed of the stars should slow toward the hub of a galaxy, because most of the gravity influencing their motion would come from the other stars in the galaxy.

Once the speed of the entrapped material is measured, astronomers can calculate the mass of the black hole using the simple laws of gravity, just as the orbital speed of the Moon can be used to calculate Earth's mass. If it turns out that there is far more mass present than there are stars, the matter must be tucked away in something that is invisible and compact.

Similar observations have been made with ground-based telescopes since the mid-1980s; but having to look through the Earth's turbulent atmosphere severely limits the accuracy of such telescopes for detecting and measuring a large, central mass. While the ground-based data give ambiguous lower limits to the central mass, NASA Hubble Space Telescope observations are decisive for accurately measuring the mass and ruling out all other possible explanations.

The first black hole confirmation was nailed down when the space telescope uncovered a spiral disk of gas swirling around the hub of the giant elliptical galaxy M87 (called Virgo A, located in the Constellation Virgo). The shape alone suggested that the material was caught in a gravitational whirlpool. Using Hubble's spectrographs, astronomers were able to measure the velocity of the gas by a method known as Doppler shift. As the disk spins like a carousel, one side of it approaches us and is blueshifted, while the other side rotates away and is redshifted.

Astronomers concluded that the gas is whirling at more than a million miles an hour. This information can be used to calculate how much mass is packed into the core of M87. It turns out that the mass of two billion Suns is compressed into a region of space no bigger than our solar system. Hubble has made similar observations in two other elliptical galaxies, NGC 4261 and NGC 3115. These monstrous black holes weigh in, respectively, at 200 million solar masses and two billion solar masses.

The Giant Elliptical Galaxy NGC 4261

NGC 4261:Trail of Black Holes and Neutron Stars Points to Ancient Collision

bright lenticular galaxy, NGC 3115

NGC 3115

Surveys of galaxy nuclei in both active and quiescent galaxies suggest black holes are common to virtually all galaxies. The mystery is how the black holes formed in the first place. The abundance of quasars in the early universe, objects at the hearts of galaxies that pour out a torrent of radiation, suggest that monstrous black holes must have formed very early, though it is still not known how this happened.

Hubble images of quasars show that they reside in a variety of galaxies, both spiral and elliptical. Many but not all of the quasar host galaxies, are engaged in a collision or interaction with other bypassing galaxies. The infall of gas resulting from such collisions fuels the monster black holes.