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NGC 1261
NGC 1261
	NGC 1261 from Hubble Legacy Archive
Observation data (J2000.0 epoch)
Class	II
Constellation	Horologium
Right ascension	03h 12m 16.21s
Declination	–55° 12′ 58.4″
Distance	53.5 kly (16.4 kpc)
Apparent magnitude (V)	8.63
Apparent dimensions (V)	12.9′
Physical characteristics
Absolute magnitude	−7.81
Mass	3.41×105 M☉
Metallicity	= −1.38±0.14 dex
Estimated age	10.24 Gyr
Other designations	GCL 5, ESO 155-SC11, Caldwell 87, Melotte 19
	See also: Globular cluster, List of globular clusters

NGC 1261 (also known as Caldwell 87) is a globular cluster of stars in the southern constellation of Horologium, first discovered by Scottish astronomer James Dunlop in 1826. The cluster is located at a distance of 53 kilolight-years from the Sun, and 59 kilolight-years from the Galactic Center.^[3] It is about 10.24^[7] billion years old with 341,000^[6] times the mass of the Sun. The cluster does not display the normal indications of core collapse, but evidence suggests it may have instead passed through a post core-collapse bounce state within the past two billion years. The central luminosity density is 2.22 L_☉·pc⁻³, which is low for a globular cluster.^[8] Despite this, it has a Shapley–Sawyer Concentration Class of II,^[1] indicating a dense central concentration.

A total of 22 RR Lyrae variables have been discovered in this cluster, along with two long-period variables, three SX Phoenicis variables, and an eclipsing binary.^[9] It was determined to be an Oosterhoff type I cluster, based on the periods of fourteen of the RR Lyrae variables.^[10] 18 probable blue straggler candidates have been identified.^[5]

References

^ ^a ^b Shapley, Harlow; Sawyer, Helen B. (August 1927), "A Classification of Globular Clusters", Harvard College Observatory Bulletin, 849 (849): 11–14, Bibcode:1927BHarO.849...11S.
^ ^a ^b Goldsbury, Ryan; et al. (December 2010), "The ACS Survey of Galactic Globular Clusters. X. New Determinations of Centers for 65 Clusters", The Astronomical Journal, 140 (6): 1830–1837, arXiv:1008.2755, Bibcode:2010AJ....140.1830G, doi:10.1088/0004-6256/140/6/1830, S2CID 119183070.
^ ^a ^b Paust, Nathaniel E. Q.; et al. (February 2010), "The ACS Survey of Galactic Globular Clusters. VIII. Effects of Environment on Globular Cluster Global Mass Functions", The Astronomical Journal, 139 (2): 476–491, Bibcode:2010AJ....139..476P, doi:10.1088/0004-6256/139/2/476, hdl:2152/34371, S2CID 120965440.
^ ^a ^b "NGC 1261". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2020-04-08.
^ ^a ^b ^c Kravtsov, V.; et al. (June 2010), "Wide-field multi-color photometry of the Galactic globular cluster NGC 1261", Astronomy and Astrophysics, 516: 8, arXiv:1005.0007, Bibcode:2010A&A...516A..23K, doi:10.1051/0004-6361/200913449, S2CID 55542820, A23.
^ ^a ^b Boyles, J.; et al. (November 2011), "Young Radio Pulsars in Galactic Globular Clusters", The Astrophysical Journal, 742 (1): 51, arXiv:1108.4402, Bibcode:2011ApJ...742...51B, doi:10.1088/0004-637X/742/1/51, S2CID 118649860.
^ ^a ^b Forbes, Duncan A.; Bridges, Terry (May 2010), "Accreted versus in situ Milky Way globular clusters", Monthly Notices of the Royal Astronomical Society, 404 (3): 1203–1214, arXiv:1001.4289, Bibcode:2010MNRAS.404.1203F, doi:10.1111/j.1365-2966.2010.16373.x, S2CID 51825384.
^ Simunovic, Mirko; et al. (November 2014), "The Blue Straggler Star Population in NGC 1261: Evidence for a Post-core-collapse Bounce State", The Astrophysical Journal Letters, 795 (1): 5, arXiv:1410.0693, Bibcode:2014ApJ...795L..10S, doi:10.1088/2041-8205/795/1/L10, S2CID 119294014, L10.
^ Salinas, R.; et al. (September 2016), "An AO-assisted Variability Study of Four Globular Clusters", The Astronomical Journal, 152 (3): 10, arXiv:1605.06517, Bibcode:2016AJ....152...55S, doi:10.3847/0004-6256/152/3/55, S2CID 119304354, 55.
^ Wehlau, A.; Demers, S. (May 1977), "NGC 1261 and the Oosterhoff type I systems", Astronomy and Astrophysics, 57: 251–256, Bibcode:1977A&A....57..251W.