Mark Striebeck
First Light: M51
M51 is a spiral galaxy in a distance of 23 million light years and has a diameter of 76,000 light years - which means it's slightly smaller than our own galaxy (105,700 light years). It was discovered by Charless Messier on October 13, 1775 - though he only discovered the main galaxy. The smaller galaxy (NGC 5159) was discovered 6 years later by Pierre Méchain. The most prominent feature of this galaxy is the encounter with the smaller galaxy (at the bottom in the image). Although it looks like a frontal encounter, the smaller galaxy (NGC 5159) is actually passing behind the larger galaxy. This process has been going on for hundreds of millions of years.