M2 - Globular Cluster in Aquarius

Discovered by Jean-Dominique Maraldi in 1746.

Globular cluster Messier 2 (M2, NGC 7089) was discovered by Maraldi on September 11, 1746. Charles Messier independently rediscovered and cataloged it exactly 14 years later, on September 11, 1760, as a "nebula without stars." William Herschel was the first to resolve it into stars.

M2 has a diameter of about 175 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 6 to 8 minutes of arc in diameter, with a bright, compressed central region of about 5'. On typical photographs it can be traced to about 12.9 arc minutes, and deep photos reveal that it extends out to a diameter of 16.0 arc minutes.

As most globular clusters, M2's central part is pretty compressed: The dense central core of globular cluster M2 is only 0.34 arc minutes or about 20 arc seconds in diameter, corresponding to a diameter of 3.7 light years. Its half-mass radius is 0.93 arc minutes (56 arc seconds, or 10 light years linearly). On the other hand, its tidal radius is large: 21.45 minutes of arc, corresponding to a radius of 233 light years beyond which member stars would escape because of tidal gravitational forces from the Milky Way Galaxy.

*Much of the information regarding the Messier Objects and their origins has been graciously provided by www.seds.org/messier/