M51

Constellation: Canes Venatici

Distance: 31 million light years

Discovered by Charles Messier in 1773, M51 was the first deep sky object recognized to have a spiral structure in 1845, long before the nature of galaxies was recognized by Edwin Hubble. M51 is viewed face-on from earth. The arms of this grand design spiral galaxy demonstrate active star formation, with young blue star clusters and red glowing H-II regions.

On the left side of M51 in this image is the smaller irregular galaxy NGC 5195 (seen as yellow), a companion galaxy with which M51 is interacting. About 70 million year ago, these 2 galaxies had a close encounter, with NGC 5195 now located behind M51. The interaction is causing one arm of M51 to splay outward towards its companion, and is also causing strewing of a stream of stars outward from the smaller galaxy.