Eta Carinae, a star about 8500 light years from earth, is thought to be the most massive and luminous star in our Galaxy. The star contains about 100 times the mass of the sun and generates about 6 million times the sun's light output. For hundreds of years it was one of the brighter stars in the sky. In the mid-1800's the star exploded and for a time became the second brightest star in the sky. During this explosion Eta Carinae ejected into space more matter than is contained in our entire solar system. Today, this debris can be seen as a dark cloud, called the homunculus, surrounding the star. The homunculus blocks much of Eta Carinae's starlight, making the star appear faint to the naked eye. But most of the absorbed light is re-radiated into space as heat. The homunculus absorbs so much energy from Eta Carinae that it presently provides more heat to earth than any other astronomical object outside the solar system.
Eta Carinae is also a strong source of X-radiation, produced by gas at extremely high temperatures. The ROSAT images of Eta Carinae show that much of this gas is distributed around the star in a huge shell about 4,000 times as large as our solar system. The ROSAT observations indicate that the temperature of the gas in this shell is about 2 million degrees - that is, about 100 times the surface temperature of the star. How can so much material be heated to such high temperatures? Astronomers think that collisions between matter ejected from Eta Carinae and stationary material surrounding the star play the key role. "We know that matter in the homunculus is moving away from Eta Carinae at tremendous velocities as a result of the explosion of the star in the last century", Corcoran said. "As the debris in the homunculus collides with gas and dust in the surrounding environment, the speed of the debris is converted into energy which heats the gas. The 2 million degree shell of gas which we see in the ROSAT images suggests that the material in the outer part of the homunculus is traveling at speeds of about 200,000 miles per hour when the collision takes place."
ROSAT also revealed the presence of a point-like source of much hotter gas very close to Eta Carinae. The presence of such a source was inferred using data obtained with the Einstein X-ray observatory in the 1980's, but ROSAT provides the first unambiguous images of this hot source. The temperature of this central source is at least 20 million degrees. If produced by a collision, this extremely high temperature implies that the speed of the colliding material is an amazing 6 million miles per hour - about 1% of the speed of light! Although such speeds seem incredible, gas velocities in this range have in fact been observed near Eta Carinae (and other massive stars as well). Massive stars like Eta Carinae possess strong, fast "stellar winds" produced as radiation from the surface of the star drives portions of its outer atmosphere into space. Measured wind velocities from other massive stars show that stellar winds can indeed achieve speeds of a few million miles per hour. If Eta Carinae possesses such a wind, then the collision of this wind with stationary dust in a portion of the homunculus near the star could produce the observed X-rays.
The most unexpected result of the ROSAT observations is the discovery that, in the 4 month interval from August 1992 to December 1992, this hot central source doubled its output of X-rays. Although X-rays from Eta Carinae have been observed since the 1960's, this is the first time such a dramatic X-ray variation has been reported. "At present it's unclear whether this is the first time the star's X-ray emissions ever varied, or whether previous instruments simply were not sensitive to previous changes, or were not looking at the star at the right time", said Corcoran. "ROSAT has sufficient sensitivity to make this observation, and ROSAT has been monitoring this star on and off for about five years. This longevity is crucial." Subsequent observations of Eta Carinae by ROSAT in June 1994 showed that the central source was bright at that time, although it seemed to have faded somewhat from the peak seen in December 1992.
The physical cause of the X-ray variability cannot be pinned down precisely because the brightening has only been seen once. Corcoran and his colleagues pointed out that, if the hot central source is produced by a collision between a fast stellar wind and stationary dust, then the observed variation could mean a change by a factor of three in the amount of material in the stellar wind. If this idea is correct, then Eta Carinae must have been undergoing a period of increased mass ejection. However, the observed X-ray change could also be produced if the temperature of the hot central source changed, or if X-ray absorbing material moved in front of the hot source. Intriguingly, other astronomers using radio and optical telescopes have also reported variations which occurred at roughly the same time as the X-ray change. All this evidence could mean that Eta Carinae is entering a period of renewed activity, perhaps a precursor to more violent activity like the explosion which rocked the star in the mid-1800's. Other explanations are possible, cautions Corcoran, and it's too early to rule anything out. "We don't know if Eta Carinae is truly a unique object or if it's in a short-lived evolutionary stage that all massive stars eventually go through. That's the bottom line. Continued monitoring with ROSAT and detailed observations with other telescopes should go a long way towards solving this 150 year-old mystery."
For More Information, Please Contact: Dr. Michael F. Corcoran corcoran@barnegat.gsfc.nasa.gov Dr. Jean H. Swank swank@lheavx.gsfc.nasa.gov Dr. Robert Petre petre@lheavx.gsfc.nasa.govFigure Captions:
FOR RELEASE: 9:20 a.m. EDT, TUESDAY, JUNE 13, 1995
"A set of low resolution, false color X-ray images of Eta Carinae obtained by ROSAT. The top 2 images show the X-ray emission from million-degree gas which surrounds Eta Carinae in a large oval shell. The image on the left was obtained in June of 1992 and the image on the right in December 1992. The lower 2 images show X-rays from gas at temperatures about 10-100 times hotter than the gas in the shell. This hot source was relatively faint in June of 1992 but was considerably brighter in the December 1992 observation. This material was presented to the American Astronomical Society meeting in Pittsburgh, PA on June 13, 1995." PHOTO CREDIT: M. F. Corcoran, Universities Space Research Association.
"High-resolution false color X-ray images of Eta Carinae from 1992 (left) and 1994 (right). X-rays are emitted from hot gas in an oval shell around Eta Carinae. A bright source of X-rays at the center of the shell shell is apparent in the 1994 image but not in the 1992 image. The bright source to the top of each image is an X-ray emitting massive star close to Eta Carinae. This material was presented to the American Astronomical Society meeting in Pittsburgh, PA on June 13, 1995." PHOTO CREDIT: M. F. Corcoran, Universities Space Research Association.
