Claire, Here's information from anothe site: http://lheawww.gsfc.nasa.gov/users/corcoran/eta_car/eta_car.html Extremely massive stars are key astronomical objects, as they play a role in chemical enrichment and galactic evolution. They mark the end of their stellar lives as supernovae explosions in which a single supernova can equal the entire radiant output of a galaxy of a billion stars. Recently the extreme members of this class have been suggested by Paczynski (199 to produce the "hypernovae" which might explain the bursts of gamma radiation which have been an astronomical mystery for 30 years. The energy emitted in a "hypernova" is astounding; perhaps the equivalent to the radiant energy output of an entire universe of galaxies. Such extraordinary explosions require stellar precursors of unusually large mass, and so should be relatively rare. Alarmingly, the Milky Way possesses one possible member of this putative class, the massive, luminous, and relatively nearby star, Eta Carinae. Eta Carinae is both an extremely massive star and an extremely unstable one. It's notorious for erupting in the mid-19th century, and is surrounded by the ejecta from this eruption. The Hubble Space Telescope has an impressive image of Eta Car and this ejecta . The entire field is beautiful at X-ray energies, as you can see from this ROSAT High Resolution Imager image. For comparison, here's the same field in the optical. Recent observations in the optical and near IR by Augusto Damineli and co-workers suggest that certain emission lines vary periodically with a cycle of 5.52 years. The exact cause of this periodicity is not really known, but the most likely explanation is that the period reflects the orbital motion of one star around another star: Eta Car may be two stars, not one!. If Eta Car really is a binary system, then our understanding of the evolution of the star and the nature of its instabilities will need to be radically revised. So this is a crucial question: Is the star really a binary, or could single-star models be found to explain the spectral variations? X-ray observations can help resolve this important issue. Eta Car is a strong X-ray source. X-rays are produced as the ejecta expands into the circumstellar medium near the star at speeds of 100-1000 km/s. There is also a mysterious point-like source of hard emission centered on the star itself. Recent X-ray observations with the ROSAT PSPC and HRI show that the X-ray emission from this point-like source varies by a factor of 3 on timescales of months (Corcoran et al., 1995, ApJ, 445, L121, or see the press release). And my favorite photo of Eta Carinae: http://lheawww.gsfc.nasa.gov/users/corcoran/eta_car/ETACAR_wfpc2.gif Bill
