Messier 4

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M4
Observation data: J2000 epoch
Globular Cluster M4
Class:	IX
Constellation:	Scorpius
Right ascension:	16^h 23^m 35.41^s^[1]
Declination:	-26° 31′ 31.9″^[1]
Distance:	7.2 kly (2.2 kpc)
Apparent magnitude (V):	+7.12^[1]
Apparent dimensions (V):	36′.0
Physical characteristics
Mass:	( M_☉)
Radius:	35 light years
V_HB:
Estimated age:	13 billion years
Notable features:	1°.3 west of Antares
Other designations:	NGC 6121^[1]
See also: Globular cluster, List of globular clusters

Messier 4 (also known as M4, Globular Cluster M4 or NGC 6121) is a globular cluster in the constellation Scorpius. It was discovered by Philippe Loys de Chéseaux in 1746 and catalogued by Charles Messier in 1764.

M4 was the first globular cluster in which individual stars were resolved. Just visible to the naked eye, M4 is conspicuous in even the smallest of telescopes as a fuzzy ball of light. It is also the one of the easiest globular clusters to find with it only being 1.3 degrees west of the bright star Antares with both objects being visible in a wide field telescope. Modestly sized telescopes will begin to resolve individual stars of which the brightest in M4 are of apparent magnitude 10.8.

At the distance of 7,200 light-years that has been determined for M4 it is perhaps the closest globular cluster to our Solar system. It is also a rather loosely concentrated cluster of class IX. M4 appears about the same size as the Moon on the sky which, given its distance, yield a spatial dimension of some 75 light years across. At least 43 variable stars have been observed in this cluster.

In 1987 a millisecond pulsar was discovered in M4 with a period of 3.0 milliseconds or about ten times faster than the Crab Pulsar. Photographs taken with the Hubble Space Telescope in 1995 have revealed white dwarf stars in M4 that are among the oldest known stars in the Milky Way Galaxy at an age of 13 billion years. One such white dwarf has been found to be a binary star with a pulsar companion, PSR B1620-26 and a planet orbiting it with a mass of 2.5 times that of Jupiter.