smaller than a light-year in diameter. Even if a sufficient number of stars were somehow crammed into such a small volume, the resulting stellar system would be so dense that the stars would quickly collide with each other and coalesce into a single unstable, massive object.

For these reasons, most astronomers believe that quasars and active galaxies can be powered only by gravitational energy released at the center of the system. According to current ideas, a massive black hole, with a mass of a million to a billion times that of our sun, inhabits the middle of an active galaxy or quasar. Surrounding gas and stars fall under the gravitational grip of the central black hole. As gas plunges toward the black hole, it releases gravitational energy, which is then transferred into high-speed particles and radiation. Matter falling toward a black hole can convert 10 percent of its mass into energy before it enters the black hole and is never heard from again.

A key to understanding quasars and active galaxies is the mechanism for feeding gas to the central black hole. Is it constant or intermittent? What triggers it? Possible sources of gas include ambient gas in the central regions of the galaxy, the gravitational shredding of hapless stars that wander too close to the black hole, the disintegration of stars by collisions with each other, and the agitation of one galaxy by a close encounter or merger with another. For the more luminous active galaxies and the less luminous quasars, gas must be fed to the central black hole at the rate of about a sun's worth of mass per year. Isolated black holes, no matter how massive, produce very little energy. Thus an understanding of the environment of the central black hole and how it is fueled may be crucial to understanding why some galaxies are highly energetic and others are not.

How can we test the hypothesis of massive black holes? A massive black hole, even as massive as a billion times the mass of our sun, would have a diameter smaller than our solar system. At the distance of the nearest big galaxy, 2 million light-years from us, such a black hole would have an angular size of only a few billionths of a degree, too small to be seen by any telescope in the near future. However, a massive black hole might reveal itself by the way that it affects the motions and positions of surrounding stars. Trapped by the gravity of the hole, surrounding stars would huddle together closely and would hurtle through space more rapidly than if no black hole were present. Infrared observations by a telescope carried aloft in the Kuiper Airborne Observatory have revealed rapid orbital motions in the center of our galaxy, indicating the possible presence of a black hole with a mass of a few million solar masses. Hints of these effects have also been found in a number of nearby galaxies, and the Hubble Space Telescope will look at a larger sample of more distant galaxies.

Black holes might also be indirectly identified by the high-energy emission of the surrounding gas. It is believed that gas near a black hole orbits it in a flattened disk, similar to a protoplanetary disk, only hotter and much more

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