Messier 92 (NGC 6341) is a globular cluster in the constellation Hercules. It is one of the brighter globular clusters in the northern hemisphere, but it is often overlooked by amateur astronomers because of its proximity to the even more spectacular Messier 13.
M 92 was discovered by Johann Elert Bode in 1777. Charles Messier independently rediscovered it and cataloged it in 1781, along with eight other objects (M84-M91) which are all Virgo Cluster galaxies. William Herschel first resolved it into stars in 1783.
Observing Messier 92
Located on the north edge of Hercules, this impressive globular cluster deserves more attention than it gets, but is outclassed by the Great Hercules cluster, M 13, to its southwest. M 92 is harder to locate than its more famous cousin, but is still a splendid object, visible to the naked eye under very good conditions, and a showpiece for optics of every size. At visual magnitude 6.3, it is only slightly fainter than M 13, and about 1/3 smaller, with an angular diameter of 14'.
Interesting trivia on M 92: the North Celestial Pole occasionally passes within 1° of this cluster, due to the precession of Earth's axis. M 92 becomes the "North Cluster" in about 14,000 years (16,000 AD), as it was about 12,000 years ago (10,000 BC).
Physical Properties
M 92 is about 26,000 light years distant, only little farther away than its brighter apparent neighbor M 13; and it is intrinsically smaller and fainter. Its true diameter is about 100 light years; its absolute magnitude is -8.1, a luminosity of 150,000 Suns (60% that of M 13), and it may contain a mass of up to 330,000 Suns. M 92 is approaching us at 112 km/sec.
The stars of M 92 are exceptionally poor in iron and other elements heavier than hydrogen. This suggests that M 92 was formed before the gas and dust of our galaxy were enriched with heavy elements, and therefore that M 92 is exceptionally old, even for a globular cluster. M 92 may be a bit younger than M 13, or about 12 billion years old.
Only 16 variable stars have been discovered in this globular, 14 of which are of the RR Lyrae type. One is of the W Ursae Majoris type, and one of the very few eclipsing binaries found in a globular cluster. In these dense stellar agglomerates, close encounters occur frequently, so that binary systems will be disturbed, and eventually destroyed.