Paula Cleggett-Haleim
Headquarters, Washington, D.C.                                                     
May 25, 1993
(Phone:  202/358-0883)                                        
Embargoed Until 1 p.m. EDT

Jim Elliott
Goddard Space Flight Center, Greenbelt, Md.
(Phone:  301/286-6255)

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

RELEASE:  93-97

HUBBLE LOOKS AT THE HEART OF A GALAXY COLLISION

   NASA's Hubble Space Telescope has looked into the heart of a 
galaxy created by the collision of two galaxies and peering deeply 
into its nucleus, discovered a remarkable pinwheel-shaped disk of 
gas surrounded by clusters of young stars born as a result of the 
merger.

   The star clusters apparently were born as a result of the 
collision of two disk-shaped galaxies.  The galaxy merger, which 
occurred about 1 billion years ago, triggered an infall of the gas 
which fueled the birth of new stars around the center of the 
galaxy.   

   "This may unlock the key for understanding how all globular 
clusters formed in ellipticals," said Dr. Brad Whitmore of the 
Space Telescope Science Institute (STScI), Baltimore, Md. "The 
Hubble observation also shows how tiny disk-like structures might 
have formed in many other galaxies."  

   This discovery provides some of the best evidence to date for 
explaining the origin of giant elliptical galaxies.  For more than 
a half century, astronomers have theorized about how such galaxies 
formed.  Some theories propose that ellipticals formed from 
collisions between disk galaxies -- flattened stellar systems 
resembling the Milky Way galaxy.   

   These results are being presented in a press conference today 
at NASA Headquarters, Washington, D.C., by Whitmore and Dr. 
Francois Schweizer of the Carnegie Institution of Washington.  
Co-investigators are Claus Leitherer, Kirk Borne and Carmelle 
Robert of STScI.  
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Pinwheel of Stars and Gas 

   The striking Hubble image shows a spiral pattern at the 
galaxy's core, surrounded by bright star clusters.  "I knew I had 
a major result within 10 seconds of looking at the Hubble 
picture," said Whitmore.  

   The pinwheel shaped disk has an uncanny resemblance to a 
face-on spiral galaxy, yet it is only 10 thousand light-years 
across -- about 1/20 the size of the total galaxy.  The gas and 
stars in the disk swirl around the nucleus, making a spiral 
pattern like cream poured in a cup of coffee.  The mini-spiral 
contains enough gas to make 8 billion stars like the sun.  Though 
several of the clusters were first spotted from ground- based 
telescopes, their true nature was uncertain until the Hubble 
observations.   

   Hubble's resolution is so good that the astronomers can measure 
the diameters (0.04 arc seconds, the apparent size of a dime at a 
distance of 80 miles) of the bright star clusters seen in the same 
image as the spiral disk.  They turn out to be about 60 light 
years across, the same size as globular clusters that orbit the 
Milky Way galaxy.  

   The globular clusters found in NGC 7252 are considered the 
progenitors of similar clusters that orbit the Milky Way galaxy.  
Since globular clusters normally contain ancient red giant stars, 
they provide a fossil record of the formation and evolution of 
galaxies.  Globular clusters contain about 1 million stars each, 
arranged in a tight, spherical swarm and generally are found to be 
about 15 billion years old.  

   However, the "ultra-luminous clusters" found in NGC 7252 
contain hot bluish stars.  Because these blue stars are 
short-lived, the clusters in NGC 7252 are estimated to be mostly 
between 50 and 500 million years old.   

   The blue stars make the globular clusters up to several hundred 
times brighter than the clusters that orbit the Milky Way galaxy.  
If the Milky Way's globular clusters were as bright, they could be 
seen with the naked-eye and would be brighter than the stars in 
the Big Dipper.  

   In the 1920's, American astronomer Edwin Hubble classified 
galaxies according to their spiral or elliptical shape.  A key 
difference is that stars are concentrated in a disk in spirals, 
but are distributed in a diffuse, roughly spherical distribution 
in ellipticals.   

   Since Edwin Hubble's time, astronomers have sought an 
explanation for why there are two different types of galaxies.  
During the past decade, the hypothesis that spiral galaxies can 
collide and merge to form elliptical galaxies has become 
increasingly popular.  
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   Located 300 million light-years away in the constellation 
Aquarius, NGC 7252 has been considered the prototypical example of 
a merger between two disk-shaped galaxies.  The galaxy has a pair 
of long tails that are unambiguous evidence of the effects of 
gravitational tidal forces from a galaxy merger.    

   The galaxy NGC 7252 is nicknamed the "Atoms-for-Peace" galaxy 
because its stars form a bizarre loop-like structure that 
resembles a schematic diagram of an electron orbiting and an 
atomic nucleus.  (In December 1953. U.S. President Dwight D. 
Eisenhower made his "Atoms for Peace" speech to foster peaceful 
applications of nuclear energy.  

   If globular clusters can be born during galaxy collisions, it 
reinforces the theory that disk galaxies merge to make giant 
elliptical galaxies.  One argument against this theory is that 
elliptical galaxies have more globular clusters than expected if 
disk galaxies were simply combined, since disk galaxies have 
relatively few clusters.  

Hubble Picture Helps to Solve Mystery 

   The new Hubble Space Telescope observation solves this dilemma 
by showing that when disk galaxies collide they can form new 
globular clusters.  Rather than being a problem for the merger 
scenario, an increase in the number of globular clusters is a 
natural consequence of galaxy mergers.  

   The existence of a "mini-disk" also fits with the merger 
scenario since similar disk-like features appear to exist in many 
elliptical galaxies.  Another clear indication that the material 
originated from the collision of two galaxies is that the 
mini-spiral is rotating in a direction opposite to the rest of the 
galaxy.  

   This discovery is the latest in a series of disk-like 
structures that Hubble has uncovered at the cores of galaxies.  
Previously, HST  found a giant disk of cool dust and gas orbiting 
a suspected black hole in the active galaxy NGC 4261 and 
discovered an edge-on "donut" of dust in the spiral galaxy M51.  

   The astronomers predict that in a few billion years the gas in 
NGC 7252 will be exhausted.  The galaxy will look like a normal 
elliptical galaxy with a small inner disk.    
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      NOTE TO EDITORS:  Color and black and white images are 
available to media from NASA's Broadcast and Imaging Branch, 
358-1741.  The photo numbers are:  B&W:  93-H-205, -206                
Color:  93-HC-190