M4, Globular Cluster in Scorpius

Image by Jim Dixon
M4 is one of the nearest globular clusters in the sky; according to newer results , 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.

M4 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, as its classification in concentration class IX indicates. M4 recedes from us at 65 km/sec and contains at least 43 known variables.