OJBs Web Site: M4

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M4

Basic information about M4: a globular cluster (class IX, indicating a very open cluster) in Scorpius, also known as NGC 6121. Its right ascension 16:23.6, and declination is -26:32. The distance is 7200 light years (making it one of the closest globular clusters). Magnitude is 5.6, and diameter is 26.3 arc minutes (apparent diameter similar to the full Moon, true diameter is about 70 light years).

A very easy object close to Antares (just over 1° away) and visible to the naked eye if the sky is dark. Red colored stars (note the picture above isn't color) are easily resolved with a relatively small telescope. Seems to show a "bar" of stars in the center (see both photographs above). This cluster is quite large but the brightness is relatively low.

The left photo above is taken at a professional observatory and the one on the right was one I took through a 30cm f/5 reflector. I used Fuji P1600D professional slide film push processed to 3200 asa. The exposure time was 3 minutes - I think a longer exposure would have been useful.

To find M4 you need find the constellation Scorpius. This should be easy because it is one of the largest and brightest in the sky. The brightest star in Scorpius is the red supergiant Antares (apparent magnitude 1.03, true brightness 387000 Suns, diameter 2700 times the Sun!). Next to this, towards the "claw" end of the Scorpion is Sigma Scorpii. M4 is about half way between these two and a bit to the south as shown on the chart above.

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