All Eyes on Eta Carinae

Artist's concept of Eta Carinae binary
An artist's concept of Eta Carinae as a binary star containing two hot, blue-white supergiants. In this proposed model, winds from the stars collide, forming an X-ray-emitting shock front (thick white arc at center) that wraps around the less powerful of the two stars.
Courtesy André Fonseca Silva.
If astronomers running one of the most complex observing campaigns ever attempted are correct, the wildly variable, far-southern star Eta Carinae is about to fulfill a tantalizing prediction: that it should nearly disappear to X-ray "eyes" after erratically brightening for several months.

Thus far it appears the predictions are correct, and intensive observations with everything from the Hubble Space Telescope to amateur equipment seem poised to write a new chapter in this famous star's history. But it may be some time before the experts agree on what their copious data really mean, says Ted Gull (NASA/Goddard Space Flight Center), who is focusing on ultraviolet spectroscopy with Hubble and the Far Ultraviolet Spectroscopic Explorer.

Eta Carinae inner nebula
This image of Eta Carinae, obtained last October by the Hubble Space Telescope's Advanced Camera for Surveys, shows not only the dusty Homunculus nebula, but dust-piercing ultraviolet light from the star itself. Though this image shows fine detail in a sky patch only about 20 arcseconds wide, it lacks the resolution needed to determine whether Eta Carinae is one star or two.
Courtesy the HST Treasury Team, STScI, and NASA.

Some 7,500 light-years from Earth, Eta Carinae is one of the most massive and luminous stars known. In the 1840s astronomers watched it undergo what University of Minnesota astrophysicist Kris Davidson, the lead scientist on the Hubble campaign, calls "the biggest explosion any star is known to have survived." Flaring from relative obscurity to become nearly the brightest star in the sky for many months, Eta Carinae shed several solar masses worth of material, which is now seen as the twin-globed "Homunculus" nebula (shown at right).

In the mid-1990s Brazilian astronomer Augusto Damineli (University of São Paolo) discovered that Eta Carinae's spectrum changes with clocklike precision every 5½ years. He predicted that the changes would next recur when 1997 drew to a close. They did; and the star's X-ray brightness simultaneously ramped up — only to crash abruptly.

The increasingly popular explanation is that Eta Carinae is two stars, not one, in an eccentric 5½-year orbit that somehow causes the source of the X-rays either to be eclipsed or to turn off when the two stars are closest together. This scenario makes a clear prediction: the late-1997 behavior should stage a repeat performance now.

The spectral changes documented in 1997 indeed are returning on schedule, says Damineli, and Eta Carinae's high-energy light curve (measured by the Rossi X-Ray Timing Explorer satellite) has obligingly brightened and declined in recent months. However, "there have been some surprises," admits Michael F. Corcoran (Universities Space Research Association), who is leading the X-ray observing campaign. In particular, though Eta Carinae's X-rays appeared to have peaked in late May, the star abruptly flared in mid-June. "It's not really clear what's causing that," Corcoran admits, though he believes that the star is still on track to bottom out this weekend.

Eta Carinae's X-rays presumably originate from million-degree gases in a shock front of some kind. The result of particles blasting into a gaseous medium at supersonic speeds, a shock front might form when two very massive stars approach one another and their fierce winds collide. However, Davidson stresses, a shock also could be generated "even if there is no companion star" — a slow, dense wind from a solitary star's equator could bump up against a faster, more rarefied wind from its poles.

For his part, Damineli is convinced that the clocklike repeatability of Eta Car's spectral shifts can be explained only by orbital motion. "There is no theoretical or observational basis to support a single luminous star," he told Sky & Telescope earlier this week. But the evidence for binarity, however compelling, remains indirect. Stronger signs will be sought in the spectra that Hubble, FUSE, RXTE, the Chandra X-Ray Observatory, the XMM-Newton X-ray satellite, the gamma-ray-sensing Integral spacecraft, and ground-based instruments will obtain in the coming days, weeks, and months. The answers will bear heavily on astronomers' understanding of how extremely massive stars form and evolve.

Want to know more? Read "Eta Carinae's Year of Glory" in the July 2003 issue of Sky & Telescope, now on newsstands.