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.

Messier 4 globular cluster by Hubble Space Telescope (credit:- NASA, The Hubble Heritage Team (AURA/STScI))

Finder Chart for M4 (credit:- freestarcharts)

Finder Chart for M4 - pdf format (credit:- freestarcharts)

Finder Chart for M6 (also shown M4, M7, M8, M19, M28, M62 and M69) (credit:- freestarcharts)

Finder Chart for M6 (also shown M4, M7, M8, M19, M28, M62 and M69) - pdf format (credit:- freestarcharts)

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

Messier4
NGC6121
Object TypeGlobular cluster
ConstellationScorpius
Distance (light-years)7,200
Apparent Mag.+5.6
RA (J2000)16h 23m 35s
DEC (J2000)-26d 31m 32s
Apparent Size (arc mins)26 x 26
Radius (light-years)27
Age (years)12,200M
Number of Stars>20,000
Notable FeatureLocated only 1.3 degrees west of Antares

Sky Highlights - March 2017

Comet
Comet 41P/Tuttle-Giacobini-Kresak now visible with binoculars as it heads towards perihelion

Mercury
Mercury heading towards greatest elongation east

Minor Planet
Vesta now visible with binoculars and small telescopes.

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for March 2017

Northern Hemisphere
Evening
West:- Venus (mag. -4.8 to -4.1 - first half of month), Mars (mag. +1.3 to +1.5), Uranus (mag. +5.9), Mercury (mag. -1.5 to -0.4 - second half of month)
Midnight
Southeast:- Jupiter (mag. -2.3 to -2.5)
Morning
Southwest:- Jupiter
Southeast:- Saturn (mag. +0.5)

Southern Hemisphere
Evening
West:- Venus (first half of month), Mars, Uranus
Midnight
North:- Jupiter
East:- Saturn
Morning
West:- Jupiter
Northeast:- Saturn
East:- Neptune (mag. +8.0 - second half of month)

Deep Sky
Naked eye / binoculars:-
Melotte 111 - Mel 111 - The Coma Star Cluster (Open Cluster)
Messier 44 - M44 - The Praesepe (Open Cluster)

Telescopes:-
Messier 67 - M67 - Open Cluster
Messier 51 - M51 - The Whirlpool Galaxy (Spiral Galaxy)
Messier 97 - M97 - The Owl Nebula (Planetary Nebula)
Messier 101 - M101 - The Pinwheel Galaxy (Spiral Galaxy)
Messier 65 – M65 – Spiral Galaxy
Messier 66 - M66 - Intermediate Spiral Galaxy
Messier 95 - M95 - Barred Spiral Galaxy
Messier 96 - M96 - Intermediate Spiral Galaxy
NGC 4244 - Spiral Galaxy
NGC 4565 - Needle Galaxy - Spiral Galaxy

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