M4 is one of the nearest globular clusters in the sky; its distance is perhaps
only about 7,200 light years. M4 can be detected by the naked eye under very
dark skies (1.3 degrees west of Antares, visual brightness 5.6 mag), and is
prominent with the slightest optical aid.
As a remarkable detail, M4 displays a central "bar" structure, well
visible in my image, roughly from slightly above left to below 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. 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. M4 recedes at 65 km/sec and contains at least
43 known variables.
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.
I take this image through a Celestron refractor 102mm f/5 on Pixcel
237 CCD. Image is composite of six 20-second unguided exposures (average combine
in CCDSoft v5).
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