Right Ascension	16 : 23.6 (h:m)
Declination	-26 : 32 (deg:m)
Distance	6.8 (kly)
Visual Brightness	5.6 (mag)
Apparent Dimension	26.3 (arc min)
Discovered by	Philippe Loys de Cheseaux in 1746

M4 is one of the nearest globular clusters in the sky; according to newer results (here adopted from W.E. Harris' database), its distance is perhaps only about 7,000 light years, which may be the smallest for a globular; the only serious competitor is NGC 6397 in the Southern constellation Ara, yet this one seems to be very slightly more remote now (7,200 light years). M4 can be detected by the naked eye under very dark skies (1.3 degrees west of Antares), and is prominent with the slightest optical aid.

M4 displays a central "bar" structure, visible in the photo, roughly from slightly below left to slightly above right. It 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 brown-ish appearance on color images. Its angular diameter is more than 26 minutes of arc, nearly that of the full Moon; this corresponds to a linear diameter of about 55 light years. It is one of the most open, or loose, globulars. M4 recedes at 65 km/sec and contains at least 43 known variables.

In 1987, the first millisecond pulsar was discovered in this globular cluster. This pulsar, 1821-24, is a neutron star rotating (and pulsating) once every 3.0 milliseconds, or over 300 times per second, which is even 10 times faster than the Crab pulsar in M1. A second millisecond pulsar was found in M28 later in the same year.

"Cluster of very faint stars. With a small telescope it looks like a nebula." Charles Messier from his Catalogue

Close by: Antares the red supergiant star is 1 1/2 degrees east of M4, and NGC 6144 1/2 degrees nw of Antares.

Main information courtesy S.E.D.S.