EMBARGOED UNTIL: 1:00 a.m. (EST) November 2, 2000
PHOTO NO.: STScI-PRC00-34

A BIRD'S EYE VIEW OF A GALAXY COLLISION

What appears as a bird's head, leaning over to snatch up a tasty 
meal, is a striking example of a galaxy collision in NGC 6745. 
A large spiral galaxy, with its nucleus still intact, peers at the 
smaller passing galaxy (nearly out of the field of view at lower 
right), while a bright blue beak and bright whitish-blue top feathers 
show the distinct path taken during the smaller galaxy's journey. 
These galaxies did not merely interact gravitationally as they 
passed one another, they actually collided.

When galaxies collide, the stars that normally comprise the major
portion of the luminous mass of each of the two galaxies will almost 
never collide with each other, but will pass rather freely between 
each other with little damage. This occurs because the physical 
size of individual stars is tiny compared to their typical separations, 
making the chance of physical encounter relatively small. In our own 
Milky Way galaxy, the space between our Sun and our nearest stellar 
neighbor, Proxima Centauri (part of the Alpha Centauri triple system), 
is a vast 4.3 light-years. 

However, the situation is quite different for the interstellar media 
in the above two galaxies - material consisting largely of clouds 
of atomic and molecular gases and of tiny particles of matter and 
dust, strongly coupled to the gas.  Wherever the interstellar clouds 
of the two galaxies collide, they do not freely move past each other 
without interruption but, rather, suffer a damaging collision.  High 
relative velocities cause ram pressures at the surface of contact 
between the interacting interstellar clouds. This pressure, in turn, 
produces material densities sufficiently extreme as to trigger 
star formation through gravitational collapse. The hot blue stars 
in this image are evidence of this star formation.

This image was created by the Hubble Heritage Team using NASA Hubble Space 
Telescope archive data taken with the Wide Field Planetary Camera 2 in 
March 1996. Members of the science team, which include Roger Lynds (KPNO/NOAO) 
and Earl J. O'Neil, Jr. (Steward Obs.), used infrared, red, visual and 
ultravoilet filters to image this galaxy system. Lynds and O'Neil 
are currently using the Hubble data along with ground-based radio 
observations to further study the interactions within NGC 6745.

Image Credit: NASA and The Hubble Heritage Team (STScI/AURA)
Acknowledgment: Roger Lynds (KPNO/NOAO) 

EDITOR'S NOTE:  For additional information, please contact
Dr. Roger Lynds, Kitt Peak National Observatory, P.O. Box 26732, 
Tucson AZ 85726-6732, (phone) 520-318-8000, (fax) 520-318-8360, (e-mail)
rlynds@noao.edu  and
 
Dr. Keith Noll, Space Telescope Science Institute, 3700 San Martin Drive, 
Baltimore, MD 21218, (phone) 410-338-1828, (fax) 410-338-4767, 
(e-mail) noll@stsci.edu.
 

Electronic image files are available on the Internet at
http://heritage.stsci.edu and
http://oposite.stsci.edu/pubinfo/pr/2000/34 and via links in
http://oposite.stsci.edu/pubinfo/latest.html
http://oposite.stsci.edu/pubinfo/pictures.html and
http://hubble.stsci.edu/go/news


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