M10 is a fine globular cluster that's located in the constellation of Ophiuchus. One of the largest constellations, Ophiuchus straddles the celestial equator and contains a host of globular clusters of which Messier catalogued seven of them. The brightest and best of them is M10 (mag. +6.6), which can be spotted with binoculars, appearing like an out of focus fuzzy star.
Charles Messier discovered M10 on May 29, 1764, describing it as a "nebula without stars". Ten years later, German astronomer Johann Elert Bode noted it as a "very pale nebulous patch without stars". Both Messier and Bode used telescopes that suffered in quality and hence were unable to resolve the cluster. It was not until William Herschel using better and larger instruments was able to spot individual member stars. He described it as a "beautiful cluster of extremely compressed stars". The best time of the year to observe M10 is during the months of May, June and July.
Locating M10 is not the easiest task as the surrounding area of sky is devoid of bright stars. Start by locating the brightest star in Ophiuchus, Rasalhague (α Oph - mag +2.1). Join the stars of the constellation in a curve heading westwards and southwards until arriving at two close 3rd magnitude stars, Yed Prior (δ Oph - mag. +2.7) and Yed Posterior (ε Oph - mag. +3.2). M10 is located about 12 degrees east of Yed Prior with star 30 Oph (mag. +4.8) one degree east of M10.
M10 makes a superb target for all telescopes. A small 80mm (3.1-inch) scope reveals an obvious non-stellar fuzzy ball of light spread over about 8 arc minutes. Through 150mm (6-inch) or 200mm (8-inch) instruments the globular appears dense with a large nebulous central core. A number of stars are resolvable especially in the outer halo, which extends some 15 arc minutes in diameter. A larger 300mm (12-inch) telescope reveals many more stars and adds volume to the view. On good nights, it's possible to see stars across the entire face of the cluster. The fainter and smaller globular cluster M12 is located 3 degrees northwest of M10. In total, M10 spans about 20 arc minutes of apparent sky.
M10 is 14,300 light-years distant and is estimated to be 11.4 billion years old. Although relatively close as far as globulars go, it's intrinsically small spanning only 84 light-years diameter and therefore not as spectacular as might be expected. For example, other globulars such as M13 in Hercules are a much grander sight, despite been much further away.
It's estimated M10 contains 100,000 stars. However, an extremely low number are variable stars and to date only 4 have been discovered.
M10 Data Table
|Object Type||Globular cluster|
|RA (J2000)||16h 57m 09s|
|DEC (J2000)||-04d 05m 58s|
|Apparent Size (arc mins)||20 x 20|
|Age (years)||11.4 Billion|
|Number of Stars||100,000|