Fooled by gravity.

Four years ago, astronomers heralded the discovery of the brightest object in the heavens. A galaxy dubbed FSC 10214+4724, it appeared to shine 100 trillion times as brightly as the sun (SN: 6/29/91, p.406).

But now the spotlight on this distant body has dimmed.

Independent observations from the W.M. Keck Telescope atop Hawaii's Mauna Kea and the Hubble Space Telescope reveal that a cosmic mirage led astronomers astray. The new images indicate that an object in the foreground acts as a gravitational lens for light emitted by FSC 10214+4724, stretching it into the shape of an arc and brightening it considerably. In fact, the galaxy may have only one-tenth to one-thirtieth the brightness originally ascribed to it.

"Its identification as the most luminous galaxy in the universe was an astronomical blooper," says Keck observer James R. Graham of the University of California, Berkeley.

He and his Berkeley colleague Michael C. Liu used the telescope last March to take a detailed near-infrared portrait of the bright body. A lower-resolution Keck image, taken 2 years earlier by other researchers, had already shown the infrared-bright galaxy to have a suspiciously curved appearance. But scientists thought the arclike shape signified the tug of a neighboring galaxy, not an optical illusion.

The new, sharper image, however, looks like a textbook example of a gravitational lens, Liu and Graham assert. They note, for example, that the object believed to be a massive foreground galaxy lies at the center of the arc's curve--just where a lensing galaxy should reside.

The team subtracted the presumed lensing galaxy from the Keck image and found a faint mirror image of the arc on the opposite side of the galaxy. Theory predicts that such a pair of images should straddle the lensing galaxy.

The researchers say they can't confirm the lensing scenario until observations establish whether the presumed foreground object indeed lies in front of FSC 10214+4724 and at the right distance from Earth. In an earlier study, Peter R. Eisenhardt of NASA's Jet Propulsion Laboratory in Pasadena, Calif., and his colleagues report that images of even higher resolution recently taken with the Hubble Space Telescope offer compelling evidence that the bright galaxy is indeed lensed.

"I have to say that it looks very plausible that this is a gravitational lens," says Michael Rowan-Robinson of Imperial College in London. Robinson accidentally discovered the luminous galaxy during a follow-up study of objects surveyed a decade ago by the Infrared Astronomical Satellite. The adjusted brightness of FSC 10214+4724 would match that of several infrared- bright galaxies in our vicinity that seem to be evolving into ellipticals. Liu and Graham say that the more distant galaxy may rank as one of the earliest examples of a fledgling elliptical. In another intriguing Keck study, Joseph S. Miller of the University of California, Santa Cruz, and his collaborators have found evidence of a quasar at the heart of the galaxy.

"Although it's not the most luminous object, it still may inhabit a special place in the evolution of galaxies," comments Robinson.