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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.
November 2, 2000
Image Credit: NASA and The Hubble Heritage Team (STScI/AURA)
Acknowledgement: Roger Lynds (KPNO/NOAO)