Right Ascension	21 : 33.5 (h:m)
Declination	-00 : 49 (deg:m)
Distance	37.5 (kly)
Visual Brightness	6.5 (mag)
Apparent Dimension	12.9 (arc min)

Discovered by Jean-Dominique Maraldi in 1746.

M2 has a diameter of about 150 light-years, contains about 150,000 stars, and is one of the richer and more compact globular clusters, as its classification in density class II indicates. This cluster is of notable ellipticity (ellipticity 9, or form E1), as can be noted in our photograph; it is extended in position angle 135 deg. At about 37,500 light years (according to W.E. Harris' database), it lies well beyond the Galactic Center. Visually it is of apparent magnitude 6.5 and about 7 minutes of arc in diameter, with a bright, compressed central region of about 5'. Photos reveal that it extends out to a diameter of 12.9 arc minutes. Its brightest stars are red and yellow giants of magnitude 13.1, while its horizontal branch stars have an apparent brightness of 16.1. The cluster's overall spectral type has been given with F0.

Of its 21 known variables, most are so-called "cluster variables" of RR Lyrae type, with short periods of less than a day. Three of them, however, are "classical" Cepheids of type II (W Virginis stars) with periods of 15.57, 17.55 and 19.30 days and about 13th magnitude; these have been studied by H. Arp and G. Wallerstein. One variable is a RV Tauri star whose apparent magnitude varies between 12.5 and 14.0 with a 69.09 day period; this star has alternating deep and shallow minima, and was discovered in 1897 by the French amateur A. Chevremont.

M2 had been discovered by Maraldi on September 11, 1746; Messier independently rediscovered it exactly 14 years later, on September 11, 1760. William Herschel was the first to resolve it into stars.

With its visual magnitude of 6.5 mag, M2 is a difficult object for naked-eye observing (just not visible under "average" conditions), but an easy target for the slightest optical aids like binoculars or opera glasses. A four-inch obstruction-free telescope (refractor or schiefspiegler) doesn't resolve this cluster, but only shows some of the brightest member stars spread over the mottled nebulous background image caused by the unresolved stars. With an 8-inch, this globular cluster is partly resolved into stars, well into the center under good viewing conditions. Larger scopes, 10-inch up, fully resolve this cluster. A peculiar dark lane crosses the north-east edge of the cluster, suggestions of which are visible in our image; larger telescopes (16-inch up) show several other, less prominent darker features or regions.

M2 is found rather easily from Alpha and Beta Aquarii, as well as Epsilon Pegasi. It is 5 degrees north of Beta Aquarii, on the same declination as Alpha Aqr.

Observing Reports for M2 (IAAC Netastrocatalog)

References:

• Harris, William E., Catalogue of Milky Way Globular Cluster Parameters. AJ, 112 (1996), 1487. Revision of June 22, 1999. Also note our Milky Way Globular Clusters online catalogue using these data.

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Hartmut Frommert (spider@seds.org)
Christine Kronberg (smil@lrz.uni-muenchen.de)

Last Modification: 9 Dec 1999, 22:58 MET