Messier 92
An ancient globular cluster in Hercules, often overshadowed by M13.
Messier 92 (also cataloged as M92, M 92, or NGC 6341) is a globular star cluster located in the northern constellation Hercules. Though it ranks among the brighter globular clusters in the northern sky—both in how it appears from Earth and in its true luminosity—it frequently gets less attention from amateur stargazers because it lies close to the even more prominent Messier 13, which is about 20% nearer.
Johann Elert Bode first spotted it on December 27, 1777, and his discovery appeared in the *Berliner Astronomisches Jahrbuch* in 1779. Charles Messier independently found it on March 18, 1781, and listed it as the 92nd object in his catalog. William Herschel became the first to distinguish individual stars within it in 1783.
Under excellent conditions, M92 is visible to the naked eye. Even in heavily light-polluted skies, a small telescope reveals it as a fuzzy patch; darker skies allow finer resolution. Compared to M13, M92 is only slightly dimmer but covers about one-third less area.
This cluster is among the oldest in the Milky Way. It lies roughly 26,700 light-years from the Solar System, about 33,000 light-years from the Galactic Center, and sits some 16,000 light-years above or below the galactic plane. Its half-light radius—the region containing half its emitted light—measures 1.09 arcminutes, while its tidal radius, the outermost standard boundary, spans 15.17 arcminutes. The cluster appears only slightly flattened, with its minor axis about 89% ± 3% the length of its major axis.
Like most globular clusters, M92 contains very little of the heavier elements beyond hydrogen and helium—astronomers describe it as having low metallicity. Its iron abundance relative to the Sun is [Fe/H] = –2.32 dex, meaning it holds only about 0.5% of the Sun’s iron content on that logarithmic scale. This puts its estimated age at 11 ± 1.5 billion years. The cluster’s true diameter is about 108 light-years, and its mass may equal roughly 330,000 times that of the Sun.
M92 has not yet undergone core collapse and is not guaranteed to do so; its core radius is about 2 arcseconds. It is classified as an Oosterhoff type II (OoII) globular cluster, a group of metal-poor clusters that contain longer-period RR Lyrae variable stars. The 1997 catalog of variable stars in globular clusters listed 28 candidate variables here, though only 20 have been confirmed.
Quick Facts
- Epoch
- J2000
- Class
- IV
- Constellation
- Hercules
- Ra
- 17 · 17 · 07.39
- Dec
- +43 · 08 · 09.4
- Dist Ly
- 26.7 e3ly
- Appmag V
- 6.4
- Radius Ly
- 54 ly
- Metal Fe
- –2.32
- Names
- M92, NGC 6341, GCl 59
Facts from the source article.
Lore & Background
Messier 92 was discovered by Johann Elert Bode on December 27, 1777, and published in the Berliner Astronomisches Jahrbuch during 1779. It was inadvertently rediscovered by Charles Messier on March 18, 1781, and added as the 92nd entry in his catalogue. William Herschel first resolved individual stars in 1783. The cluster is one of the galaxy's oldest, with an estimated age range of 11 ± 1.5 billion years. It lies about 26,700 light-years from the Solar System, 33,000 light-years from the Galactic Center, and about 16,000 light-years above or below the galactic plane. Its true diameter is 108 light-years, and it may have a mass corresponding to 330,000 suns. The cluster appears only slightly flattened, with its minor axis about 89% ± 3% of the major. It has low metallicity, with an iron abundance of [Fe/H] = –2.32 dex, or 0.5% of the solar value. Messier 92 is an Oosterhoff type II globular cluster, meaning it belongs to the group of metal-poor clusters with longer period RR Lyrae variable stars. As of 2001, there are 17 known RR Lyrae variables in the cluster. Ten X-ray sources have been detected within the half-mass radius, half of which are candidate cataclysmic variable stars. The cluster is approaching us at 112 km/sec. Due to precession, the Earth's North Celestial Pole periodically passes less than one degree from this cluster; it was a 'Polarissima Borealis' about 12,000 years ago (10,000 BC) and will be again in about 14,000 years (16,000 AD). The cluster hosts at least two stellar generations (1G and 2G), with two distinct groups of 2G stars (2GA and 2GB) having higher helium abundances than the 1G stars.
Reader's Guide
Messier 92 is notable as one of the galaxy's oldest globular clusters, with an estimated age of 11 ± 1.5 billion years. Despite being one of the brighter globular clusters in the northern hemisphere, it is often overlooked by amateur astronomers because it lies only about 20% closer in angular distance to the bright cluster Messier 13. When compared to M13, M92 is only slightly less bright but about one-third less extended. It is visible to the naked eye under very good viewing conditions, and with a small telescope it can be seen as a nebulous smudge even in severely light-polluted skies. The cluster's low metallicity—its iron abundance is 0.5% of the Sun's—places it among the metal-poor Oosterhoff type II group, characterized by longer period RR Lyrae variable stars. Its multiple stellar populations reveal at least two generations of stars, with distinct helium abundance differences. The cluster is not yet in core collapse, and its core radius is about 2 arcseconds. Its historical role as a 'Polarissima Borealis' due to precession adds to its significance.
Did You Know?
- The cluster's iron abundance is only 0.5% of the Sun's, indicating very low metallicity.
- About 12,000 years ago (10,000 BC), Messier 92 was a 'Polarissima Borealis' because the North Celestial Pole passed less than one degree from it.
More in Globular Clusters 1-24
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