Astronomy, Guide to Space

Messier 2

20 May , 2009 by

Object Name: Messier 2
Alternative Designations: NGC 7089, GC 4678, Bode 70
Object Type: Class II Globular Cluster
Constellation: Aquarius
Right Ascension: 21 : 33.5 (h:m)
Declination: -00 : 49 (deg:m)
Distance: 37.5 (kly)
Visual Brightness: 6.5 (mag)
Apparent Dimension: 16.0 (arc min)


Locating Messier 2: Messier 2 is located approximately 5 degrees (about 3 finger widths) north of Beta Aquarii, on the same declination as Alpha Aquarii. In binoculars it will appear as a large, fuzzy “ball” with little or no resolution. In a small telescope, it will begin resolution of individual stars around the outside edges and resolution improves significantly as aperture approaches 6″ or more. M2 is sufficiently bright enough to be seen in urban lighting conditions and can alternately be found by looking about 10 degrees (a fist width) south/southwest of Epsilon Peg (Enif). For those with large telescopes, look for a dark dust lane which crosses the north-east edge of this globular cluster. It’s a nice observing challenge!

What You Are Looking At: As one of the largest known globular clusters, Messier 2 is a rich, round concentration of gravitationally bound stars which orbits the galactic core and has a diameter of about 175 light-years. M2 is believed to contain about 150,000 stellar members. Because its members are so tightly packed together, it has a density classification of II. Positioned well beyond the galactic center, M2’s tidal influence spans about 233 light years and it is believed to be as much as 13 billion years old. Inside are at least 21 known variable stars, most of RR Lyrae type, with short periods of less than a day. Three are classic Cepheids and one variable is a RV Tauri star – located at eastern edge of the cluster, slightly north.

History: M2 was first discovered by Jean-Dominique Maraldi in 1746 while observing a comet with Jacques Cassini whose notes state: “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 center, 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.”

Later, the object was independently recovered by Charles Messier who wrote: “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 [104] times; the center 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).”

However, it was William Herschel who finally resolved Messier 2 into the object we recognize today. Say Herschel’s notes: “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.”

Of course, John Herschel saw it as “It is like a heap of fine sand!” which is perhaps as apt an description in a large telescope as can be rendered!

B&W image thanks to Palomar Observatory, courtesy of Caltech, color image courtesy of Hubble Space Telescope.

Tammy was a professional astronomy author, President Emeritus of Warren Rupp Observatory and retired Astronomical League Executive Secretary. She’s received a vast number of astronomy achievement and observing awards, including the Great Lakes Astronomy Achievement Award, RG Wright Service Award and the first woman astronomer to achieve Comet Hunter's Gold Status. (Tammy passed away in early 2015... she will be missed)

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