One of the brightest and most spectacular stellar objects in the Milky Way, known as eta Carinae, is really two stars, making it the most massive orbiting binary star system ever discovered.

Augusto Damineli, a visiting fellow at the University of Colorado at Boulder, said the type of changes in eta Carinae's spectrum last seen in mid-1992 were predicted to recur in cycles of 5.5 years. They began again, right on schedule, in December 1997, confirming the precise periodicity of the event.

"This indicates the object likely is two orbiting stars rather than a single star," Daminelli said.

The two stars each weigh about 70 times as much as the sun and revolve around each other in a highly eccentric orbit at a distance greater than that from the sun to Saturn once every 5.5 years, according to calculations by Damineli and CU-Boulder astronomy professor and JILA Fellow Peter Conti.

Spectral changes such as fading emission lines of elements like helium and argonium, and large variations in the flux of x-rays and radio waves occur when the stars make their closest pass to one another.

"The key to this mystery is that these changes are predictable like a clock," Damineli said. "We cannot predict the behavior of a single luminous star with clock-like accuracy. The only way to explain the precise timetable of the spectral changes we see i n eta Carinae is through the mechanical orbit of a companion star."

A paper by Damineli, co-authored by astronomers Otmar Stahl and Andreas Kaufer of Germany's Heidelberg University and Dalton Lopes of Brazil's National Observatory, was presented at the American Astronomical Society's nat ional meeting in Washington, D.C. today (Jan. 8).

Eta Carinae is located about 7,500 light-years from Earth, said Damineli, whose team has made most of their observations of eta Carinae over the years from the National Laboratory of Astrophysics in Itajuba, Brazil and the European Southern Observatory, a series of astronomical facilities in Chile.

An astronomy professor at Brazil's Sao Paulo University, Damineli is currently a visiting fellow at JILA. Headquartered at CU-Boulder, JILA is a joint institute of CU and the National Institute of Standards and Technology.

Eta Carinae has been of great interest to astronomers since it erupted about 150 years ago with the power of a supernova, becoming the second brightest star in the sky after Sirius. "But eta Carinae survived the event and continues to release about 5 mill ion times more radiative power than our sun, which remains a big mystery to astronomers and physicists," Damineli said.

Damineli and his colleagues determined the elliptical orbit of the two stars by sudden changes in eta Carinae's velocity as recorded by changes in spectral light called the Doppler shift. "The star makes a very fast turn for about a month at the top of th e elliptical orbit, then slows again," he said. "When we see fast turns like this, we know we are seeing stars in elliptical orbits."

The two stars pass within about twice the distance from the Earth to the sun, although the brightness of the companion star is indistinguishable from eta Carinae's brightness, he said.

When the stars make their closest pass, winds from eta Carinae blowing at more than 300 miles per second collide with companion star winds of about 700 miles per second, increasing the ejection of x-rays and obscuring the emissions of colliding gases to observers.

Damineli speculated that the eruption of eta Carinae in 1843 may have been caused by huge "tidal waves" in the surfaces of the stars. At that time, the radius of eta Carinae increased by 10 times, causing the smaller star to dive into eta Carinae's atmosp here.

The interaction is likely to have triggered the explosion of eta Carinae and shaped the bipolar lobes and equatorial disk now visible in the ejected nebula. [Contact: Augusto Damineli]

8-Jan-1998