Messier 72: Globular Cluster in Aquarius

Messier 72 is a modest globular cluster in the western corner of Aquarius. It is small and faint, but it gives you a real sense of depth: it lies far beyond the center of our galaxy, and its light has been traveling for more than 50,000 years.

What it is

M72 is a globular cluster about 54,300 light-years away, one of the most distant in Messier's list. Its 6.6-arcminute glow corresponds to roughly 100 light-years across. To look as bright as it does from so far away, it must be a fairly luminous cluster. It is also loosely packed; SEDS notes that only a few Messier globulars, such as M71 and M55, are less concentrated. Astronomers have confirmed 49 variable stars in it, most of them RR Lyrae stars. Its neighbor in the sky, M73, is not a cluster at all but a small pattern of four stars.

When to see it

M72 is best on late summer and early autumn evenings. It crosses the meridian around midnight in early August and around 10 p.m. in early September. From 40°N it reaches only about 37° above the southern horizon, so pick a night with a clear southern sky. From the Southern Hemisphere it passes high overhead.

How to find it

Start at Algedi (Alpha Capricorni), the wide naked-eye double at the northwest corner of Capricornus, about 22° south of Altair. M72 lies almost due east of Algedi, at the same declination, about 8½° away. Another route starts at Epsilon Aquarii (Albali, magnitude 3.8): M72 is about 3½° south-southeast of it. Once you have M72, M73 is about 1⅓° farther east.

What you will see

Binoculars show M72 at best as a faint, nearly star-like smudge under dark skies. A 70–100 mm telescope shows a small, round, evenly lit patch of haze about 2 arcminutes wide, with no stars resolved. In a 200 mm telescope the glow grows and gains a grainy texture, and at high power the faintest outer stars may just begin to flicker into view; its brightest stars are around 14th magnitude. Under suburban skies it is a dim, small target. It is one of the harder globulars to resolve, so aperture pays off here more than for most.

History

Pierre Méchain discovered M72 on the night of August 29–30, 1780. Charles Messier looked for it on October 4 and 5 of that year, saw it as a faint nebula, and added it to his catalog. On the same occasion he measured the nearby four-star group that became M73.