M4 is a magnificent large apparent size globular cluster located amongst the rich Milky Way star fields of the constellation of Scorpius. With an apparent magnitude of +5.6, it can be spotted with the naked eye from dark sites. Even the simplest optical aid reveals an obvious non-stellar fuzzy object. In addition, not only is M4 bright but it's also one of the easiest globulars to find. It's located just 1.3 degrees west of striking red giant Antares (α Sco – mag. +1.0), the brightest star in Scorpius.
M4 was discovered by Philippe Loys de Chéseaux in 1746, who listed it as number 19 in his catalogue. It was also included in Lacaille's catalogue as Lacaille I.9 and subsequently added by Charles Messier to his famous list on May 8, 1764. M4 holds the distinction of being the first globular cluster ever to be resolved into stars by a telescope. Messier himself achieved the task, describing it "as a cluster of very small faint stars". Incidentally, this was the only globular cluster he could manage to resolve with his modest instruments.
Scorpius is best seen from tropical and Southern Hemisphere latitudes where it appears high in the sky during May, June and July. At least part of the constellation can be seen from most Northern Hemisphere latitudes, but it never rises particularly high above the horizon.
On close naked eye inspection, M4 appears as a slightly unusual star. Through 10x50 binoculars the centre of the cluster is relatively bright surrounded by a thin halo of light that hints at resolution. In reality, M4 is an easy globular to resolve in small scopes due to its large size (26 arc minutes) and loose unconcentrated structure. On nights of good seeing, a 100mm (4-inch) scope at high magnifications resolves the entire face of the cluster into a multitude of stars. Medium to large aperture telescopes reveal a mass of stars including a strange central bar type structure from roughly below left to above right of center, which was first noted by Sir William Herschel in 1783. The bar consists of 11th magnitude stars.
The reason why M4 is bright and large is simply due to distance. At only 7,200 light-years it's a stone's throw away, making it one of the closest globular clusters to our Solar System. Currently the only other real contenders for the title of closest globular are NGC 6397 in the far southern constellation of Ara, which is about the same distance as M4 and recently discovered (in 2006) FSR 1767 globular at an estimated 4,900 light years.
M4 Data Table
|Object Type||Globular cluster|
|RA (J2000)||16h 23m 35s|
|DEC (J2000)||-26d 31m 32s|
|Apparent Size (arc mins)||26 x 26|
|Number of Stars||>20,000|
|Notable Feature||Located only 1.3 degrees west of Antares|