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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)...
Click image to go to a high resolution view.
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| 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. |
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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.
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