Messier 2 (also designated NGC 7089) is a globular cluster in Aquarius, five degrees north of the star Beta Aquarii. It was discovered by Jean-Dominique Maraldi in 1746 and is one of the largest known globular clusters - the showpiece of the constellation!
Discovery and Observation
M 2 was discovered by the French astronomer Jean-Dominique Maraldi in 1746 while observing a comet with Jacques Cassini. Charles Messier rediscovered it in 1760, but described it as a "nebula without stars." William Herschel was the first to resolve individual stars in the cluster, in 1783.
With a visual magnitude of magnitude 6.5, Messier 2 is just visible to the naked eye under extremely good conditions. Binoculars or small telescopes will identify the cluster as non-stellar, while larger telescopes will resolve individual stars, of which the brightest are of magnitude 13.1.
Messier 2 is a large, bright globular cluster, well-compressed, with an intense core. The 6'-diameter core is symmetric, with a slight N-S elongation. There is a profusion of stars in the outer corona, and perhaps a hundred stars can be resolved across the disk against a background haze. Star chains meandering out from the core extend the halo's span to a diameter of 16'. Several dark lanes are visible, the most prominent located in the NE portion.
Physical Properties
M 2 is about 37,500 light years distant, and lies well beyond the Galactic Center It is about 175 light-years in diameter, and contains about 150,000 stars. It is one of the richer and more compact globular clusters, and is notably elliptical in shape. The dense central core has a diameter of only 3.7 light years. On the other hand, its tidal radius, beyond which member stars would escape due to gravitational forces from the Milky Way Galaxy, is 233 light years.
The age of M 2 has been estimated at about 13 billion years. M 2 is approaching us at 5.3 km/sec. It is moving on a highly eccentric (e=0.76) orbit, which carries it out to an enormous distance of 171,000 light-years from the galactic center, and 165,000 light-years above and below the Galactic plane.