An Oversized Star Acts Up

James Glanz

CHICAGO--Is it a distress signal or just a boisterous how-do-you-do? A star called Eta Carinae is flashing a mysterious message, and theorists are struggling to make sense of it. Visible to the naked eye in the southern sky, Eta Carinae is one of the biggest stars known to astronomers; shrouded in gas and dust, it is perhaps 100 times the mass of the sun, and so bright that if it gets any brighter it should blow itself apart. But observations announced at a meeting of the American Astronomical Society here last week show that over the last 2 years, it has indeed brightened --by a factor of more than two--while remaining intact. And whereas such a star would be expected to expand and cool when it brightens, Eta Carinae has heated up.

 "Here's a very massive star doing some weird stuff," says Craig Wheeler, an astrophysicist at the University of Texas, Austin. So erratic is the behavior that some astronomers are speculating that, rather than being on the verge of blowing up because of its own brilliance, the mammoth star may be about to collapse, triggering an even bigger explosion called a supernova.

 Astronomers at the University of Minnesota and the NASA Goddard Space Flight Center in Greenbelt, Maryland, picked up the brightening in four measurements made by the orbiting Hubble Space Telescope's Imaging Spectrograph (STIS) between December of 1997 and February of 1999. The star, which Kris Davidson of Minnesota says "reminds us a little of a geyser," has acted up before. A tremendous eruption in the 1840s belched up several solar masses of material that formed a dumbbell-shaped cloud around the star, which astronomers call the Homunculus. That event, during which the entire structure was about 10 times as bright as it is now, was followed by a smaller burp in the 1890s and a gradual brightening in this century, probably because the central star is shining through more and more clearly as the Homunculus expands and its veil of gas and dust thins.

 But this time around the star itself has brightened. When the brightening turned up in the STIS spectra, "we started questioning, 'Is this real or is it an instrumental effect?' " says Goddard collaborator Theodore Gull. The team was reassured when they checked lower resolution images by other telescopes and discovered that ground-based astronomers had missed a smaller, but rapid, overall brightening, says Roberta Humphreys of the University of Minnesota.

 The brightening remains mysterious, however, because the star is thought to be very close to its "Eddington limit," where light exerts so much outward pressure that gravity is just barely able to hold the star together. So any further brightening should produce an outrush of material. But an expanding burst of gas--although still too small to be seen directly--would cool like gas rushing out of a spray can. The cooling would strengthen the star's infrared signal and turn down the ultraviolet. But the full STIS spectra showed just the opposite pattern.

 "One explanation is that the star got hotter" without changing size, says Humphreys --although theorists don't know how a star could do that. Whatever the cause, astronomers are wondering what comes next. Perhaps Eta Carinae is about to pop off as it did in the 1840s, or perhaps it is about to collapse and blow up as a supernova. Stars of that mass are also the conjectured progenitors of hypernovae--even larger explosions that might produce the cosmic blasts called gamma ray bursts. "It really is a Rosetta stone of some kind," says Wheeler. "We just don't know of what."

Volume 284, Number 5421 Issue of 11 Jun 1999, p 1748 
©1999 by The American Association for the Advancement of Science.