Kinds of Black Holes
Galactic Black Holes
Scientists believe that gigantic black holes inhabit the cores of most galaxies, hence the term "galactic" black holes. Evidence discovered in the 1980's suggests that holes like this power most quasars and radio galaxies, and also inhabit the cores of the majority of large, normal galaxies such as our own galaxy, the Milky Way, and our neighbor Andromeda. It is believed that they are in some small galaxies, too, like M32.
The big question is: "Do we really have a monster like that at the center of our own galaxy?" The evidence can be interpreted many ways, and scientists are far from sure. This is what they have found out, though: there are gas clouds at our galaxy's core orbiting around an object weighing three solar masses, and this heavy object gives off peculiar yet not too strong radio waves coming from something only the size of our solar system. These observations support a black hole, but that's not the only explanation for them. Don't worry, though: we are located far away from the black hole, on one of our galaxy's outer arms.
How Are They Formed?
Stars and interstellar gas are flung into the heart of the galaxy, gases by the effect of friction and stars by the gravitational forces which causes one star that passes by another to get flung inward toward the core, while the other gets flung outward. The overall effect of all this is to drive some gas and stars into the center of the galaxy.
As this continues to happen, the gravity of the clump they form gets stronger and stronger. Eventually, the gravity may become strong enough to overcome the star's internal forces and a black hole will be formed. Or, in a different situation, stars massive enough in the center clump may form their own small black holes, which can collide with the other small black holes at the center until one large black hole is finally formed. Scientists have calculated the time it takes for all this to happen makes it possible that most galaxies have formed gigantic black holes at their centers long before now.
Galactic black holes often have two "jets" shooting out opposite sides of the center. This effect can be created by gravitational energy released by gas in the accretion disk, which produces a "jet" or "beam" of electrons that spiral out at almost the speed of light.
New Evidence to Support the Theory
Scientists now recognize that black holes come in at least two distinct categories: stellar black holes, which form from the collapse of stars, and these galactic black holes that lurk in a galaxy's core, some of which may have existed for 90% of the history of our universe. The galaxy with the most evidence to support the existence of a black hole at its center is undoubtedly the giant M87, where the Hubble Space Telescope discovered swirling gases stretching out as far as 500 light-years around a monstrous black hole. This is discussed in more detail in the section Black Holes Proven By Hubble Telescope.
More and more evidence is piling up for the existence of other galactic black holes, too. Two newly discovered black holes, at the centers of the galaxies NGC 3379 and NGC 3377, are located in the constellation Leo. The first mentioned weighs in at 50 million solar masses, the other 100 million solar masses. The third newly discovered hole is at the core of NGC 4486b, in the Virgo constellation. Observations made by the Hubble Space Telescope in orbit showed a big increase in the speeds of stars located near the cores of these galaxies, the kind that would be produced by galactic black holes. Look at the table below for information on these and other black holes that have been found.
A Census of Galactic Black Holes
Only 11 of the extremely dense objects called galactic black holes have been definitively detected to date.
Milky Way ---- 28,000 2 million NGC 224 (M31) Andromeda 2.3 million 30 million NGC 221 (M32) Andromeda 2.3 million 3 million NGC 3115 Sextans 27 million 2 billion NGC 4258 Canes Venatici 24 million 40 million NGC 4261 Virgo 90 million 400 million NGC 4486 (M87) Virgo 50 million 3 billion NGC 4594 (M104) Virgo 30 million 1 billion
Galaxy Constellation Distance
NGC 3377 Leo 32 million 100 million NGC 3379 Leo 32 million 50 million NGC 4486b Virgo 50 million 500 million
**The above tables were originally printed in the Tuesday, January 14, 1997 edition of The New York Times, page C7 in the Science Times section.**
Using data from the Hubble Space Telescope, the Canada-France-Hawaii telescope on the island of Mauna Kea in Hawaii, and other findings, scientists have been able to develop computer models of the speed patterns caused by a galactic black hole. Using this information, they did a fast survey of 15 other galaxies near to our own and 14 of them showed signs similar to the computer model. On the basis of these findings, Dr. Douglas Richstone of the University of Michigan guesses that almost all galaxies contain black holes at their centers. From these findings, scientists also came to the conclusion that the strength of galactic black holes is related to the size of the galaxy it lives in.