Messier 72

Discovered 1780 by Pierre Méchain.

Messier 72 (M72, NGC 6981) is one of the apparently smaller and fainter globular clusters in Messier's catalog. It is situated in the very western part of constellation Aquarius, close to the the group of four stars, M73.

Globular cluster M72 was discovered by Pierre Méchain in the night of August 29-30, 1780. Consequently, Charles Messier looked for it on the following October 4 and 5, and included it in his catalog.

M72 is one of the more remote of Messier's globular clusters: At about 53,000 light years, it lies a considerable distance beyond the Galactic Center. It is of 9th or 10th apparent magnitude, but as it is so distant it is one of the more intrinsically luminous globular cluster. However, M72 is not very concentrated (Shapley classified it as class IX); among Messier's globulars, only M71 (class X-XI) and M56 (class X) are even less concentrated. M72 is approaching us quite rapidly, at 255 km/sec, and has the considerable number of 42 known variables, mostly RR Lyrae stars. Its diameter is about 106 light years and appears to us under an angle of 6.6 arc minutes.

According to the Deep Sky Field Guide to Uranometria 2000.0, the brightest star in M72 is about 14.2 mag, while Kenneth Glyn Jones, quoting Helen Sawyer Hogg, gives the average of the 25 brightest stars as 15.86. The horizontal branch level magnitude is 16.9 (Uranometria 2000.0).

There are several ways to locate M72: Either find 4.5-mag 3 Aqr and 4-mag Epsilon Aqr from Delphinus; M72 is 3 deg S, 1.5 deg E of Epsilon. Or locate M73, the group of four stars, from Nu Aquarii; then M72 is 1.5 deg W and little N. Or find it 9 deg E of 4-mag Alpha Cap.

M72 is a pale nebulous patch of light, very small and of grainy texture in a 4-inch, which shows only the 2' diameter core region. Larges scopes show it to a larger extent. This globular is of notable even brightness, fainting very gradually to the edges. It is hard to resolve in amateur telescopes; in the 8-inch, only the extreme edges show suspicions of resolved stars. A close pair of stars is situated to the south of this cluster.

A 6th mag star (HD 198431 at RA 20:50.6, Dec -12:32, spectral type K1 III) is 40' to the West of M72. Situated just about the same distance beyond this star from M72 is the very faint Local Group galaxy Aquarius Dwarf (RA 20:46.9, Dec -12:51), which requires larger telescopes to be observed. To the East of M72, about 1.5 deg, Messier 73 can be found, a Y-shaped group of 4 stars which is generally considered inconspicuous.

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