M2 is, under extremely good conditions, just visible to the naked eye.Binoculars or asmall telescope will identify this cluster as non-stellar, while larger telescopes will resolve individual stars, of which the brightest are ofapparent magnitude 6.5.[12]
M2 is about 55,000light-years distant fromEarth. At 175 light-years in diameter, it is one of the larger globular clusters known. The cluster is rich, compact, and significantlyelliptical. It is 12.5 billion years old and one of the older globular clusters associated with theMilky Way galaxy.[7]
Data fromGaia has led to the discovery of an extended tidalstellar stream, about 45 degrees long and 300 light-years (100 pc) wide, that is likely associated with M2. It was possibly perturbed due to the presence of theLarge Magellanic Cloud.[13]
Messier 2 is located within our Milky Way galaxy, and is one of the oldest clusters of stars designated to the Milky Way. Like most globular clusters, M2 is found within thegalactic halo, specifically in the southern galactic cap. This places it right below the southern pole of the Milky Way.[14]
M2 is defined as an Oosterhoff type II globular cluster. Oosterhoff type is a classification system of globular clusters originally observed byPieter Oosterhoff in where globular clusters are generally separated into two types. Oosterhoff type is determined bymetallicity, age, and average pulsation period of type abRR Lyrae variable stars of the cluster. A cluster metallicity below −1.6, an age above 13 billion years,[15] and an average RRab Lyrae pulsation period around .64 days indicates a type II cluster.[16] This .64 day value, coupled with a metallicity of −1.65, provides evidence that M2 follows the Oosterhoff Gap phenomena. This is an observed gap in the grouping of type I and type II clusters in the Milky Way on a metallicity vs average RRab pulsation period plot.[17]
M2 is a bit of an anomaly in reference to Oosterhoff type. While it satisfies the metallicity and RRab Lyrae pulsation period conditions, it actually has an age of 12.5 Gyr, well below the cutoff age of 13 Gyr normal for a Oosterhoff type II cluster. This is unexpected because age of a cluster is generally determined from metallicity. However, this abnormality is explained in an article by Marín-Franch.[7]
^Shapley, Harlow; Sawyer, Helen B. (August 1927). "A Classification of Globular Clusters".Harvard College Observatory Bulletin.849 (849):11–14.Bibcode:1927BHarO.849...11S.
^Helmi, A; van Leeuwen, F; McMillan, P J; Massari, D; Antoja, T; Robin, A; Lindegren, L; Bastian, U (2018). "Gaia Data Release 2: Kinematics of globular clusters and dwarf galaxies around the Milky Way".Astronomy and Astrophysics.616. Gaia Collaboration: A12.arXiv:1804.09381.Bibcode:2018A&A...616A..12G.doi:10.1051/0004-6361/201832698.S2CID260496632.
^"Messier 2".SEDS Messier Catalog. Retrieved21 July 2024.
^Stobie, R. S. (1971). "On the Difference Between the Oosterhoff Types i and II Globular Clusters".The Astrophysical Journal.168: 381.Bibcode:1971ApJ...168..381S.doi:10.1086/151094.
^Kuehn, Charles A.; Smith, Horace A.; Catelan, Marcio; Jeon, Young-Beom; Nemec, James M.; Walker, Alistair R.; Kunder, Andrea; Dame, Kyra; Pritzl, Barton J.; De Lee, Nathan; Borissova, Jura (2013-10-01). "RR Lyrae in the LMC: Insights Into the Oosterhoff Phenomenon".arXiv:1310.0553 [astro-ph.SR].
