Globular Cluster Messier 2

Messier 2
Messier 2: Globular cluster in Aquarius; Takahashi Mewlon 250 CR (f=2500 mm / f10), SBIG ST-8300; 20L x 600 sec 1×1, 8R, 13G, 14B 2×2 x 600 secs; Bernese Highlands; © 2016 Bernhard Blank, Dragan Mihajlovic

History

The beautiful, bright globular star cluster Messier 2 was first sighted by G. Maraldi in 1746 when he was looking for Comet Chéseaux. At the time, the appearance of the fleeting tail stars was more popular than it is today. Any nebulous items found during their search and pursuit were entered in the star map simply to avoid confusion with comets. In 1760 Charles Messier came across this globular cluster again and added it to his list as the second comet-like object.

Physical Properties

Messier 2
Messier 2: Image taken with the Hubble Space Telescope [222]

Visually, M 2 appears as a spherical star cluster with an angular diameter of about 7 arcminutes - but stars up to a diameter of 11 arcminutes can still be seen in photographs. The total integrated brightness is about 6.4 mag, the brightest stars reach about 13.1 mag, which is about three magnitudes above that of the horizontal branch in the HR diagram.

At a distance of around 50'000 light years, the globular cluster is a good bit further away than M 13 in the constellation Hercules or M 5 in Serpentis Caput. The current diameter is given as about 150 light years. With around 100'000 members, it is one of the richer globular clusters. With a Shapley/Sawyer concentration class of 2, M 2 is one of the more compact star clusters. It is particularly impressive due to its location in a region of the sky that is relatively poor in stars. The brightest stars are yellow and red giants with an absolute magnitude of -3 mag. The total absolute brightness of M 2 is -10 mag. The integrated spectral class of the cluster is F0. The radial speed is very low at around 3 km/s. [4]

«Revised New General Catalogue and Index Catalogue» Dr. Wolfgang Steinicke, 2021 [277]
DesignationNGC 7089
TypeGCL (II)
Right Ascension21h 33m 27.2s
Declination-00° 49' 22"
Diameter16.00 arcmin
Visual magnitude6.6 mag
Dreyer Description!!, globular, B, vL, gpmbM, rrr, st eS
IdentificationM 2, GCL 121

Finder Chart

The globular cluster Messier 2 is located in the constellation Aquarius, roughly five degrees north of the 2.9 mag bright star Sadalsuud (β Aquarii) about the same declination as the two stars Sadalmelik (α Aquarii, 3.0 mag) and Sadachbia (γ Aquarii, 3.8 like). As with almost all retrieval procedures, the choice of a large field eyepiece with the highest possible field of view (more than one degree) is recommended. In the months of August to October, M 2 is highest in the sky at night.

Chart Messier 2
Chart created using SkySafari 6 Pro and STScI Digitized Sky Survey. [149, 160]

References

4«Burnham's Celestial Handbook: An Observer's Guide to the Universe Beyond the Solar System» by Robert Burnham; Dover Publications, Inc.; Voume I: ISBN 0-486-23567-X; Volume II: ISBN 0-486-23568-8; Volume III: ISBN 0-486-23673-0
149SkySafari 6 Pro, Simulation Curriculum; skysafariastronomy.com
160The STScI Digitized Sky Survey; archive.stsci.edu/cgi-bin/dss_form
222The largest of its kind; esahubble.org/images/potw1913a (2021-01-10)
277«Historische Deep-Sky Kataloge» von Dr. Wolfgang Steinicke; klima-luft.de/steinicke (2021-02-17)