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X-Rays Reveal Gigantic Star May Be Twins

Donald Savage
Headquarters, Washington, DC                       January 7, 1998
(Phone:  202/358-1547)                    EMBARGOED UNTIL 9 AM EST

William Steigerwald
Goddard Space Flight Center, Greenbelt, MD
(Phone:  301/286-5017)

RELEASE:  98-1

X-RAYS REVEAL GIGANTIC STAR MAY BE TWINS

     One of the Milky Way Galaxy's largest stars may in fact be a 
double star system, according to recent research by a team of 
astronomers using NASA's Rossi X-ray Timing Explorer (RXTE) 
spacecraft.  The team's results will be the subject of a 
presentation Jan. 7 in Washington, DC, at the winter meeting of 
the American Astronomical Society.

     The team, led by Dr. Michael Corcoran of the Universities 
Space Research Association, Columbia, MD, bases its conclusions on 
unusual variations in the intensity of X-rays emitted by hot gas 
near the star, called Eta Carinae, which is located about 7,500 
light years from Earth.  They believe that the variations are 
caused by the presence of a massive companion star in orbit around 
Eta Carinae.

     The new work offers insight into the origin and evolution of 
a class of stars called luminous blue variables, which are the 
most massive stars known.

     "Stars such as these shine so intensely that, sometimes, they 
become unstable and blow their outer layers off," said Corcoran.  
"That's what happened to Eta Carinae.  During the mid-1800's, it 
blasted an amount of material equivalent to the mass of our entire 
solar system into space.  The gas and dust that make up this 
material formed a shell that surrounds the star and now blocks it 
from direct view.  We have taken what amounts to an X-ray of this 
shell and found that what's inside may really be two stars."

     While using the Rossi Explorer to monitor the X-ray emission 
from Eta Carinae every week for a period of two years, the team 
found that X-rays emitted by hot gas near the star initially 
increased over a period of months and then rapidly diminished in 
intensity in a matter of days.  

     Such variability is highly unusual and has never before been 
observed for Eta Carinae.  The simplest explanation is that the 
variability of the X-ray emission is due to the presence of a 
massive stellar companion orbiting the star, bound to each other 
by the force of gravity.

     The presence of such a companion has recently been claimed 
based on variations in near-infrared and optical spectra by Dr. 
Augusto Damineli and collaborators at the University of Colorado 
at Boulder.  However, the presence of the "companion" star 
remained controversial, since the spectrum (a means of measuring 
the properties of objects by splitting their light into its 
component colors) of Eta Carinae is notoriously variable, and 
since the spectral features originate in a very complex medium.  
As a result, the "binary model" for Eta Carinae has not yet been 
generally accepted by the astronomical community.  The X-ray 
variations may help change this situation.

     "We believe the orbit of the companion star is elongated into 
an ellipse," Corcoran said, "which alternately moves it closer to 
and further away from Eta Carinae over the five-and-a-half year 
orbital period.  When the stars are close, the two stellar winds 
slam together, which creates a shock wave that heats the gas 
tremendously, to about 60 million degrees, and it emits large 
amounts of X-rays.  When they are further away, this shock wave 
diminishes, along with the X-rays.  This agrees pretty well with 
our RXTE X-ray measurements."

     The spacecraft's data may have helped close one mystery, but 
at the same time, they have opened another.

     "Strange peaks in the X-ray emission intensity seem to occur 
every 85 days," notes Kazunori Ishibashi of the University of 
Minnesota.  "While the first peaks detected were relatively weak, 
their strength has recently risen as the overall X-ray emission 
from Eta Carinae has brightened."

     "The most puzzling unknown is what causes the 85 day X-ray 
period," Davidson said.  "It may be the rotation of the star, or 
the star may pulsate in that time, or it might even be the orbit 
period of a third object in the system, a possibility that makes 
some astronomers uncomfortable."

     With at least 50 times more mass than the Sun, luminous blue 
variable stars like Eta Carinae are the most massive known.  If 
Eta Carinae is really a double star system, each is estimated to 
be 70 times more massive than the Sun, according to Damineli's 
binary star model.

                         - end -

Note to Editors: Images to support this story are available at the 
following Internet locations:
        FTP://PAO.GSFC.NASA.GOV/newsmedia/JAN_AAS/BH