All galaxies that have central bulges are now believed to have black holes at their cores, our Milky Way included. Indeed, a very dark mass weighing about 2.6 million Suns lies at the Milky Way's exact center, revealing itself by its gravitational effects on surroundings stars. A supermassive black hole is the obvious candidate.

But it's not the only possible candidate. Some astronomers have considered what else might fit the evidence. A massive cluster of dark stars could conceivably do, or maybe a pool of neutrinos, or a gathering of some even more exotic kind of dark matter.

One puzzle that has prompted such speculation is the object's curious darkness and silence, compared to the wildly energetic turmoil surrounding the central black holes in many galaxies. The object's position is marked only by the weak radio source Sagittarius A* ("A-star") and a feeble X-ray glow discovered by the Chandra X-Ray Observatory in 1999. If it's a black hole, it's either consuming surprisingly little matter considering the richness of the stuff in the galaxy's center, or it's swallowing matter so efficiently that hardly a burp of energy escapes from vanishing down the hole.

Now all such alternative theories can be tossed out the window, announce Frederick Baganoff (MIT) and 10 colleagues in today's issue of Nature. It's a black hole after all.

The smoking gun turned up in a new Chandra observation. The supersensitive satellite found X-rays from Sagittarius A* varying drastically in as little as 10 minutes. In that short a time, light (and X-rays) travels hardly more than 150 million kilometers, the distance from the Earth to the Sun (1 astronomical unit). Therefore the X-ray source can't be much larger than that. According to general relativity, anything that small with 2.6 million solar masses must quickly collapse to become a black hole.