The mysterious quasars that have baffled scientists ever since their discovery in 1963 may be black holes so massive they contain the equivalent of 1 billion suns.

"There is a consensus now emerging that the quasars we've seen are supermassive black holes at the centers of galaxies," Dr. Martin J. Rees of the Institute of Astronomy at Cambridge University in England told the annual meeting of the American Association for the Advancement of Science yesterday.

Typically, each of the more than 2,000 quasars discovered so far matches the annual energy output of the sun in just a few seconds. Most quasars outshine the galaxies in which they're found and in 1975 one quasar had an increase in luminosity in one week that was equal to 10,000 stars being turned on at once.

Astronomers have long thought that quasars were due to some new phenomenon that could not be explained by classical physics, but Rees said yesterday that the number of astronomers who still believe that is rapidly dwindling.

"My own view is that of a reluctant conservative," Rees said. "I had hoped that a fundamentally new phenomenon could explain quasars but more and more I am accepting the orthodox view that quasars can be explained by conventional physics and Einstein's Theory of Relativity."

The way Rees sees it, quasars were born when clusters of stars died out early in time and collapsed together to form an enormous black hole, equal in mass to anywhere from 100 million to 1 billion suns. As much mass as they have, these "supermassive" black holes are so tightly condensed that they cover no more of the sky than our own solar system does.

"Even this makes these supermassive objects bigger than conventional black holes," Rees went on. "Most black holes are no wider across than 50 kilometers."

Why do quasars shine so brightly if they are black holes and black holes are invisible? Rees pointed out that many quasars are at the centers of galaxies and seem physically linked to these galaxies. Rees said that the quasars are almost surely sucking gas and even entire stars out of their galaxies into themselves as whirlpools draw debris into their wells.

"By the time this gas falls into the well it is moving near the speed of light and has been heated to enormous temperatures," Rees said. "If only 10 percent of this heat was radiated away as light, it would be enough to account for the luminosities we see from quasars."

While only a little more than 2,000 quasars have been found in the cosmos, Rees said they could be common to every one of the millions of galaxies in the heavens. Rees said he believes that quasars have relatively short lifetimes and that most are now dead, having exhausted their fuel because their galaxies were swept clean of gas or because the quasars themselves gobbled up every nearby star in their galaxies.

"Most quasars that have ever been died out long ago," Rees said. "It is possible that the universe is littered with the corpses of dead quasars."

Though a few quasars have been found in nearby galaxies, most are so deep in space that they lie beyond most of the observable universe.

"What we are seeing are objects that were born as long as 10 billion years ago," Rees said. "We are looking backwards in time through four-fifths of the time since the Big Bang created the cosmos."