The Antennae Galaxies, NGCs 4038 and 4039, are a pair of interacting galaxies in the constellation Corvus approximately 50 million light years away. They are currently going through a starburst phase, in which the collision of clouds of gas and dust, with entangled magnetic fields, causes rapid star formation.
About 1.2 billion years ago, the Antennae were two separate galaxies. NGC 4038 was a barred spiral galaxy and NGC 4039 was a spiral galaxy. They collided about 900 million years ago and passed through each other around 300 million years ago with stars being thrown out into the two streamers that extend far beyond the original galaxies, resulting in the antennae shape.
(Text derived from Wikipedia)