Messier 12

Lore & Background

Messier 12 was discovered by Charles Messier on May 30, 1764, who recorded it as a 'nebula without stars'. In dark conditions, it can be faintly seen with binoculars, but resolving its stars requires a telescope with an aperture of 8 inches (20 cm) or greater. Through a 10-inch (25 cm) scope, the granular core appears 3 arcminutes in diameter, surrounded by a 10 arcminute halo of stars.

The cluster is located roughly 3° northwest of the cluster M10 and 5.6° east southeast of star Lambda Ophiuchi, near the 6th magnitude star 12 Ophiuchi. With a Shapley-Sawyer rating of IX, it is rather loosely packed for a globular and was once thought to be a tightly concentrated open cluster. Thirteen variable stars have been recorded in this cluster. M12 is approaching Earth at a velocity of 16 km/s.

A study published in 2006 concluded that this cluster has an unusually low number of low-mass stars. The authors surmised that they were stripped from the cluster by passage through the relatively matter-rich plane of the Milky Way.

Reader's Guide

Messier 12 is significant as a globular cluster that challenges typical expectations of such objects due to its loose structure, reflected in its Shapley-Sawyer rating of IX. Its historical misclassification as a tightly concentrated open cluster underscores the evolving understanding of stellar groupings. The 2006 study revealing a deficit of low-mass stars provides a key insight into the dynamical evolution of globular clusters, suggesting that interactions with the Milky Way's plane can strip away less massive members. This makes M12 a valuable case study for how galactic environment shapes cluster populations. Its proximity to M10, only a few thousand light-years away, offers a comparative perspective on two clusters with similar apparent magnitudes (about magnitude 4.5 from each other). For amateur observers, M12 is accessible with binoculars under dark skies and reveals its granular core and halo with moderate telescopes, making it a rewarding target for visual astronomy.

Did You Know?

• Charles Messier discovered M12 on May 30, 1764, describing it as a 'nebula without stars'.
• M12 has a Shapley-Sawyer rating of IX, making it unusually loosely packed for a globular cluster.
• A 2006 study found M12 has an unusually low number of low-mass stars, possibly stripped by passage through the Milky Way's plane.