Messier 45 is a young open cluster located in Taurus. Also know as the Pleiades, or the Seven Sisters. Visible to the naked eye, the cluster has been known since ancient times. It's curious that Charles Messier included it in his catalog of non-comet objects as there is little chance of it being confused with a comet. Originally thought the reflection nebula surrounding the stars was left over debris from their formation, but at 100 million years of age, it should have been dispersed. It is now assumed the cluster is just passing through a dust cloud. Light from the hot O type blue stars is reflected off the surrounding dust creating the reflection nebula around the stars.
The cluster contains over 1,000 stars and is around 444 light years away.