Sarah Keegan
Headquarters, Washington, D.C.                                            
August 27, 1993
(Phone:  202/358-1547)

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:  93-156

Hubble Identifies a Source of Mysterious X-Ray Bursts

   Astronomers using the NASA Hubble Space Telescope have taken 
an image for the first time of a double star that is emitting 
enormous amounts of x-ray energy and previously only could be 
"seen" by instruments that detect x-rays.

   "I was just doing a routine study of the dynamics and 
population of stars in the cluster which harbors this binary 
(double star) system," said Dr. Ivan King of the University of 
California at Berkeley.  "My decision to take an ultraviolet 
image was a shot in the dark."

   He noted that the double star never could have been seen 
with ground-based telescopes.  However, Hubble's high 
resolution and ultraviolet sensitivity clearly distinguish the 
star from the others around it.  King's picture was obtained by 
the European Space Agency's Faint Object Camera aboard Hubble.

   The binary system is in the heart of a dense globular 
cluster of stars called NGC 6624.  It is about 30,000 light 
years from Earth in the constellation Sagittarius.

   One binary system member is a neutron star, the corpse of a 
massive star that exploded eons ago.  It is extremely dense and 
compact, and exerts a powerful gravitational pull.  Its 
companion, which has less mass, is a white dwarf, the remnant 
of a burned-out star which once was like Earth's sun.
                           - more -
                             - 2 -


   The two stars are only about 100,000 miles apart and 
complete one orbit of each other every 11 minutes, making it 
the fastest binary system known.

   During this deadly dance, the neutron star's intense gravity 
strips helium from its companion, accumulating a disk of the 
gas around the neutron star.  This continuous infall of gas 
produces a steady stream of x-ray energy.  The energy, or 
radiation, heats the gas disk to temperatures greater than 
100,000 degree Fahrenheit.

   When enough super-hot helium accumulates in the disk, it 
spontaneously ignites with the force of an enormous nuclear 
fusion bomb.  The outburst of energy is of far greater 
intensity than the steady infall emission and is called an 
"x-ray burster."  Astronomers have identified about 30 x-ray 
bursters in the universe, most near the center of the Milky Way 
galaxy.

   Astronomers are interested in binary systems like this to 
gain greater insight into the physics of matter falling onto 
gravitationally powerful, compact objects such neutron stars 
and black holes.  This mechanism is thought to be the primary 
source of energy in a variety of exotic objects in the 
universe, ranging from x-ray sources in the Milky Way to 
quasars, the most energetic objects known.

   Kings plans to use Hubble's High Speed Photometer to look 
for brightness changes as the binary goes around its 11-minute 
orbit.  He also will look optically for the mysterious 
flickering at 30-40 times per second that has been seen in 
x-rays.  

   "There is only a remote hope of seeing something in the 
millisecond range, but we can't afford not to look," he says.  
HST eventually will be used to make spectroscopic observations 
that should confirm the nature of the star.

   The Hubble Space Telescope is a project of international 
cooperation between NASA and the European Space Agency.

                            - end -

NOTE TO EDITORS:  Images are available to media from NASA's 
Broadcast and Imaging Branch, 202/358-1900.  Photo numbers are:  
B&W:  93-H-400, 
-401 and Color:  93-HC-375, -376.