Messier 4, NGC 6121

Messier 4 (NGC 6121), in Scorpius, is one of the closest and brightest globular clusters in the sky.

M 4 was discovered by Philippe Loys de Cheseaux in 1746, and included in Lacaille's catalog as Lacaille I.9. Charles Messier cataloged it in 1764, and was the first to resolve it into a "cluster of very small [faint] stars." M 4 was the only globular cluster Messier could resolve with his moderate instruments, and thus the first globular cluster to be resolved. About 20 years later, William Herschel was able to resolve all of Messier's globulars with his larger telescopes.

Amateur Observation

M 4 is one of the easiest globular clusters to find, located only 1.3° west of Antares; both objects are visible in a wide field telescope. It is a round diffuse patch in binoculars, and even a 4-inch telescope will begin to resolve individual stars, the brightest of which are of magnitude 10.8.

M 4 is a bright mass of glittering stars. It displays a central "bar" of 11th-magnitude stars about 2-1/2' long, first noted by William Herschel in 1783, running north to south through the cluster's center. Its many jagged star chains extend out to a diameter of 16'. The core is bright, though less concentrated than those of other globular clusters, and well-resolved. In deep photographs, M 4 appears 36' across - larger than the full Moon!

Nearby (50' to the ENE) and even closer to Antares (only 30' NW), the fainter globular cluster NGC 6144 (magnitude 10.4, 3.3' in diameter) can be found. To observe it, Antares should be excluded from the field of view, so that it cannot outshine that faint globular.

Properties and Evolution

M 4 is estimated to be only 7,200 light years away, and might well be the nearest globular to Earth. Its angular diameter, as seen on deep photographs, corresponds to a linear diameter of 75 light years. The tidal radius, where gravitational forces of the Milky Way would cause member stars to escape, is estimated at about 70 light-years, so this globular dominates a spherical volume 140 light-years in diameter.

M 4 would be one of the most splendid globulars in the sky if it were not obscured by heavy clouds of dark interstellar matter. Interstellar absorption also reddens the color of the light from the cluster, and gives it a slightly orange or brownish appearance on color images. M 4 is rather loosely concentrated cluster, decidedly below average in luminosity; its absolute magnitude of -6.8 correspond to a modest luminosity of 44,000 suns.

M 4 is receding from us at 70.4 km/sec, and contains at least 43 known variable stars. The cluster has an abundance of iron relative to hydrogen that is about 8.5% of the Sun's. With an estimated age of 12.2 billion years, M 4 is one of the oldest known globulars, and may have undergone at least two separate cycles of star formation.

M 4 is following an eccentric orbit through the Milky Way, with e=0.80. The orbital is inclined 23° from the galactic plane. When passing through the Milky Way's disk, the cluster undergoes tidal shock during each passage, which repeatedly sheds stars; thus the cluster may have been much more massive in the past.

In 1987, a millisecond pulsar was discovered in M 4 with a period of 3.0 milliseconds - about ten times faster than the Crab Nebula pulsar. Photographs taken with the Hubble Space Telescope in 1995 revealed white dwarf stars in M 4 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 part of a triple system, with a pulsar companion, PSR B1620-26, and a planet of 2.5 times the mass of Jupiter, in orbit around it.