Astronomy Picture of the Day - A Messier Moment for Tempel 2

Explanation: Which of these is not a comet? You guessed it - the one on the right is a globular star cluster. The diffuse greenish coma of periodic comet 10P/Tempel 2 is at left in the frame. In fact the globular star cluster is Messier 30, also known as M30, or the 30th entry in astronomer Charles Messier's catalog of things which are not comets. The well-known 18th century astronomer kept a list of objects he observed, now his famous Messier Catalogue of Nebulae and Star Clusters, which did not move from night to night against the background stars and...