The M7 Open Star Cluster in Scorpius . ... Credit Copyright: . Credit Copyright: N. A. Sharp , REU Program , AURA , NOAO , NSF Explanation: M7 is one of the most prominent open clusters of stars on the sky. The cluster, dominated by bright blue stars, can be seen with the naked eye in a dark sky in the tail of the constellation of Scorpius . M7 contains about 100 stars in total, is about 200 million years old, spans 25 light-years across, and lies about 1000 light-years away. ...