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M 92
19 of 111

M 92

February 19, 2005
Globular Cluster M92 (NGC 6341), class IV, in Hercules

Right Ascension: 17 : 17.1 (h:m)
Declination: +43 : 08 (deg:m)
Distance: 26.7 (kly)
Visual Brightness: 6.4 (mag)
Apparent Dimension: 14.0 (arc min)

Discovered 1777 by Johann Elert Bode.

Globular cluster M92 is one of the original discoveries of Johann Elert Bode, who found it on December 27, 1777. Charles Messier independently rediscovered it and cataloged it on March 18, 1781, the same day as he cataloged another 8 objects, all of them Virgo Cluster galaxies (M84-M91). It was William Herschel who first resolved it into stars in 1783.

According to newer sources, M92 is about 26,000 light years distant, only little more than its brighter apparent neighbor M13. From its HRD (or CMD), it may be a bit younger than M13 as its turnoff point is shifted to the brighter and bluer end. A semi-recent estimate of M92's age has given a value of about 16 billion years (anyway more than 14 billion years), see e.g. the diagram in Sky & Telescope, January 1996, p. 22 (text on p. 20). However, this value is now again under discussion because of the general modifications of the distance scale of the universe, implied by results of ESA's astrometrical satellite Hipparcos: These results suggest that M92, as well as most other globular clusters, may be at a 10 per cent larger distance; therefore, the intrinsical brightness of all their stars must be about 20 % higher. Considering the various relations which are important for understanding stellar structure and evolution, they should also be roughly 15 % younger, in a preliminary off-hand estimate (or about 12-14 billion years). For M92, W.E. Harris' globular cluster database lists an only little modified value of 26,700 (former 26,100) light years, though, so that the quoted result might stay valid after all.

M92 is a splendid object, visible to the naked eye under very good conditions and a showpiece for every optics. It is only slightly less bright but about 1/3 less extended than M13: its 14.0' angular extension corresponds to a true diameter of 109 light years, and may have a mass of up to 330,000 suns.

Only about 16 variables have been discovered in this globular, 14 of which are of RR Lyrae type, while one of them is one of the very few eclipsing binaries in globular clusters, of W Ursae Majoris type. Although Burnham claims it is not well understood why eclipsing binaries are so rare in globulars, it appears to the present author that there may be a simple answer: In these dense stellar agglomerates, close encounters occur frequently, so that binary systems will be disturbed, and on the long term, will be destroyed.

M92 is approaching us at 112 km/sec.

Interesting trivia on M92: As the ecliptical coordinates for M92, Longitude = 249.9 deg, Latitude = 65.9 deg, indicate, the Earth's North Celestial Pole occasionally passes closer than 1 degree of this cluster, at periods of the precession of Earth's axis (about 25,800 years). So this cluster becomes a "Polarissima Borealis", or "North Cluster", in about 14,000 years (16,000 AD), as it was about 12,000 years ago (10,000 BC).


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