M10 is a fine globular cluster that's located in the constellation of Ophiuchus. One of the largest constellations, Ophiuchus straddles the celestial equator and contains a host of globular clusters of which Messier catalogued seven of them. The brightest and best of them is M10 (mag. +6.6), which can be spotted with binoculars, appearing like an out of focus fuzzy star.

Charles Messier discovered M10 on May 29, 1764, describing it as a "nebula without stars". Ten years later, German astronomer Johann Elert Bode noted it as a "very pale nebulous patch without stars". Both Messier and Bode used telescopes that suffered in quality and hence were unable to resolve the cluster. It was not until William Herschel using better and larger instruments was able to spot individual member stars. He described it as a "beautiful cluster of extremely compressed stars". The best time of the year to observe M10 is during the months of May, June and July.

Locating M10 is not the easiest task as the surrounding area of sky is devoid of bright stars. Start by locating the brightest star in Ophiuchus, Rasalhague (α Oph - mag +2.1). Join the stars of the constellation in a curve heading westwards and southwards until arriving at two close 3rd magnitude stars, Yed Prior (δ Oph - mag. +2.7) and Yed Posterior (ε Oph - mag. +3.2). M10 is located about 12 degrees east of Yed Prior with star 30 Oph (mag. +4.8) one degree east of M10.

M10 globular cluster by the Hubble Space Telescope (credit:- NASA, The Hubble Heritage Team (AURA/STScI))

Finder Chart for M10 (credit:- freestarcharts)

Finder Chart for M10 - pdf format (credit:- freestarcharts)

M10 makes a superb target for all telescopes. A small 80mm (3.1-inch) scope reveals an obvious non-stellar fuzzy ball of light spread over about 8 arc minutes. Through 150mm (6-inch) or 200mm (8-inch) instruments the globular appears dense with a large nebulous central core. A number of stars are resolvable especially in the outer halo, which extends some 15 arc minutes in diameter. A larger 300mm (12-inch) telescope reveals many more stars and adds volume to the view. On good nights, it's possible to see stars across the entire face of the cluster. The fainter and smaller globular cluster M12 is located 3 degrees northwest of M10. In total, M10 spans about 20 arc minutes of apparent sky.

M10 is 14,300 light-years distant and is estimated to be 11.4 billion years old. Although relatively close as far as globulars go, it's intrinsically small spanning only 84 light-years diameter and therefore not as spectacular as might be expected. For example, other globulars such as M13 in Hercules are a much grander sight, despite been much further away.

It's estimated M10 contains 100,000 stars. However, an extremely low number are variable stars and to date only 4 have been discovered.

M10 Data Table

Object TypeGlobular cluster
Distance (light-years)14,300
Apparent Mag.+6.6
RA (J2000)16h 57m 09s
DEC (J2000)-04d 05m 58s
Apparent Size (arc mins)20 x 20
Radius (light-years)42
Age (years)11.4 Billion
Number of Stars100,000

Sky Highlights - July 2017

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for July

Meteor Shower
Southern Delta Aquariids (Aquarids) meteor shower peaks on July 29

Northern Hemisphere
West:- Mercury (mag. -0.5 to +0.3) (second half of month)
Southwest:- Jupiter (mag. -2.0)
South:- Saturn (mag. +0.2)
West:- Jupiter
South:- Saturn
East:- Neptune (mag. +7.8)
Southwest:- Saturn
South:- Neptune
Southeast:- Uranus (mag. +5.8)
East:- Venus (mag. -4.1)

Southern Hemisphere
West:- Mercury (second half of month)
Northwest:- Jupiter
East:- Saturn
West:- Jupiter
North:- Saturn
East:- Neptune
West:- Saturn
North:- Neptune
Northeast:- Venus, Uranus

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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