M72 is a distant, magnitude +9.3, globular cluster located in the constellation of Aquarius. At 55,000 light years from Earth, it's way beyond the galactic center and consequently one of the faintest and smallest (apparent size) of Messier's globulars. Hence, M72 appears as a very faint diffuse ball of light in small telescopes that's easy to miss.
M72 was discovered by Pierre Méchain on August 29, 1780. His friend Charles Messier found it a few days later on October 4th and included it in his catalogue. With their instruments - good quality at the time although low quality by today's commercially standards - both Méchain and Messier had difficulty in deciding exactly what M72 was. In the end they believed it to be a faint nebula rather than a star cluster. A few years later, British astronomer Sir William Herschel using a larger instrument was able to determine the true nature of M72, resolving it into stars.
Of note, on the same night as he catalogued M72, Messier discovered M73. This curious item, located just over a degree east of M72, was described by Messier as a cluster of four stars with some nebulosity. However, it's now believed to be just an asterism of four stars with no associated nebulosity.
Locating M72:- There are several ways to locate this object but none are particularly easy; amateur astronomers often regard M72 as one of the more difficult Messier objects to find. Since it's positioned in the southern part of Aquarius towards the constellation boundary, one method is to start with neighbouring Capricornus. Once familiar with the stars and general shape of Capricornus, focus on theta (θ Cap - mag. +4.1). Then image a line from θ Cap traveling in a northwesterly direction for 9 degrees. Once there you will reach a slightly brighter star, magnitude +3.8 Albali (ε Aqr). Just over halfway along this line is M72.
The globular is best seen during the months of July, August and September and ideally from tropical, Southern Hemisphere regions where it appears high in the sky.
Due to its vast distance and faintness, even large 80mm binoculars struggle to pick out M72, appearing as just a star like point of light. Small 100mm (4-inch) telescopes fair little better, with only the brighter core region readily visible. Overall this is a tough cluster to resolve with amateur instruments and even a 250mm (10-inch) telescope only hints at resolving the extreme edges of M72. However, what is noticeable is the even brightness across the face of M72. Very large amateur telescopes of 400mm (16-inch) aperture or more show M72 looking rather like M13 as seen with a small scope.
M72 has a spatial diameter of 104 light-years and is believed to contain at least 100,000 stars. Of these, 42, a considerable number are know variables (mostly RR Lyrae stars). It's estimated to be 9.5 Billion years old.
In summary, distant globular M72 is a challenging object for amateur astronomers. It's visible in small scopes but to get the most from this far off cluster, dark skies together with a medium / large size telescope are essential. This not very compact cluster is difficult to resolve but intrinsically luminous, hence why we can seen it at all from such a far distance with small scopes.
M72 Data Table
|Object Type||Globular cluster|
|RA (J2000)||20h 53m 28s|
|DEC (J2000)||-12d 32m 13s|
|Apparent Size (arcmins)||6.6 x 6.6|
|Radius (light years)||52|
|Number of Stars||>100,000|