September 19, 2013
The Ptolemy Cluster, an open star cluster in Scorpius
Image Credit: N.A.Sharp, REU program/NOAO/AURA/NSF
The Ptolemy Cluster (also known as Messier 7 or NGC 6475) is an open star cluster of some 25 light-years across that lies about 980 light-years away from Earth in the constellation of Scorpius (the Scorpion). It is approaching us at approximately 14 kilometers per second.
This southernmost Messier object has been known since antiquity; it was first recorded by the 1st-century Greek-Roman astronomer Ptolemy (hence its name), who described it as a nebula in 130 AD.
The Ptolemy Cluster is a bright cluster that contains some 80 stars brighter than magnitude 10, with the highest degree of concentration toward the center. The total number of stars in this cluster is still unknown, but is expected to be at least a few hundred. Anyway, its total mass, including the interstellar medium, is about 735 times the mass of our Sun. The tidal radius of the cluster is 40.1 light-years. The cluster contains a similar abundance of elements – other than hydrogen and helium – as our Sun, so it is likely that it contains many Sun-like stars with rocky Earth-like planets.
The cluster with its bright, blue members are projected on a crowded background of dense Milky Way star fields and dark dust clouds. However, it’s incredibly difficult to determine which stars are small, dim and red and part of this cluster, and which ones are background stars. The yellow giant star just below-and-to-the-right of center in the image above is the brightest in the cluster: a yellow giant that’s entering the final stages of its life.
With an age of around 220 million years, the Ptolemy Cluster is considered to be a middle-aged cluster. As it ages, the handful of bright stars that attract the eye will vanish as supernovae, and the remaining faint stars, which are much more numerous, will hardly be recognizable as a group.
The cluster is easily detectable with the naked eye, close to the “stinger” of Scorpius.
This color composite was created from CCD images taken at the Burrell Schmidt Telescope of the Warner and Swasey Observatory of the Case Western Reserve University, located on Kitt Peak in southwestern Arizona. The contributing images were taken in June and July of 1995 during the Research Experiences for Undergraduates (REU) program operated at the Kitt Peak National Observatory and supported by the National Science Foundation.