Downtown

Thursday, September 7, 1995

Go outside on any clear evening in summer or early fall. Look to the southern sky, where the long curve of the scorpion's tail sweeps low. Just to its left is the bright constellation Sagittarius, supposedly an archer, but to modern eyes it looks exactly like an old-fashioned teapot. Tilt your head to the right so the teapot is level. Look just above and to the right of the teapot's spout, where you might expect to see steam rising. In fact, there does seem to be a lot of steam about. There's no bright star to see there, just a lot of hazy-looking clouds. But it's a very special point in the sky. That's the center of a pinwheel of two hundred billion stars. You are looking downtown, toward the center of our galaxy. Why can't we see it?

Out in the burbs where I live, there are few lights and the sky is dark except to the southeast where the glow of the City looms above the ridge. Our view of the galaxy is much the same. Out here on the outskirts (27,000 lightyears out) there aren't many stars and the sky is dark. But when we look in toward the center our view is blocked by clouds of dust like smoke. Glowing beyond the obscuring dark clouds is the immense central bulge of the Milky Way, consisting of billions of stars. We know something's over there, but we can't see what it is. Now, after decades of trying, scientists are piercing the veil using radio telescopes that see through the smoke. The center turns out to be a very strange place. But don't consider moving there.

First, it's crowded. In our neighborhood there's about one star in every 275 cubic lightyears; the nearest is about four lightyears away. But at the center of the galaxy there are at least three hundred and fifty stars within one lightyear of the center. Assuming that we're only seeing the brightest stars of the cluster, it must be much more crowded than that. If Earth were circling one of those stars, there would be no night. There would be thousands of suns in the sky.

Second, it's windy. The innermost stars are bathed in a violent wind of gas streaming away from the center, travelling at one and half million miles per hour. Even at the distance from which we view them, many of the stars can be seen to have long tails behind them as the wind tears away their atmospheres. Any planets they originally had would have been eroded away to nothing by the wind.

Third, it's hot and noisy. Light, heat, radio, and x-rays pour out from the center, heating everything in the area and bathing it in hard radiation. The clouds of dust glow blue-white and the gas is ionized cherry pink.

Fourth, there's a monster in the neighborhood. The wind and the radiation are coming from a strange object at the very center called Sag A* (pronounced "saj ay-star"), the brightest radio emitter in the galaxy. Aside from being hot and bright, it is heavy - very heavy. Estimates of its mass range from five hundred to a million times the mass of our sun. It is the object around which the other two hundred billion stars orbit. The bad news is, it's inviting everybody in.

Sag A* is almost certainly a black hole, an object so massive that it pulls everything into itself, even light. The gas, dust, stars, and planets in its vicinity are pulled relentlessly by its overwhelming gravity. They spiral in toward it, forming a disk of infalling material around the it's equator, rather like Saturn's rings. As they fall, the incoming stars get squeezed by the object's gravity. Their temperatures rise to inconceivable levels as they are crushed to the size of a planet, a grape, a grain of sand. Their diameters decrease to infinitesimal size, and then beyond. They pass through the event horizon that surrounds the black hole, leaving our universe forever. The radiation we see at Sag A* does not come from the black hole itself, but from this accretion disk. The noise we hear on our radios is the high-pitched death screams of stars. Entire star systems are being crushed out of existence, many with planets, perhaps even the seats of ancient civilizations, with all their learning and their art and their history.

The thing about big black holes is that once you have one, it has you. You can't get rid of it, and it can only grow. As stars fall into it, it gets bigger and heavier and pulls the rest all the harder. So there's no stopping it. If Sag A* is a massive black hole, then it's growing. Given enough time, it will consume the entire galaxy.

So keep an eye on that pretty patch of steam above the teapot's spout, for there lies our doom. You're looking into the eye of the Eater of Stars.

copyright 1996 by Brian K. Crawford