|Right Ascension||16 : 23.6 (h:m)
|Declination||-26 : 32 (deg:m)
|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 7,200 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 (7,500 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.
As a remarkable detail, M4 displays a central "bar" structure, well visible in our 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 58 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.
Globular cluster M4 was discovered by de Cheseaux in 1745-46 and listed by him as No. 19, and included in Lacaille's catalog as Lacaille I.9.
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.
In August 1995, the Hubble Space Telescope has photographed white dwarf stars in M4, which are among the oldest stars in our Milky Way Galaxy.
Dust clouds and nebulae around Rho Ophiuchi near Antares and M4; material of these clouds is obscuring the cluster.
Bill Arnett's Antares & Rho Ophiuchi nebula pages include M4: photo page, info page.
Last Modification: 9 Dec 1999, 22:58 MET