Massive black holes let there be light.

Stars in the night, blazing their light, can't hold a candle to ... a black hole?

The darkest objects in the heavens may produce a sizable fraction of the light in the cosmos. Two new studies suggest that supermassive black holes, the invisible monsters believed to lurk at the hearts of many galaxies, generate at least 5 percent and perhaps as much as half of all the radiation in the universe.

Because of its huge gravity, a black hole swallows everything around it, including light. Just before the point of no return, called the event horizon, gas spiraling toward the hole attains high velocities and enormous temperatures. Before this hot, high-speed gas accretes onto the hole and vanishes forever, it emits a swan song--intense radiation ranging from visible light to X rays.

This light, researchers say, may illuminate much of the cosmos. Quasars, the brilliant beacons that supermassive black holes create at the core of some galaxies, are thought to shine in this way. Although astronomers have suspected for more than 20 years that black holes make a substantial contribution to heavenly light, "the data are better now, so one can ask sharper questions" and refine estimates, notes E. Sterl Phinney of the California Institute of Technology in Pasadena.

To calculate how much radiation is generated around supermassive black holes, Phinney homed in on patches of sky containing extensively studied quasars and other, more-muted fireworks believed to be powered by black holes. He then estimated how much light in these patches comes from stars and how much from accretion onto black holes. Light from the two sources can be distinguished because quasars appear more pointlike, Phinney notes.

Assuming that the sources of light in the observed regions are representative of the entire universe, Phinney estimates that black hole accretion accounts for 5 to 20 percent of the light in the cosmos. Stars would produce the remaining light. He reported the finding earlier this month at a workshop on black holes at the European Southern Observatory in Garching, Germany.

In another study, Andrew C. Fabian of the University of Cambridge in England estimates the contribution of black holes to cosmic radiation based on the average density of supermassive black holes near our galaxy and the intensity of the sky's X-ray background. Neither stars nor the known population of quasars can account for the background, and Fabian argues that it's produced by a vast population of quasars that have not yet been detected because they lie behind a veil of dust. "For every ordinary quasar, about 10 more obscured ones are needed," he says.

According to this model, 10 to 50 percent of the light in the universe comes from accretion onto black holes, he reported in mid-September at a meeting on X-ray astronomy in Bologna, Italy. Astronomers had previously come up with estimates of only a few percent because they based their calculations on the relatively few quasars seen in visible light, Fabian says.

Looking for hidden quasars isn't easy. Dust absorbs both ultraviolet and visible light and reradiates it as far-infrared radiation. Astronomers, however, have few high-resolution telescopes to detect this wavelength band. NASA's Space Infrared Telescope Facility, scheduled for launch in 2001, should provide "the ultimate check on Fabian's model," Phinney says.

Aside from illuminating the universe, a population of previously unknown quasars could complicate galaxy formation, Fabian speculates. For example, quasar winds might blow gas out of young galaxies. Such winds might also hinder the growth of the slender arms and disks of spiral galaxies, notes Roger D. Blandford of Caltech.