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Messier 10 is a globular cluster—a dense, roughly spherical collection of hundreds of thousands of ancient stars held together by gravity—located in the constellation Ophiuchus. Astronomers study globular clusters like Messier 10 to better understand the early history of our galaxy and how stars age over billions of years.
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Messier 10 or M10 (also designated NGC 6254) is a globular cluster of stars in the equatorial constellation of Ophiuchus. The object was discovered by the French astronomer Charles Messier on May 29, 1764, who cataloged it as number 10 in his catalogue and described it as a "nebula without stars". In 1774, German astronomer Johann Elert Bode likewise called it a "nebulous patch without stars; very pale". Using larger instrumentation, German-born astronomer William Herschel was able to resolve the cluster into its individual members. He described it as a "beautiful cluster of extremely compressed stars". William Parsons, 3rd Earl of Rosse thought he could distinguish a dark lane through part of the cluster. The first to estimate the distance to the cluster was Harlow Shapley, although his derivation of 33,000 light years was much further than the modern value.
The tidal radius of M10 is 19.3 arcminutes, which is about two-thirds of the apparent diameter of the Moon. Viewed through medium-sized telescopes it appears about half that size (8–9 arcminutes), as its bright core is only 35 light-years across. It has a core radius of 48 arcseconds and a half-mass radius of 147 arcseconds (2.5 arcminutes). M10 has a spatial diameter of 83 light-years and is estimated to be 14,300 light-years away from Earth. It figures a degree west of 30 Ophiuchi, a center-of-constellation orange star.