M92, mag. +6.4, is a bright globular cluster located in the northern part of the constellation of Hercules. Despite being almost visible to the naked eye, it's often overlooked due to its close proximity to more spectacular, M13.
M92 is one of the original discoveries of Johann Elert Bode who found it on December 27, 1777. He described it as "a nebula that's more or less round with a pale glow". Charles Messier independently rediscovered it and catalogued it on March 18, 1781. Incidentally, this proved to be Messier's most productive night, during which he discovered another 8 objects all of them Virgo Cluster galaxies (M84 to M91). As with many globulars, it was William Herschel who first resolved it into stars. To date, only about 16 variables have been discovered in M92 of which 14 are of the RR Lyrae type, and one is a rare globular eclipsing W Ursae Majoris type binary.
M92 is located a few degrees north of the Keystone asterism in a relatively blank area of Hercules. It can be found by drawing an imaginary line from eta Herculis (η Her - mag. +3.5) to iota Herculis (ι Her - mag. +3.8). About 3/5ths of the way along this line is M92. When viewed through 10x50 binoculars this cluster appears distinctly non-stellar, like an out of focus star or a hazy patch of light. It has a brighter core, which can be seen with direct vision but much easier with averted. Small to medium size telescopes start to resolve some of the outer stars in the surrounding halo. Compared with M13, the core of M92 is compact and therefore more difficult to resolve. This is particularly noticeable with larger scopes of the order of 250mm (12-inch) aperture or greater. When viewed through such instruments, M92 appears as a large bright ball of stars with dozens of members resolved across the surface, although fewer towards the centre.
Due to the effect of precession, the North Celestial Pole (NCP) occasionally passes within a degree of M92. This last occurred about 10,000 BC and will happen again in about 14,000 years time. Around the year 16,000 AD, M92 will become "Polarissima Borealis" or the "North Cluster" object.
In total, M92 has an apparent diameter of 14 arc minutes. It's located 26,700 light-years from Earth and has a spatial diameter of 108 light-years. It's best seen from Northern Hemisphere locations during the months of June, July and August.
M92 Data Table
|Object Type||Globular Cluster|
|RA (J2000)||17h 17m 07s|
|DEC (J2000)||+43d 08m 10s|
|Apparent Size (arc mins)||14 x 14|
|Age (years)||14.2 Billion|
|Number of Stars||250,000|