Messier 4 or M4 (also designated NGC 6121) is a globular cluster in the constellation Scorpius. It has an apparent visual magnitude of 5.6 and its angular diameter is 26.3 arc-minutes. M4 lies at an estimated distance of 7200 light years. The Equinox 2000 coordinates are RA= 16h 23.6m, Dec= -26° 32´ which makes M4 best seen during the summer. The Messier Summer Star Chart shows the position of all Messier objects visible during that season.

The image above shows the uncropped view of M4 through the Takahashi E-180 Astrograph (North is up). A 3x enlargement of this image appears to the right. The bright star on the left is Antares.

In spite of its inclusion in the Messier Catalog, this globular cluster was actually discovered by J-P. de Chéseaux in 1746. Charles Messier added it to his catalog in 1764. It was the only globular cluster that Messier was able to resolve into individual stars.

According to Recio-Blanco et al.(2005), the distance of M4 is 5640 light years and its diameter is 57 light years. Its estimated mass is 100,000 solar masses and it contains 65 variable stars.

M4 is located just 1.3° west of first magnitude star Antares or Alpha Scorpii (seen to the left in the image above). A fainter globular cluster NGC 6144 (mag 10.4, diameter 3.3') is located 0.83° ENE of M4 and just 0.33° NW of Antares. M4 is one of the nearest globular clusters and is the only one that Charles Messier could resolve into stars. Because of its position deep in the Milky Way, M4 is significantly obscurred by interstellar dust and gas.

For more information, see the Messier Catalog as well as specific entries for M4 in Wikipedia and SEDS.

Messier's Description of M4

May 8, 1764
`Cluster of very small [faint] stars; with an inferior telescope, it appears more like a nebula; this cluster is situated near Antares and on its parallel. Observed by M. de la Caille, and reported in his catalog. Reviewed January 30 and March 22, 1781. (Diam 2.5')'

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