M10 - NGC 6254

Messier 10 (NGC 6254) is a bright and well-resolved globular cluster in the constellation Ophiuchus, located approximately 14,300 light-years from Earth. It is one of several globular clusters that populate this region of the sky, which lies in the general direction of the galactic center. M10 contains several hundred thousand stars within a diameter of about 83 light-years, though its visible core region is considerably smaller. The cluster shines at an apparent magnitude of 6.6 and spans about 20 arcminutes across, roughly two-thirds the apparent size of the full Moon. It was discovered by Charles Messier on May 29, 1764, just one day after he found the nearby M9. Compared to many globular clusters, M10 has a relatively loose core concentration, classified as Class VII on the Shapley-Sawyer scale, which means that amateur telescopes can begin to resolve individual stars across much of the cluster's face. A 4-inch telescope shows a grainy texture at the edges, while 6 to 8 inches of aperture reveals a beautiful spray of stars cascading away from a brighter central condensation. M10 is estimated to be about 11.4 billion years old and contains a relatively small number of known variable stars compared to some of its peers. The cluster has an interesting stellar composition, with a notable deficiency of the faintest, lowest-mass red dwarf stars in its outer regions, likely a result of mass segregation where heavier stars sink toward the center while lighter stars are pushed outward and eventually stripped away by tidal forces. M10 and its neighbor M12, located just over 3 degrees away, make an attractive pair for a single evening's observation.

Spans about 83 light-years in diameter with hundreds of thousands of stars, located 14,300 light-years from Earth at magnitude 6.6.