Messier 10: Globular Cluster in Ophiuchus

Messier 10 is the brightest of the globular clusters in the middle of Ophiuchus and one half of a well-known pair with M12. Both fit into the same low-power binocular sweep, so you can compare a compact cluster with a looser one on the same summer night.

What it is

M10 is a globular cluster, a ball of very old stars bound by gravity and orbiting in the halo of the Milky Way. At about 16,500 light-years it is one of the nearer globulars to us, and its 20′ apparent size works out to roughly 95 light-years across. It is more concentrated toward the center than its neighbor M12, which lies about 3° away and at almost the same distance from us. The two clusters are close on the sky but are not a physical pair; they simply lie in the same direction. Other globulars in Ophiuchus include M9, M14, M19 and M107.

When to see it

M10 is a summer object. From about 40°N it is highest in the south around 11 p.m. daylight time in late June, and it is well placed in the evening through July and August. It climbs to about 46° above the horizon at 40°N. Because it lies just south of the celestial equator, it is equally easy to see from the Southern Hemisphere, where it rides higher in the winter sky.

How to find it

Ophiuchus has no very bright stars, so start from the pair Delta and Epsilon Ophiuchi (Yed Prior and Yed Posterior), two stars of about 3rd magnitude close together in the western part of the constellation. From Delta Ophiuchi, go about 8° east and slightly north to reach M12. M10 then lies about 3.3° southeast of M12. A useful check is the 5th-magnitude star 30 Ophiuchi, which sits about 1° east of M10 at almost the same declination.

What you will see

In binoculars M10 is a round glow with a brighter center, easy from a dark site and still visible from the suburbs. A 70–100 mm telescope shows a bright, round haze, and at 100× or more the edges begin to look grainy, with a few stars popping out. A 200 mm or larger telescope resolves much of the cluster into stars, with a bright core and chains of stars in the outer halo. Under suburban skies the core holds up well, though the faint outer stars fade. No filter helps with star clusters.

History

Charles Messier discovered M10 in late May 1764, the night before he found M12, and described it as a round nebula without stars. William Herschel resolved it into stars in the 1780s.