CHICAGO-- Astronomers are struggling to make sense of the unusual behavior of Eta Carinae, a star in the southern sky. Observations announced at a meeting of the American Astronomical Society here last week show that over the last 2 years, the star has brightened by a factor of more than two--even though it was already so bright that by now it should have blown itself apart. And while stars would be expected to expand and cool when they brighten, Eta Carinae has heated up. "Here's a very massive star doing some weird stuff," says astrophysicist Craig Wheeler of the University of Texas, Austin.
Eta Carinae, some 100 times more massive than the sun, 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 was followed by a smaller burp in the 1890s and a gradual brightening this century, probably because the central star is shining through more and more clearly as the Homunculus expanded and its veil of gas and dust thinned.
But this time the star itself has brightened, according to astronomers at the University of Minnesota, Twin Cities, and the NASA Goddard Space Flight Center in Greenbelt, Maryland, who watched Eta Carinae using a spectrograph aboard the Hubble Space Telescope between December of 1997 and February of 1999. The brightening is mysterious because the star is thought to be very close to its "Eddington limit," where light exerts so much outward pressure that gravity can barely hold the star together. So any further brightening should produce an outrush of material. But an expanding burst of gas--though 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 Hubble spectra showed just the opposite pattern.
Baffled by the cause of what they see, 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 Eta Carinae's mass are 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."