Beast in the MIDDLE, The

Muse, May/Jun 2004 by Irion, Robert

The astronomers applied these laws to the orbits of the galaxy's inner stars. Based on how fast the stars were moving, they calculated that the hidden object at the center of the Milky Way must be about 3.7 million times heavier than the sun. Nothing besides a black hole could weigh that much and yet occupy such a tiny region of space. Those tight orbits were a smoking gun, solid proof of the existence of a massive black hole. The teams published their results in scientific journals in 2002.

Today, most astronomers are convinced a supermassive black hole lies at the center of our galaxy. In fact, the case for this supermassive black hole is stronger than the case for supermassive holes elsewhere. That's because the new high-tech telescopes see the stars in the Milky Way more clearly than those in other galaxies.

Now astronomers are busy trying to figure out how these giant black holes were created. The most popular idea is that galaxies and black holes formed together early in the history of the universe. Perhaps clumps in a giant cloud of gas collapsed to form the stars of a baby galaxy while a particularly dense clump spawned a black hole. Then the galaxy and the black hole grew together for billions of years, the black hole swallowing gas and stars in the galaxy, while the galaxy itself absorbed gas and other galaxies in the neighborhood. The selling point of this idea is the hole would have been very big to start with and so could have quickly bulked up to super-massive size.

Other astronomers think that supermassive black holes might have grown from one-star black holes after all. The idea is these small black holes collided and merged with other black holes, forming ever more massive holes that then "settled" into the centers of galaxies. But some astronomers say this process is too slow and would never have produced black holes millions of times more massive than a typical star. Astronomers hope more powerful space telescopes will show which answer is right. But either way, black holes are no longer laughable mathematical oddities. Instead, they are clearly central players in the greatest dramas of the universe.

If astronomers are still guessing about what happened at the beginning, they're pretty much certain what will happen at the end. Supermassive black holes will outlast us all. Billions of years from now, the galaxies will gradually fade away. All of the stars will use up their fuel, and starlight will become dim and then flicker out entirely. In that dark universe, supermassive black holes will be the only remaining objects. Every once in a while, something will wander close enough to them to be swallowed. But most of the time, they will drift through space-massive, invisible, and utterly alone.

Robert Irion lives near the ocean in Santa Cruz, California. He writes about astronomy for Science, Discover, Sky & Telescope, and other magazines. He once fell into a black hole, but it was only a dream.