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). 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.
From its color-magnitude diagram, Halton Arp (1962) has estimated the age of M2 as about 13 billion years and to be about the same as that of globular clusters M3 and M5.
According to Christine Clement (2012), 42 variable
stars have been discovered in globular cluster M2 to that time. Of
these 42 known variables, the first two have been discovered by Bailey
in 1895 (Pickering and Bailley 1895), and a total of 8 until 1897, 10
to 1902. Most of them are so-called "cluster variables" of RR Lyrae
type, with short periods of less than a day (23 of type RR0, 15 of type
RR1). Three of them, however, are "classical" Cepheids of type II (W
Virginis stars) with periods of 15.57, 17.55 and 19.30 days
respectively, and an apparent visual brightness of about 13th
magnitude. These stars have been studied by H.C. Arp
(1955) and G. Wallerstein (1970). One variable (No.
11) is a RV Tauri star whose apparent magnitude varies around mag 12.1
with an amplitude of 1.1 mag and a period of 67.0 days; this star has
alternating deep and shallow minima, and was discovered in 1897 by the
French amateur A. Chèvremont. It lies at the eastern edge of the
cluster, slightly toward the North. This star is the brightest variable
known in M2, an orange-red giant beyond its red-giant Mira variable
stage (a "post-AGB star"). It is supposed to rapidly approach the end
of its nuclear life, having ejected most of it envelope, and in process
to generate a proto-planetary nebula; it is thought to develop to a
planetary nebula and eventually a white dwarf star considerably soon
(in astronomical time scales). RV Tauri stars are also found in the
Messier globular cluster M5, M56, and
possibly M28. Pulsars have been searched, but not been found
in M2 up to now.
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