Paula Cleggett-Haleim
Headquarters, Washington, D.C.                                                         
June 9, 1993
(Phone:  202/358-0883)                                   EMBARGOED 
UNTIL 1:00 p.m. EDT 

Jim Elliott
Goddard Space Flight Center, Greenbelt, Md.
(Phone:  301/286-6256) 

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

RELEASE:  93-109

HUBBLE FINDS EVIDENCE OF STELLAR CLOSE ENCOUNTERS

     NASA's Hubble Space Telescope (HST) has discovered a group of 
stars that apparently have been canabalized of their cooler outer 
gas layers by other passing stars, resulting in stellar "naked 
cores" with surface temperatures five times hotter than Earth's 
sun.

     "This is amazing.  These objects represent a totally new 
population of very blue stars," says Guido De Marchi, of the Space 
Telescope Science Institute (STScI), Baltimore, Md., and the 
University of Florence, Italy.  "When we started wondering what 
they could be, we realized that they may be among the first 
observed stars to have been stripped." 

     The stars are located deep in the core of M15, one of the 
densest globular clusters known.  A globular cluster is a "beehive 
swarm" of several hundred thousand stars held together by each 
other's gravity.  If the cluster is exceptionally dense, chances 
are increased for close encounters of stars, in which bodies with 
strong gravitational attraction could strip other stars of their 
outer material.

     "If our planet were there, we would see 100,000 stars closer 
than Proxima Centauri, the closest star to Earth's sun," said De 
Marchi.  "The night sky would look simply fantastic."

     De Marchi and Dr. Francesco Paresce of the European Space 
Agency, explain that this could only have happened if stars are so 
crowded together in the core they can be stripped of much of their 
gaseous envelopes by the gravitational pull of bypassing stars. 

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     This stellar cannibalism could only take place where stars 
are so crowded together that chances for close encounters are 
exceptionally high, they said.  De Marchi and Paresce interpret 
the existence of this new class of stars as possible evidence that 
the center of the globular cluster has contracted to an extremely 
dense condition called "core collapse."  

     This research by De Marchi and Paresce is being announced at 
a press briefing today at the meeting of the American Astronomical 
Society in Berkeley, Calif. 

     The astronomers were surprised to discover about 15 hot blue 
stars segregated at the very core of M15.  Their surface 
temperatures are above 60,000 degrees Fahrenheit (the sun's 
surface is 11,000 degrees Fahrenheit).   

     This discovery was possible only with the Hubble Space 
Telescope because it can resolve stars at the dense core of M15 
that are only a blur from the ground.  The observations also 
required Hubble's sensitivity to ultraviolet light to distinguish 
the hot stars from the surrounding cooler stars. 

     Such very hot blue stars can be made in several ways besides 
stellar stripping, such as magnetically stirred-up super massive 
stars, white dwarfs, or planetary nebulae.  However, the 
researchers are quick to point out that none of these scenarios 
explain why the stars are so concentrated and so numerous only at  
M15's core. 

     "This rules out a number of other hypotheses," says De 
Marchi.  He explains that all the blue stars lie within a 1 
light-year radius at the very core of the cluster.  What's more, 
90 percent of them are concentrated at the very center of this 
volume, within a 4/10th light-year radius. 

Close Encounters Of The Stellar Kind 

     According to this scenario, the new population of blue stars 
was once the cores of red giant stars.  Such stars expand to 
enormous sizes late in their lives, due to changes in the nuclear 
"burning" at their cores.  If the sun were the size of a red giant 
it would engulf the inner solar system out to the diameter of 
Mars' orbit.

     Red giant stars are so distended that they have a weak 
gravitational hold on their outer envelope of cool gas.  If a 
normal main sequence star passes within a few stellar radii it can 
rob gas from the red giant.  This stripping process can, in 
theory, expose a star's core -- the nuclear fusion "engine" that 
powers stars. 

     However, conditions where stars are so crammed together are 
unusual.  For example, in the Earth's stellar neighborhood the 
stars are typically a million times farther apart than the 
distance between the sun and Earth.  

     Conversely, due to the relentless pull of gravity, the stars 
at the core of M15 have converged so that they are at about 500 
times the distance between the Earth and the sun.
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     The astronomers used Hubble Space Telescope's Faint Object 
Camera to probe the core of M15 (15th object in the Messier 
Catalog) which is located 30,000 light-years away in the 
constellation Pegasus.  M15 is visible to the naked eye as a hazy 
spot 1/3rd the diameter of the full Moon. 

Core Collapse 

     Globular clusters are compact "beehive swarms" of several 
hundred thousand stars loosely held together under the mutual pull 
of gravity.  The stars are deflected by gravity if they pass near 
each other.  During such close encounters a smaller, less massive 
star steals momentum from the larger star. 

     Because of these near-collisions, the massive stars lose 
momentum and "fall" toward the center of the cluster, like marbles 
rolling to the bottom of a funnel.  Given enough time, massive 
stars should accumulate at the cluster's center.  Theoretically, 
this could become a runaway collapse where stars quickly crowd 
together. 

     Previous Hubble observations suggest that the cluster 
probably contains powerful energy "storage batteries" in the form 
of double star systems, which prevent the core from imploding all 
the way down to a black hole.  The rapid orbits of two stars about 
each other in tight binary systems create a powerful reservoir of 
kinetic energy.  A few double stars can stir up the motion of 
in-falling stars.  This would cause the core to rebound, like 
squeezing and relaxing a rubber ball.  

     Astronomers have long sought evidence for core collapse at 
the heart of very dense clusters like M15.  To estimate the true 
stellar density from ground-based visible light photographs, 
however, has been difficult.  The Hubble observation does not tell 
whether the core is still collapsing or rebounding. 
 
     Previous research by a team led by Paresce found that another 
class of unusual blue star, dubbed blue stragglers, also dwell at 
the cores of some clusters.  However, even the "stragglers" are 
not as hot nor as blue as the new population of blue stars in M15.  
Most of the blue stragglers are probably double stars that 
gravitationally capture each other.  The capture stirs-up the 
stragglers' nuclear fuel.  The star "resets its clock" to relive a 
bright and hot youth. 

     The researchers plan to use Hubble to peer into the cores of 
other globular star clusters to see if this new class of star 
dwells elsewhere as well. 

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NOTE TO EDITORS:    A photo to illustrate this release is 
available through NASA's Broadcast and Imaging Branch by calling 
202/358-1900.  The photo number is:

        Color:  93-HC-240             B&W:  93-H-263