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 the Messier globulars. In small telescopes it appears as only a dim ball of light, 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 and subsequently included it in his catalogue. With their telescopes - which were good quality at the time but not comparable to today's amateur instruments - 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, Sir William Herschel using larger and better reflectors resolved it into stars and determined its true nature.
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 without nebulosity.
There are several ways to locate M72 but none are particularly easy, with many astronomers regarding it as one of the more difficult Messier objects to find. Since it's located towards the constellation boundary, one method is to start with neighbouring Capricornus. First focus on theta Capricorni (θ Cap - mag. +4.1) and then image a line moving in a northwesterly direction for about 9 degrees to Albali (ε Aqr - mag. +3.8). Just over halfway along this line is M72.
The globular is best seen from tropical and Southern Hemisphere locations during the months of July, August and September.
Due to its faintness even large 80mm binoculars struggle to pick out M72, where it appears at best 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 for most amateur instruments and even 250mm (10-inch) scopes only hint at resolving the extreme edges. However, what is noticeable is the brightness across the face of M72 is even. With very large amateur telescopes of 400mm (16-inch) aperture or more, this object appears rather like M13 when seen through small scopes.
M72 has a spatial diameter of 104 light-years and is believed to contain at least 100,000 stars. Of these a considerable number, 42, are known variables (mostly RR Lyrae stars). It's estimated to be 9.5 billion years old.
M72 Data Table
|Object Type||Globular cluster|
|RA (J2000)||20h 53m 28s|
|DEC (J2000)||-12d 32m 13s|
|Apparent Size (arc mins)||6.6 x 6.6|
|Age (years)||9.5 Billion|
|Number of Stars||>100,000|