27.2 Supermassive black holes: what quasars really are  (Page 3/6)

Modern estimates show that there is a mass of at least 3.5 billion M _Sun concentrated in a tiny region at the very center of M87. So much mass in such a small volume of space must be a black hole. Let’s stop for a moment and take in this figure: a single black hole that has swallowed enough material to make 3.5 billion stars like the Sun. Few astronomical measurements have ever led to so mind-boggling a result. What a strange environment the neighborhood of such a supermassive black hole must be.

Another example is shown in [link] . Here, we see a disk of dust and gas that surrounds a 300-million- M _Sun black hole in the center of an elliptical galaxy. (The bright spot in the center is produced by the combined light of stars that have been pulled close together by the gravitational force of the black hole.) The mass of the black hole was again derived from measurements of the rotational speed of the disk. The gas in the disk is moving around at 155 kilometers per second at a distance of only 186 light-years from its center. Given the pull of the mass at the center, we expect that the whole dust disk should be swallowed by the black hole in several billion years.

Another galaxy with a black-hole disk.

But do we have to accept black holes as the only explanation of what lies at the center of these galaxies? What else could we put in such a small space other than a giant black hole? The alternative is stars. But to explain the masses in the centers of galaxies without a black hole we need to put at least a million stars in a region the size of the solar system. To fit, they would have be only 2 star diameters apart. Collisions between stars would happen all the time. And these collisions would lead to mergers of stars, and very soon the one giant star that they form would collapse into a black hole. So there is really no escape: only a black hole can fit so much mass into so small a space.

As we saw earlier, observations now show that all the galaxies with a spherical concentration of stars—either elliptical galaxies or spiral galaxies with nuclear bulges (see the chapter on Galaxies )—harbor one of these giant black holes at their centers. Among them is our neighbor spiral galaxy, the Andromeda galaxy , M31. The masses of these central black holes range from a just under a million up to at least 30 billion times the mass of the Sun. Several black holes may be even more massive, but the mass estimates have large uncertainties and need verification. We call these black holes “supermassive” to distinguish them from the much smaller black holes that form when some stars die (see The Death of Stars ). So far, the most massive black holes from stars—those detected through gravitational waves detected by LIGO—have masses only a little over 30 solar masses.