From the Hubble Space Telescope
What appears as a bird's head is a 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. 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 the chance of physical encounter being 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.
Image Title: A Bird's Eye View of a Galaxy Collision
based on press release for PHOTO NO.: STScI-PRC0034
NASA and The Hubble Heritage Team (STScI/AURA)
Acknowledgment: Roger Lynds (KPNO/NOAO)
Last Modified On: Friday, December 15, 2000