All eyes on Eta Carinae: a new spectacle?

One of the most massive, luminous, and unstable stars in the heavens, Eta Carinae has been a showstopper since the 1840s, when astronomers watched this behemoth undergo a giant explosion. A new study suggests that in January the star could put on another stellar light show.

Astronomers are gearing up for what could be the largest campaign of ground-based and space observations ever devoted to a single star. The chances are slim that next month's predicted spectacle will reprise Eta Carinae's bravura performance of 150 years ago, when it hurled into space a pair of ballooning, mirror-image gas clouds. Nevertheless, by taking multiwavelength portraits of Eta Carinae at a time when its X-ray output--and perhaps other emissions--are likely to be at a peak, observers hope to answer such basic questions as whether this hour-glass-shaped object is actually two stars locked in a gravitational embrace.

The data take on added significance because Eta Carinae is one of the most likely stars in our galaxy to explode as a supernova, notes Michael F. Corcoran of NASA's Goddard Space Flight Center in Greenbelt, Md. "When you're studying Eta Carinae, you're studying a star just before it becomes so unstable that it blows itself to bits," he says.

Since early 1996, Corcoran and his colleagues have used a NASA craft, the Rossi X-ray Timing Explorer, to monitor Eta Carinae weekly. They report in the Dec. 11 Nature that radiation from the star exhibits two previously unsuspected patterns. Every 85 days, Eta Carinae undergoes small-scale outbursts at X-ray energies. This activity is superimposed on another, more intriguing trend: an increase in the intensity of high-energy emissions, 3,000 to 10,000 electronvolts, which has accelerated since last January. That increase is expected to peak next month.

Earlier work hinted that the rise in high-energy X rays is periodic, peaking every 5.5 years--exactly in synch with a previously detected minimum in the intensity of near-infrared emissions from helium atoms in Eta Carinae. These periodicities suggest that Eta Carinae may in fact be two stars that orbit each other every 5.5 years. The two members of this stellar system would make their closest approach in January.

In this scenario, the X rays emitted by Eta Carinae are generated when fierce winds from the two stars collide. As the stars approach each other, the radiation should intensify.

Mario Livio of the Space Telescope Science Institute in Baltimore suggests another test of the two-star model. Ultraviolet emissions should sharply decline as one star blocks the other just before their closest approach. Other forms of radiation may also vary.

Even if Eta Carinae is a single star, the X-ray peak may provide a field day for the four X-ray telescopes now in space, says Livio. There remains an outside chance, he and other astronomers note, that the rise in X-ray emissions may trigger a giant explosion. The 1840 outburst, notes Corcoran, coincided with what would have been a peak in X rays.

If there are fireworks, a slew of telescopes will be poised to record them.