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
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.
Credit: NASA and The Hubble
Heritage Team (STScI/AURA)
Acknowledgement: R. Lynds (KPNO/NOAO)