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Messier 12

Globular Cluster M12 (NGC 6218), class IX, in Ophiuchus

[m12.jpg]
Right Ascension 16 : 47.2 (h:m)
Declination -01 : 57 (deg:m)
Distance 16.7 (kly)
Visual Brightness 6.7 (mag)
Apparent Dimension 16.0 (arc min)

Discovered by Charles Messier in 1764.

M12 is one of Charles Messier's original discoveries, found on May 30, 1764. Like many other globular clusters, Messier described it as "Nebula without stars", as did Bode 10 years later; a consequence of the modest resolving power of their instruments. William Herschel was the first to resolve it into stars in 1783.

Messier 12 (M12, NGC 6218) appears nearly as a twin of its apparent neighbor, M10, and is only slightly larger and an idea fainter. Nevertheless, it was once believed to be an intermediate type between globular and dense open clusters (like M11), as it is not very concentrated - Harlow Shapley included M12 in his concentration class IX. It is, e.g., notably much less concentrated toward the center than M10 (of class VII). At its distance of about 16,700 light years, the apparent diameter of M12 of 16.0 arc minutes corresponds to about 79 light years. This stellar swarm is approaching us at 16 km/sec. The apparent proximity, together with the similar distance (M10: 16.5 kly, M12: 16.7 kly) results in a considerably small physical distance, about 930 ly (0.93 kly, 0.285 kpc).

Helen Sawyer Hogg determined the cluster's overall spectral type as F7 and gives its color index as 0.0, and the mean magnitude of the 25 brightest stars as 13.97. The brightest stars of M12 are about mag 12.0, its horizontal branch level (of giant stars) is magnitude 14.9, according to the Deep Sky Field Guide to Uranometria 2000.0.

Alan Sandage has found 13 variables in M12, according to Kenneth Glyn Jones. This number has increased meanwhile because of new discoveries: Christine Clement (2022) reports 34 variable stars. Unusual for a globular cluster, it contains no single RR Lyrae star, but one population II Cepheid variable (W Virginis star) - this is variable the V1, with a period of 15.5 days and an amplitude of 0.8 mag. 19 are eclipsing variables, mostly of short period, and 5 very-short-period small-amplitude SX Phoenicis stars. Also, she mentions 2 millisecond pulsars, both in binary systems, according to Freire (2026).

De Marchi et.al. (2006) investigated observations of M12 with ESO's Very Large Telescope, and found a significant deficit in low-mass stars in this cluster, compared to the mass distribution in average globulars; these missing stars have escaped from the cluster, and are continuously lost, because of tidal gravitational forces in the Milky Way Galaxy's gravitational field along its orbit. These stars are and will be staying in similar orbits as the cluster and form a stellar stream. Over a very long time - the authors guess 17 to 29 Gyr from now - the tidal forces may disrupt the cluster and leave only the stream.

Holger Baumgardt et.al. (2023) investigated astrometrical data from the GAIA satellite, and from the position and velocity, derived M12's simulated orbit. They found that M12 is at a distance of about 16.7 kly (5.11) kpc from us, 14.9 kly (4.57 kpc) from Galactic Center. With an orbital period about 60 Myr, it is moving between 7.0 kly (2.15 kpc) and 15.4 kly (4.71 kpc) from the Galactic Center; this correspomds to a numerical excentricity of 0,3731 its orbit - the second least excentricity for a Messier globular cluster, only M71 has a less excentric orbit. It is currently starting to close in after apogalacticum passage about 5 Myr ago, last perigalacticum was 35 Myr in the past. From fitting the Color-Magnitude Diagram (CMD), they estimate the cluster's age at 13.0 Gyr (billion years). From the velocity distribution of its stars, they calculate a total mass of 106,000 solar masses for M12, and a half-mass radius of 13.3 ly (4.08 pc).

Globular cluster M12 is easily found either 2 deg N and 2 deg W of M10, or 2 deg N and 8.5 deg E from Delta Ophiuchi.

  • Historical Observations and Descriptions of M12
  • ESO/VLT images of M12 (see De Marchi et.al. 2006)
  • More images of M12
  • Amateur images of M12

  • Marco Castellani's data for M 12
  • Christine Clement's Catalog of Variable Stars in M12
  • Holger Baumgardt's Fundamental Data for M12 (NGC 6218)
  • SIMBAD Data of M12
  • NED Data of M12
  • Publications on M12 (NASA ADS)
  • Observing Reports for M12 (IAAC Netastrocatalog)
  • NGC Online data for M12

  • Wikipedia page for Messier 12
  • Messier 12 in Hubble's Messier Catalog

    References


    Hartmut Frommert
    Christine Kronberg
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    Last Modification: July 19, 2026
    Former Definitive Version: August 21, 2007