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 - August 2017

Total Solar Eclipse
Total Solar Eclipse of August 21st

Meteor Shower
Perseids meteor shower peaks on August 12th

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for August

Northern Hemisphere
Evening
West:- Mercury (mag. +0.4) (start of month)
Southwest:- Jupiter (mag. -1.9)
South:- Saturn (mag. +0.3)
East:- Neptune (mag. +7.8)
Midnight
Southwest:- Saturn
Southeast:- Neptune
East:- Uranus (mag. +5.8)
Morning
Southwest:- Neptune
South:- Uranus
East:- Venus (mag. -4.0)

Southern Hemisphere
Evening
West:- Mercury (first half of month)
Northwest:- Jupiter
North:- Saturn
East:- Neptune
Midnight
West:- Jupiter
Northwest:- Saturn
North:- Neptune
East:- Uranus
Morning
West:- Neptune
North:- Uranus
Northeast:- Venus

Deep Sky

Small telescopes:-
Messier 13 - M13 - Great Hercules Globular Cluster
Messier 92 - M92 - Globular Cluster
Messier 11 - M11 - The Wild Duck Cluster (Open Cluster)
Messier 7 - M7 - The Ptolemy Cluster (Open Cluster)
Messier 6 - M6 - The Butterfly Cluster (Open Cluster)
Messier 4 - M4 - Globular Cluster
Messier 8 - M8 - Lagoon Nebula (Emission Nebula)
Messier 16 - M16 - Eagle Nebula (Emission Nebula with Open Cluster)
Messier 20 - M20 - Trifid Nebula (Emission and Reflection Nebula)

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