Composite image of two colliding galaxies, known as the Antennae galaxies, or NGC 4038 and NGC 4039. The Antennae galaxies take their name from the long antenna-like arms, seen in wide-angle views of the system. These features were produced by tidal forces generated in the collision. The collision, which began more than 100 million years ago and is still occurring, has triggered the formation of millions of stars in clouds of dusts and gas in the galaxies. The most massive of these young stars have already sped through their evolution in a few million years and exploded as supernovas.