Galaxy collision in NGC 6745. Image taken by the Hubble Space Telescope using the Wide Field Planetary Camera 2 in March 1996. A large spiral galaxy has collided with a passing galaxy (at lower right). Bright blue and whitish-blue areas show the path of the smaller galaxy's journey. When galaxies collide, the stars that comprise the major luminous mass of the galaxies will pass between each other because the physical size of individual stars is small compared to their typical separations. The situation is different for the interstellar media in the above galaxies consisting of atomic and molecular gases and tiny particles of matter and dust. Through their collisions sufficient pressure is produced to trigger star formation through gravitational collapse. The hot blue stars in this image indicate star formation.