Plenty of Nothing

And nothing is plenty for some sky watchers.

Devoting a column that's supposed to be about what you can see in the night sky to something you can't see may seem to make no sense. But then, there's a lot about black holes that seems to make no sense--at first.

Although we tend to think of black holes as a purely post-Einsteinian conception, the credit for coming up with the idea belongs to eighteenth-century British geologist and astronomer John Michell, who used Newton's laws to deduce the possibility of what he called a "dark star"--a mass so substantial that not even light can escape its gravitational field. The idea first received widespread attention in French mathematician and astronomer Pierre-Simon de Laplace's wildly popular 1796 book Exposition du systeme du monde. But such speculation didn't begin to assume modern form until 1916, when the German astronomer Karl Schwarzschild seized upon the brand-new notion of relativity to produce his landmark paper "On the Gravitational Field of a Point Mass According to the Einsteinian Theory."

There the matter (or disappearance thereof) rested for the next half century: mathematically possible, even speculatively irresistible, but real? In 1967 U.S. physicist John Archibald Wheeler dubbed any such objects (should they exist) black holes, and throughout the 1960s and 1970s theorists worked out models for how these objects might reveal themselves. Since a black hole, by definition, emits no electromagnetic radiation, the only way to find one would be indirectly. This could be done by observing a signature gravitational effect on nearby visible objects or by monitoring through X-ray telescopes the "death scream" of gas that's been heated to millions of degrees as it plunges toward the event horizon--the black hole's ring of no return.

Still, observers remained understandably reluctant to come right out and say that something like Cygnus X-1--a binary star system consisting of one visible star rapidly orbiting one invisible companion that is radiating extraordinary levels of X rays--contained a black hole. As one astronomer said at a press conference on black holes just last summer, "It's taken us this long to convince ourselves they're real."

Not until May 6, 1994, did astronomers identify a black hole with relative certainty. It resided, they said, at the heart of M87, an elliptical galaxy 50 million light-years from Earth and previously best known for an unusual jet emanating from it, the first such extragalactic jet discovered (by American astronomer Heber Curtis in 1918). What convinced them of the existence of a black hole within M87 was the Hubble Space Telescope's detection of gas whipping around the core of the galaxy at 1.1 million miles per hour--meaning, according to their calculations, that whatever was driving this frenzy had to have a mass equivalent to 3 billion Suns. What's more, the mysterious jet, extending a distance of at least 5,000 light-years outside the galaxy, was probably fueled by that mass at M87's center. In the seven years since then, so many black holes have been detected that one astronomer, with only a little irony, has described identifying one as "even mundane."

Astronomers today no longer ask whether individual specimens might exist but how best to think about black holes collectively. A supermassive black hole--one with a mass equaling between 1 million and 3 billion Suns--may perhaps reside at the center of every large galaxy (including our own Milky Way, as observers demonstrated in 1998). Stellar-remnant black holes--those with a mass between three and thirty Suns--might number as many as 100 million per galaxy, including one situated only 1,600 light-years from Earth, as astronomers announced just last year. Stellar-remnant black holes are formed by the collapse of massive stars; as for supermassive black holes, nobody knows where they come from or why they seem to have a mass directly proportional to the size of their host galaxy.

But at least astronomers at long last are beginning to figure out what to ask about black holes, besides the obvious question of what happens to anything that goes into one (see "Universe" October 1995). So what's in all this for a sky watcher? Nothing, at least in terms of something to see. But in terms of something to think about--just as much the point for many amateurs studying the night sky--the answer is, Plenty. If the galaxies in the universe number 125 billion (according to the latest estimates, which are always subject to upward revision), and even if stellar-remnant black holes number "only" 50 million per galaxy, that would bring the universe's black hole population to well over 5 quintillion.

You can use binoculars or a telescope to find M87 in the Virgo cluster of galaxies, on the border between the Virgo and Coma Berenices constellations, which this month will be high in the southeastern sky after nightfall. And if, while you're staring up at the heavens, some neighborhood wit should ask the inevitable "What's up?" you can answer, "Nothing"--and then go on to explain just how much of it there is.