Messier 12: Globular Cluster in Ophiuchus

Messier 12 is the companion of M10 in the middle of Ophiuchus. The two look alike in binoculars, but a telescope shows that M12 is much looser, which makes it one of the easier globular clusters to resolve into individual stars.

What it is

M12 is a globular cluster, a gravitationally bound ball of very old stars orbiting in the halo of our galaxy. It lies about 16,700 light-years away, almost the same distance as M10, and its 16′ apparent size corresponds to roughly 80 light-years across. Its stars are spread out more evenly than in most globulars, without a tightly packed core. Studies of the cluster have suggested that it has lost many of its low-mass stars over time, stripped away by the Milky Way's gravity as it passed through the galactic disk. That may be part of why it looks so open. M10 lies about 3° away; despite their nearness on the sky, the two are not a physical pair.

When to see it

M12 is a summer object. From about 40°N it is highest in the south around 11 p.m. daylight time in mid-June, and it is well placed in the evening through July and August. It reaches about 48° above the horizon at 40°N. It lies just south of the celestial equator, so it is easy to see from the Southern Hemisphere as well.

How to find it

Start from the close pair Delta and Epsilon Ophiuchi (Yed Prior and Yed Posterior), two 3rd-magnitude stars in the western part of Ophiuchus. From Delta Ophiuchi, move about 8° east and slightly north; M12 sits there in an area without bright stars. Another route starts at Lambda Ophiuchi, a 4th-magnitude star about 6° northwest of M12. Once you have M12, M10 is about 3.3° to the southeast, and you can sweep between them in binoculars.

What you will see

In binoculars M12 is a round, soft glow, a little fainter and less concentrated than M10. A 70–100 mm telescope shows a hazy disk, and at 100× some of its brighter stars start to appear against the glow. A 200 mm or larger telescope resolves it well, showing many stars across the face of the cluster, with a loose center and dark lanes between star chains. From suburban skies it is still easy, though it looks fainter than M10 because its light is spread out. No filter helps.

History

Charles Messier discovered M12 at the end of May 1764, one night after M10, and described it as a round nebula without stars. William Herschel resolved it into stars in the 1780s.