PHOTO NO.: STScI-PRC00-34 (Heritage)

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 ultraviolet 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)
Acknowledgement: 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)  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)
Electronic image files are available on the Internet at and and via links in and