RELEASE NO: 99-064

TROUBLE BREWING IN ETA CARINAE

Eta Carinae, a massive star famous for violent outbursts, has doubled in brightness since early 1998 and now shines more brilliantly than it has in over a century. Astronomers observing the star with the Space Telescope Imaging Spectrograph (STIS) onboard NASA’s Hubble Space Telescope report that a recently discovered inner, cone-shaped nebula has brightened considerably as a consequence. If the brightening continues, Eta Carinae may have begun a massive eruption like the one in the 1840's that created its stunning dumbbell shaped nebula.

"Eta Carinae is at its brightest in visible light since 1864," said Dr. Theodore Gull of NASA's Goddard Space Flight Center, Greenbelt, Md. "An inner cone-shaped nebula has tripled its brightness since we first discovered it in March 1998. There is a lot going on inside the larger Eta Carinae nebula, and we do not understand all of the activity at this point. Some of the brightness increase may be due to the excitation of gas in the nebula by light from Eta Carinae. However, we may also be witnessing the start of another massive eruption."

The observations will be the subject of a press conference June 2 at 11 a.m. EDT during the 100th Anniversary meeting of the American Astronomical Society in Chicago.

Eta Carinae is about 7,500 light-years from Earth and appears in the southern sky in the constellation Carina. At about 100 times the mass of the Sun, Eta Carinae is a rare luminous blue variable star, a class that includes the brightest and most massive stars in the universe. Luminous blue variables are prone to violent displays because of their instability - they shine so brightly that their gravity can hardly hold them together.

In the 1840's, Eta Carinae flared brightly and, over a 20 year period, ejected enough material to create three Suns. The result of this eruption can be seen today as a spectacular double-lobed nebula of gas and dust, lit from within by an intemperate star. The STIS researchers recently found a cone-shaped nebula inside of the larger nebula.

"Eta Carinae is unquestionably the most mysterious and powerful star that we can see with the unaided eye," says Professor Kris Davidson of the University of Minnesota. "We basically understand how all of the naked-eye visible stars work - except Eta Carinae. Its new brightness increase surprised us for a couple of reasons. First, we thought we had good reasons not to expect a major outburst in the next few decades. Second, when an eruption does occur, it shouldn't look like the recent behavior - the spectrum should change, and its color should get cooler. Both are contrary to the data," said Davidson.

The extremely high resolution of the STIS instrument made the discovery possible. "We could not separate the activity of Eta Carinae from its surrounding inner nebula without the very high resolution of the STIS instrument on Hubble. Its one tenth of an arc-second resolution is like viewing a baseball from across Lake Michigan. STIS showed us the new, cone shaped inner nebula as well as revealing the brightness increase before it was obvious to astronomers on the ground," said Gull.

Astronomers in Chile, South Africa and Australia confirmed the change in April, but the effect is less dramatic for them because ground-based telescopes cannot separate the star from the surrounding outer nebula, which is half a light year across," said Prof. Roberta Humphreys, also of University of Minnesota.

Eta Carinae is one of the brightest known luminous blue variables, a mysterious category of extremely massive, extremely powerful unstable stars. The explosion that ends the life of such a massive star is even more violent than a normal supernova explosion.

"Only one star in a billion is as massive as Eta Carinae. Some theorists speculate that such an object ends its life as a hypernova, not a supernova explosion, and may produce a gamma ray burster, which occurs about once every 100 million years for each galaxy. A gamma ray burster is such an cataclysmic event that it might even be hazardous at a distance of 7500 light years," said Davidson.

The research group is planning more observations of Eta Carinae in the next two years. "The STIS instrument combines high resolution imaging with the ability to get velocity information. We plan to use this powerful combination to build a three dimensional map of the Eta Carinae nebula, which will help us understand the beat within its fiery heart," said Gull.

Editor's note: Images to support this story are available on the internet at: http://www.gsfc.nasa.gov/ftp/newsmedia/AAS/ECAR