|Right Ascension||20 : 53.5 (h:m)
|Declination||-12 : 32 (deg:m)
|Visual Brightness||9.3 (mag)
|Apparent Dimension||5.9 (arc min)
M72 is one of the more remote of Messier's globular clusters: At about 53,000 light years, it lies a considerably 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 a bit more than 90 light years.
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 E 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
larger. 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.
Last Modification: 30 Jul 1999, 22:40 MET