Eta Carinae, an enigmatic star that has mystified astronomers for 160 years with both its sheer beauty and massive size, is repeating a roughly five-year cycle of pumping out X rays. This finding supports the theory that Eta Carinae may in fact be two stars with clashing stellar winds generating varying amounts of X rays as the stars come closer to each other during a five-year orbit.
These results are presented today at the 201st meeting of the American Astronomical Society in Seattle, Wash.
The team, led by Dr. Michael Corcoran, a researcher at NASA’s Goddard Space Flight Center in Greenbelt, Md., originally proposed a two-star theory five years ago, at the 1998 winter meeting of the AAS, to explain newly uncovered X-ray variations. The scientists have now seen Eta Carinae repeating its cycle, bolstering the initial interpretation.
“If Eta Carinae is in fact two stars, the X-ray output should vary with the orbital period of a smaller star revolving around a larger star in each cycle, and this is in fact what we see,” Corcoran said.
Corcoran and his colleagues have tracked X-ray emission from Eta Carinae for the past decade, initially with the Roentgen Satellite (ROSAT) and since 1996 with NASA’s Rossi X-ray Timing Explorer (RXTE). Surprisingly, they have also found that Eta Carinae is emitting more X rays during this particular cycle than last time, even as it emits less of other types of electromagnetic radiation. This may be due to differing amounts of stellar material being blown off of each star during the 5.52-year cycle, Corcoran suggests.
Eta Carinae — found in Southern Hemisphere skies in the constellation Carina about 7,500 light years from Earth, visible with a backyard telescope — is one of the most massive and intrinsically brightest stars in the galaxy. Eta Carinae is at least 100 times as massive as the Sun, which challenges theories concerning the upper limit of stellar mass. (One light year is almost six trillion miles, or about 9.5 trillion kilometers.)
During its “Great Eruption” from 1838 to 1858, Eta Carinae was so bright that at times it rivaled Sirius, the brightest star in the sky. Eta Carinae ejected large amounts of stellar material during this eruption that has subsequently formed a double or “bipolar” nebula stretching over a half light year across, resembling a dividing cell. Since then, Eta Carinae has been a laboratory for studying the final stages of the life of a stellar giant.
“How massive can the most massive stars be, and how do such massive stars form and evolve? This is an unsolved problem and one in which Eta Car plays the starring role,” Corcoran said. “Stars as massive as Eta Car are almost impossible to form in isolation, and may need to be built up gradually from the merger of many less massive stars.”
Brazilian astronomer Dr. Augusto Damineli proposed a two-star hypothesis in 1996 to account for what then appeared as a 5.52-year cycle in the ionization state — that is, a cycle of atoms in the region losing their electrons. Supporters of the “one star” theory, however, say that Eta Carinae’s unprecedented mass could explain its perplexing emission.
Corcoran’s observations show that the X-ray emissions from the star also follow this ionization cycle, although with strong variations. Eta Carinae’s X rays, Corcoran says, could be produced by the collision of stellar winds in which a smaller companion star with fast, light winds orbits around a much more massive star with a slower wind. Calculations indicate the theorized companion star is 30 times more massive than the Sun while the primary star is fully 80 times more massive and 5 million times as luminous as the Sun. The 5.52-year orbit would bring the stars as close as five times the Earth-Sun distance and as far away as 30 times the Earth-Sun distance.
Eta Carinae’s last peak in X-ray emission was in November 1997. A large group of astronomers have planned an unprecedented campaign to observe Eta Carinae during the next X-ray peak in the summer of 2003. This campaign involves near simultaneous observations with radio, infrared, visible light, ultraviolet, X-ray and gamma-ray observatories — including observations with the Hubble Space Telescope and Chandra X-ray Observatory – which would make this one of the best observed astronomical events in history.
The Rossi Explorer, launched in 1995, is operated by NASA Goddard. Corcoran joins Goddard as part of the Cooperative Program in Space Sciences, managed for NASA by the Universities Space Research Association (USRA). For images of Eta Carinae, refer to:
