Messier 12 — Globular Cluster in Ophiuchus

M12 is a globular cluster in the constellation Ophiuchus, located about 15,700 light-years from Earth. It spans about 75 light-years in diameter and contains several hundred thousand stars. At magnitude 6.1 and with an apparent diameter of about 16 arcminutes, it is a bright and attractive target. M12 has a relatively loose structure for a globular, with a Shapley-Sawyer concentration class IX — noticeably less concentrated than its neighbor M10. The cluster is estimated to be about 12.67 billion years old.

Located about 3.4 degrees northwest of M10 in Ophiuchus. At magnitude 6.1, it is faintly visible to the naked eye under dark skies. Binoculars show a round, fuzzy glow slightly larger and more diffuse than M10. A 4-inch telescope at 100x reveals a bright, loosely concentrated cluster with a grainy texture. An 8-inch telescope easily resolves individual stars throughout, as the loose concentration makes star resolution easier than in many globulars. Best viewed from June through August, often paired with M10.

Discovered by Charles Messier on May 30, 1764, the day after he found M10. He described it as a 'nebula without stars.' William Herschel resolved it into stars using his larger telescope. For many years astronomers debated whether M12 was an extremely rich open cluster or a loose globular, but it is now firmly classified as a globular.