Don Savage
Headquarters, Washington, DC                   December 4, 1997
(Phone:  202/358-1547)

Tammy Jones
Goddard Space Flight Center, Greenbelt, MD
(Phone:  301/286-5566)

Ray Villard
Space Telescope Science Institute, Baltimore, MD
(Phone:  410/338-4514)

RELEASE:  96-279

ONE STAR'S LOSS IS ANOTHER'S GAIN:  HUBBLE 
CAPTURES BRIEF MOMENT IN LIFE OF LIVELY DUO

       Some stars in double-star systems have found a quick way to 
lose weight by dumping their extra pounds onto their companions.  
Astronomers using NASA's Hubble Space Telescope have discovered 
such a case in the double-star system Phi Persei.  A "rapid diet" 
program has trimmed an aging, once massive star to a lean one-
solar mass, while the once mild-mannered, moderate-sized companion 
has bulked up to a hefty nine-solar masses and is spinning so 
violently that it's flinging gas from its surface.  This 
observation has allowed astronomers to catch a glimpse of an 
unusual, fleeting moment in the life of a massive star in a 
double-star system. 

       "We have seen massive star binaries at the end of their 
lives, when one star has collapsed to become a neutron star," says 
Douglas Gies of the Center for High Angular Resolution Astronomy 
at Georgia State University, Atlanta.  "There are about six dozen 
of those known in our galaxy.  But what we hadn't seen before is 
the phase just prior to the collapse of the aging star.  The 
Hubble observations dramatically show how severely a star's 
material can be removed through the gravitational influence of a 
nearby companion."

       What Gies and his team have seen is an opportunistic star 
that has taken advantage of an aging, ailing partner.  After 
consuming most of its hydrogen -- the fuel that keeps its 
thermonuclear furnace running -- the aging star swelled up and 
began jettisoning its mass until only its bare core was left.  The 
companion star cannibalized the discarded material, thereby 
increasing in size.  Gies calls the stripped-down star a subdwarf, 
a type of aging star that has passed the expansion phase -- by 
swelling and puffing away its outer layers -- and is on its way to 
becoming a fading white dwarf.  Yet this aging, stripped-down 
star, which has the same mass as the Sun, is nine times hotter 
than the Sun at 95,000 degrees Fahrenheit and is very bright.

       The subdwarf would be the brightest object of its class in 
the sky (a sixth-magnitude star) if it could be seen alone.  If 
placed at the Sun's distance, it would appear 200 times brighter 
than the Sun.  However, the beefed-up companion is ten times 
brighter in visible light than the subdwarf, which is lost in its 
glare and eluded detection for many years.  

       The subdwarf was detected by the Hubble telescope's Goddard 
High Resolution Spectrograph (which was removed from Hubble last 
February during the second servicing mission), allowing scientists 
to identify its spectral signature.  Gies' analysis of these 
observations will appear in the Jan. 20, 1998, issue of the 
Astrophysical Journal.  With the Hubble data, he pieced together a 
better picture of life in this double-star system, especially how 
the beefed-up companion gained its extra mass.  

       By puffing away most of its mass, the aging, stripped-down 
star has given new life and a new identity to its companion.  The 
roughly 10-million-year-old companion has potentially doubled its 
lifetime because it has gained a vast amount of hydrogen fuel, 
which is needed to maintain its thermonuclear furnace.  By beefing 
up, the companion also has changed its identity from a normal, 
moderately massive star to a "Be" star, a type of hot star with a 
broad, flattened disk of hydrogen gas swirling around it, much 
like the rings of Saturn.  Based on measurements taken by 
astronomers at the U.S. Naval Observatory, the disk is eight times 
wider than the star.

       The disk formed from gas spun off the rapidly rotating "Be" 
star.  What causes the fast spin of "Be" stars has been a mystery 
to astronomers.  Now the Hubble telescope observations of Phi 
Persei offer at least a partial explanation:  The gas discarded 
from a nearby swelling star strikes the companion off-center, 
causing it to spin faster.

       "The companion is now rotating so fast (one million mph or 
450 kilometers per second at its equator) that the star is 
distorted into a flattened oblate figure in which gravity can 
barely maintain its hold on the star's outer layers," Gies 
explains.

       This new information about the stripped-down star and its 
companion leads scientists to speculate about their past.  Before 
the exchange of material, the stripped-down subdwarf was the more 
massive of the two, about six times more massive than the Sun.  
Its companion was slightly less bulky, about five solar masses. 
Such massive stars usually race through life at a faster pace than 
most stellar objects, ending their lives in one big supernova 
explosion.  However, stars in binary systems live differently.  
When the once massive subdwarf entered its twilight years about 
one million years ago, it swelled in size as it began using up its 
hydrogen fuel.  A single massive star would have eventually 
exploded, but the presence of the companion prevented the once-
massive star from suffering such a violent fate.  Instead, the 
once-massive star dumped most of its outer layers onto its 
companion, and now may be heading to a quiet demise. 

       A curious destiny may await this pair.  As long as the "Be" 
star doesn't break apart, it will live for another 10 million 
years because of the hydrogen fuel it acquired from its companion.  
Then it will swell during the expansion phase and possibly dump 
some of its mass back onto the subdwarf, which will have evolved 
into a white dwarf.  The subdwarf then might grow in mass and 
eventually explode as a supernova.  Or, the companion might swell 
up so much that it would engulf the white dwarf, eventually 
tossing out its material in mix-master action.

       "The beefed-up 'Be' star has won a new lease on life, but 
its ultimate fate will be determined by the corpse of its former 
companion, which remains in orbit uncomfortably nearby," Gies 
concludes.  Phi Persei is 720 light-years away in the 
constellation Perseus, visible in the autumn evening sky in the 
northern hemisphere, just north of the Andromeda Galaxy, M31.  The 
double-star system is visible as a fourth-magnitude star.

       The Space Telescope Science Institute is operated by the 
Association of Universities for Research in Astronomy, Inc. (AURA) 
for NASA, under contract with the Goddard Space Flight Center, 
Greenbelt, MD.  The Hubble Space Telescope is a project of 
international cooperation between NASA and the European Space 
Agency (ESA).

                       - end -

EDITORS NOTE:  A photo and caption are available via the World 
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Images are available at
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 http://oposite.stsci.edu/pubinfo/jpeg/phiper.jpg (JPEG).

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Higher resolution digital versions (300 dpi JPEG) of the release 
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