M2, also designated NGC 7089, GC 4678, and Bode 70, is a globular cluster in the constellation Aquarius, five degrees north of the star Beta Aquarii and is one of the largest known globular clusters at 175 light-years in diameter, at right ascension 21: 33.5 (h:m) and declination -00: 49 (deg:m). Globular clusters are spherical groups of ancient stars that are bound together by their mutual gravitational attraction. Most of the cluster’s mass is concentrated at its centre, with shimmering streams of stars extending outward into space. M2 is about 55,000 light-years distant from Earth. The cluster is rich, compact, and significantly elliptical. It is 13 billion years old and one of the older globular clusters associated with the Milky Way galaxy. M2 contains about 150,000 stars, including 21 known variable stars. These are mostly RR Lyrae variables, pulsating variable stars belonging to the spectral class A (or F), typically half as massive as the Sun, and commonly found in globular clusters. RR Lyrae variables are often used as standard candles to measure galactic distances. The brightest stars in M2 are of magnitude 13.1 and mostly yellow and red giants. The cluster’s overall spectral class is F4. Because its members are so tightly packed together, it has a density classification of II – which is reserved for clusters that are particularly rich and compact. And like most globular clusters, M2’s central region is highly compressed, measuring just 3.7 light years in diameter. It’s tidal influence, on the other hand, has a radius of 233 light years, beyond which members stars would escape due to the influence of the Milky Way’s tidal forces. The overall spectral type is F4. It has an apparent magnitude of 6.3 and is bright enough that it can even be seen with the naked eye when observing conditions are extremely good. Three Cepheid variables have also been identified in the cluster. Cepheids are luminous variable stars that also serve as indicators of galactic distance scales. These stars vary between a larger, brighter state and a smaller, denser one. Cepheids were named after Delta Cephei in Cepheus constellation, the first variable star of this type to be identified. M2 is part of the Gaia Sausage, the hypothesised remains of a merged dwarf galaxy. The best time of year to observe the cluster is between the months of July and October.
M2 was first discovered by Jean-Dominique Maraldi in 1746 while observing a comet with Jacques Cassini. According to Cassini’s notes, which detail the discovery, the two believed it to be a “nebulous star” at the time: On September 11 I have observed another one [nebulous star] for which the right ascension is 320d 7′ 19″ [21h 20m 29s], and the declination 1d 55′ 38″ south, very near to the parallel where the Comet should be. This one is round, well terminated and brighter in the centre, about 4′ or 5′ in extent and not a single star around it to a pretty large distance; none can be seen in the whole field of the telescope. This appears very singular to me, for most of the stars one calls nebulous are surrounded by many stars, making one think that the whiteness found there is an effect of the light of a mass of stars too small to be seen in the largest telescopes. I took, at first, this nebula for the comet.” The object was independently recovered by Charles Messier in 1769, though he too mistook it for something else. In his notes, which were also taken on September 11th (fourteen years later), he described the object as a nebula: “On September 11, 1760, I discovered in the head of Aquarius a beautiful nebula which doesn’t contain any star; I examined it with a good Gregorian telescope of 30 pouces focal length, which magnified hundred four  times; the centre is brilliant, and the nebulosity which surrounds it is round; it resembles quite well the beautiful nebula which is located between the head and the bow of Sagittarius: It extends 4 minutes of arc in diameter; one can see it quite well in an ordinary telescope [refractor] of 2 feet [focal length]: I compared its passage of the meridian with that of Alpha Aquarii which is situated on the same parallel; its right ascension was derived at 320d 17′ and its declination at 1d 47′ south. In the night of June 26 and 27, 1764, I reviewed this nebula for a second time; it was the same, with the same appearances. This nebula can be found placed in the chart of the famous Comet of Halley, which I observed at its return in 1759 (b).” Ultimately, it was William Herschel who finally resolved Messier 2 into the object we recognize today. This took in 1783, where – according to his notes – he was able to resolve individual stars: “The scattered stars were brought to a good, well-determined focus, from which it appears that the central condensed light is owing to a multitude of stars that appeared at various distances behind and near each other. I could actually see and distinguish the stars even in the central mass. The Rev. Mr Vince, Plumian Professor of Astronomy at Cambridge, saw it in the same telescope as described.”
Credits: Messier Objects, NASA, Universe Today, Wikipedia.