M4

M4 is one of the nearest globular clusters to earth at a distance of about 7 200 light years. It can be detected by the naked eye under very dark skies, positioned 1.3 degrees west of Antares, and is prominent with every optical aid. M4 is one of the most open, or loose, globular clusters and can be easily resolved into individual stars. As a remarkable detail, which is unique for globular clusters, M4 displays a central "bar" structure, well visible in the photo. This bar consists of 11th mag stars and is about 2.5 arcminutes long at a position angle of 12°. It was first noted by William Herschel in 1783.

M4 would be one of the most brilliant globulars in the sky if it were not obscured by heavy clouds of dark interstellar matter. Interstellar absorption also reddens the color of the cluster's light, giving it a slightly brownish appearance. Its angular diameter, seen on deep photographs, is about 36 arcminutes, more than that of the Full Moon, this corresponds to a linear diameter of about 75 light years. On typical photos it appears somewhat smaller at about 26', and visually it was estimated at 14 arcminutes.

The Antares - M4 Region, refractor photograph.

Exposure Data

Instrument: TMB 4.1" f/6.2 APO
F/stop: 6.2
Exposure: Composite of two exposures, 25 + 40 minutes
Film: Fuji Provia 400F
Exposure start: 2:16 / 0:14 MESZ
Date: May 30, 2003 / June 6, 2003
Location: Ebenwaldhöhe, Lower Austria
Conditions: Visual limiting magnitude 5.5 mag, seeing 3, temperature 16.5°C, no wind, no dew / visual limiting magnitude 5.9 mag, seeing 4, temperature 15.5°C, slight wind, no dew
Autoguider: Meade Pictor 216 XT, max. error 1 / 2
Enhancement: Adobe Photoshop 7.0

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