Messier 4

M 4
Messier 4 by Hubble Space Telescope; 3.5′ view Credit: NASA/STScI/
Observation data (J2000 epoch)
Class	IX
Constellation	Scorpius
Right ascension	16h 23m 35.22s
Declination	–26° 31′ 32.7″
Distance	7.2 kly (2.2 kpc)
Apparent magnitude (V)	+5.9
Apparent dimensions (V)	26′.0
Physical characteristics
Mass	7004670000000000000♠6.7×104 M☉
Radius	35 light years
Metallicity	${\displaystyle {\begin{smallmatrix}\left[{\ce {Fe}}/{\ce {H}}\right]\end{smallmatrix}}}$ = –1.07dex
Estimated age	(12.2 ± 0.2) Gyr
Notable features	1°.3 west of Antares
Other designations	NGC 6121
See also: Globular cluster, List of globular clusters

Messier 4 or M4 (also designated NGC 6121) is a globular cluster in the constellation of Scorpius. It was discovered by Philippe Loys de Chéseaux in 1745 and catalogued by Charles Messier in 1764. It was the first globular cluster in which individual stars were resolved.

M4 is conspicuous in even the smallest of telescopes as a fuzzy ball of light. It appears about the same size as the Moon in the sky. It is one of the easiest globular clusters to find, being located only 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.

M4 is a rather loosely concentrated cluster of class IX and measures 75 light years across. It features a characteristic "bar" structure across its core, visible to moderate sized telescopes. The structure consists of 11th magnitude stars and is approximately 2.5' long and was first noted by William Herschel in 1783. At least 43 variable stars have been observed within M4.

M4 is approximately 7,200 light years away, the same distance as NGC 6397, making these the two closest globular clusters to the Solar System. It has an estimated age of 12.2 billion years.

In astronomy, the abundance of elements other than hydrogen and helium is called the metallicity, and it is usually denoted by the abundance ratio of iron to hydrogen as compared to the Sun. For this cluster, the measured abundance of iron is equal to:

This value is the logarithm of the ratio of iron to hydrogen relative to the same ratio in the Sun. Thus the cluster has an abundance of iron equal to 8.5% of the iron abundance in the Sun. Based upon the abundance measurements, there is evidence that this cluster hosts two distinct stellar populations. Each of the populations is a group of stars that all formed at about the same time. Thus the cluster may have undergone at least two separate cycles of star formation.