M4 (NGC6121)

M4 was discovered by de Chéseaux in 1746; he described it as an object "...close to Antares...white, round and smaller than the preceding ones. I don't think it has been found before..."
The Messier's description (may, 8, 1764): "Cluster of very small stars in an inferior telescope it appears like a nebula; this star cluster is situated near Antares and on the same parallel... ".
Bode and Lacaille both thought it resembled the hazy nucleus of a comet.

M4 is the nearest (7,000 light-years away), and one of the most conspicuous of all the globular clusters.
Probably it's also the easiest to locate: merely point the telescope to Antares, and then move 1.3° west.

In the binoculars, you can see M4 and Antares together; under a very clear and dark sky, the globular is even visible to the naked eye.
M4 is one of the largest and most open globular cluster (class IX, Shapley). It shows no great central condensation, and begins to show resolution into stars in a 100 mm. refractor. In good conditions, the famous 114 mm. reflector gives a superb sight.
The most conspicuous detail is a central "bar", or "chain" of 11th mag. stars, some 2.6' in lenght; this detail was noted by Herschel in 1783. With larger apertures many other chains of stars are deiscerned around the edges of the cluster (see the above image).

Greenstein, in 1939, determined magnitude and colors for 660 stars of this globular cluster; the H-R diagram based on this survey showes that M4 is reddened by about 0.8 magnitudes by dark nebulosity in the region.
Greenstein found M4 relatively poor in red giants: only 210 stars are brighter than absolute magnitude +0.5.
The average magnitude, according Helen Sawyer Hogg, is 13.11.
Baade found in M4 37 variables with a period of 0.24 days and 41 with a period of 0.25 days.
In 1987 a pulsar of millisecond frequency was discovered in M4: the first to be observed in a globular cluster.
In 1995, M4 was the target of a Hubble Space Telescope search for white dwarf stars. Hubble revealed a total of 75 white dwarfs in one small area within M4, out of the total of about 40,000 white dwarfs that the cluster is predicted to contain. The Hubble results will allow astronomers to refine theoretical predictions of the rate at which white dwarfs cool – an important prerequisite for making reliable estimates for the age of the universe and of our Milky Way galaxy, based on white dwarf temperatures.