Chandra seems to have confirmed this, spotting high concentrations of heavy metals -- the signature of a supernova remnant -- in the ring.

When the supernova ripped through space, the researchers believe, it shoved a bounty of heavy metals and other matter into the black hole, which would have become temporarily active, spewing X-rays and other high-energy radiation. The black hole would have settled down again once the shock wave moved past.

Tom Geballe of the Gemini Observatory, who was not involved in the study, called the supernova switch theory a promising explanation.

"Indeed the blasts from supernovae do evacuate large regions of space and could result in the temporary heavy feeding, and then subsequent starvation of a nearby black hole," Geballe said. "On the other hand, at present we do see a fairly intense radio signal from the vicinity of the black hole, which indicates that it still is eating something, though perhaps only a light snack."

Geballe also agreed that supernovae might be found to serve as universal triggers for black hole activity, but he said there could be other mechanisms.

"The black hole itself may be an on/off switch," Geballe suggested. "Violent activity around it and intense radiation from its vicinity, while it is feeding heavily, could result in the ejection of surrounding material and cause a subsequent period of starvation."

The idea of a black hole on a Weight Watchers diet might seem counterintuitive. Don't black holes suck everything inward?

Not exactly. Up close, a black hole is unquestionably powerful. Its gravity accelerates incoming matter to near the speed of light. Once inside, nothing gets out -- not even light.

But from far away, a black hole has no more gravitational effect than any other object of equal mass.

Beyond a certain sphere of influence, matter is not significantly affected by the gravity of a black hole. So to feed, a black hole needs gas, dust or stars to come its way.

Noting the position of the Milky Way's central black hole, and the location and size of the supernova remnant, Maeda's team figured the feeding frenzy could have been remarkably recent in terms of the billion years or so that it takes the average supermassive black hole to evolve.

"If the gas plowed by the supernova remnant was pushed past the black hole, the spectacular interaction would very possibly have occurred as recently as a few hundred years ago," said Mark Morris, a UCLA astronomy and physics professor who worked on the study.

The Chandra observations were made in September 1999 and discussed earlier this month at a meeting of the American Astronomical Society.