This cluster forms part of the West Halo, a region that is moving outward with respect to the rest of the SMC.[7] It is located about 2.3° northwest of the SMC galactic center.[4] The cluster mass is3.6×105 times themass of the Sun.[1] The angularhalf-light radius of this cluster is27.1″,[5] and the tidal radius is143″.[3] It is positioned about~32′ from the massive globular cluster47 Tucanae, which has atidal radius of42.86′.[5]
NGC 121 is the only old globular cluster in the SMC that is similar to Galactic clusters in the Milky Way.[4] Age estimates for this cluster range from 10.5 to 11.8 billion years old, which is 2-3 billion years younger than the oldest such comparable clusters in the Milky Way.[5] The aginggiant stars in this cluster demonstrate that there are two distinct stellar populations, with the younger population being chemically enriched from the output of the first generation. The second generation forms a relatively low ~32% of the total population, but this amount is enhanced in the central portion of the cluster. This suggests the later generation is more centrally concentrated.[4][8]
^abcShara, Michael M.; et al. (December 1998). "Hubble Space Telescope Observations of NGC 121: First Detection of Blue Stragglers in an Extragalactic Globular Cluster".The Astrophysical Journal.508 (2):570–575.Bibcode:1998ApJ...508..570S.doi:10.1086/306423.
^abcdeGlatt, Katharina; et al. (April 2008). "An Accurate Age Determination for the Small Magellanic Cloud Star Cluster NGC 121 with the Hubble Space Telescope/Advanced Camera for Surveys".The Astronomical Journal.135 (4):1106–1116.Bibcode:2008AJ....135.1106G.doi:10.1088/0004-6256/135/4/1106.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^Mucciarelli, A.; et al. (September 2023). "The chemical DNA of the Magellanic Clouds. II. High-resolution spectroscopy of the SMC globular clusters NGC 121, NGC 339, and NGC 419".Astronomy & Astrophysics.677. id. A61.arXiv:2307.03470.Bibcode:2023A&A...677A..61M.doi:10.1051/0004-6361/202347120.
^Walker, Alistair R.; Mack, Peter (September 1988). "CCD Photometry of the RR Lyrae Stars in NGC 121 and the Distance to the Small Magellanic Cloud".Astronomical Journal.96: 872.Bibcode:1988AJ.....96..872W.doi:10.1086/114853.
^Fiorentino, G.; et al. (January 2008). "Detection of new variable stars in the SMC cluster NGC 121".Memorie della Società Astronomica Italiana.79: 3.arXiv:0801.2713.
Alvarado, F.; Wenderoth, E.; Alcaino, G.; Liller, W. (October 1991). "A UBVRI photoelectric sequence in the SMC globular cluster NGC 121".Astronomy and Astrophysics, Supplement Series.90: 191.Bibcode:1991A&AS...90..191A.
Da Costa, G. S.; Mould, J. R. (November 1988). "The Globular Clusters of the Dwarf Elliptical Galaxies NGC 147, NGC 185, and NGC 205. I. Abundances".Astrophysical Journal.334: 159.Bibcode:1988ApJ...334..159D.doi:10.1086/166826.
Suntzeff, N. B.; et al. (February 1986). "The stellar population of the Small Magellanic cloud near NGC 121. I. The mean metallicity, metallicity spread and radial velocity of SMC halo giants".Astronomical Journal.91:275–289.Bibcode:1986AJ.....91..275S.doi:10.1086/114007.
Stryker, L. L.; et al. (November 1985). "The main-sequence turnoff of the old SMC globular cluster NGC121".Astrophysical Journal.298:544–559.Bibcode:1985ApJ...298..544S.doi:10.1086/163639.
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