M92 is a bright globular cluster located in the northern part of the constellation of Hercules. Despite shining at magnitude +6.4 and at the very far limit of naked eye visibility, it's often overlooked by amateur astronomers because of its close proximity to the more spectacular M13.
M92 is located in a relatively blank area of Hercules north of the well-known "Keystone" asterism. It can be found by drawing an imaginary line from the NW corner "Keystone" star Eta Herculis (η Her - mag. +3.5) to star Iota Herculis (ι Her - mag. +3.8). About 60% of the way along this line is M92. When viewed through 10x50 binoculars, the cluster appears distinctly non-stellar, looking like an out of focus star or a hazy patch of light. It has a brighter core that can be seen with direct vision but easier if using averted vision. A small to medium size telescope will start to resolve some of the outer stars. With a 150mm (6-inch) or 200mm (8-inch) telescope, M92 appears slightly oval in shape with a bright core and a smattering of stars in the surrounding halo. Compared with M13, the core of M92 is more dense and compact and hence much more difficult to resolve. This is particularly noticeable in large telescopes of the order of 250mm (12-inch) or greater.
When viewed through a telescope of this size, M92 appears as a large, bright, ball of stars with dozens of bright stars resolved in the halo, across the surface of the cluster and in the dense central core. In total, it has an apparent diameter of 14 arc minutes and is best seen from the Northern Hemisphere during the months of June, July and August.
M92 is one of the original discoveries of Johann Elert Bode, who found it on December 27, 1777 and described it as "A nebula that is more or less round with a pale glow". Charles Messier independently rediscovered it and cataloged it on March 18, 1781. Incidentally, this proved to be a very productive day for Messier as he also cataloged another 8 objects, all of them Virgo Cluster galaxies (M84-M91) at the same time. As with many globulars, it was William Herschel in 1783 who first resolved M92 into stars. Of all the stars in M92, only about 16 variables have been discovered of which 14 of are of RR Lyrae type and one of them is a rare globular eclipsing binary of the W Ursae Majoris type. The cluster is located 26,700 light-years from Earth and has a spatial diameter of 108 light-years.
Due to the effect of precession, the Earth's North Celestial Pole (NCP) occasionally passes within one degree of M92. The last time this occurred was about 12,000 years ago (10,000 BC) and the next time will be in about 14,000 years. So around the year 16,000 AD, M92 will become "Polarissima Borealis", or the "North Cluster" object.
M92 Data Table
|Object Type||Globular Cluster|
|RA (J2000)||17h 17m 07s|
|DEC (J2000)||43d 08m 10s|
|Apparent Size (arc mins)||14 x 14|
|Number of Stars||250,000|