Astronomy:NGC 6426

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NGC 6426
File:320px
HST image of NGC 6426
Observation data (J2000 epoch)
ClassIX[1]
ConstellationOphiuchus
Right ascension 17h 44m 54.71s[2]
Declination+03° 10′ 12.5″[2]
Distance67 kly (20.6 kpc)[3]
Apparent magnitude (V)10.9[4]
Apparent dimensions (V)4.2′[4]
Physical characteristics
Metallicity[Fe/H] = –2.34[3] dex
Estimated age13.0±1.5 Gyr[5]
Other designationsNGC 6426[6]
See also: Globular cluster, List of globular clusters

NGC 6426 is a globular cluster of stars located in the equatorial constellation of Ophiuchus. It was discovered by the German-English astronomer William Herschel on 3 June 1786.[7] This cluster is at a distance of 67,000 light years from the Sun.[3] It has an apparent visual magnitude of 10.9 and an angular diameter of 4.2′, making it difficult to observe with a small telescope.[4]

This cluster is orbiting in the outer galactic halo at a distance of 47 kly (14.4 kpc) from the Galactic Core. It is one of the oldest and most metal-poor clusters in the Milky Way system. NGC 6426 has an angular half-light radius of 0.92′ and a tidal radius of 13.0′,[3] with a Shapley–Sawyer Concentration Class of IX.[1] It is an estimated 13.0±1.5 billion years old.[5] Chemical abundances of four members at the tip of the red giant branch suggests there may have been at least two generations of stars, with the older stars enriching a younger generation with elements including Mg, Si, and Zn. There is also some indication of hypernova enrichment of the pre-cluster medium by lighter alpha process elements.[8]

Based on the spectra of a dozen identified RR Lyrae variables, this is classified as an Oosterhoff type II cluster.[9] In 2012, a carbon star was discovered near the center of the cluster.[10]

References

  1. ↑ 1.0 1.1 Shapley, Harlow; Sawyer, Helen B. (August 1927). "A Classification of Globular Clusters". Harvard College Observatory Bulletin 849: 11–14. Bibcode: 1927BHarO.849...11S. 
  2. ↑ 2.0 2.1 Di Criscienzo, M. et al. (February 2006). "RR Lyrae-based calibration of the Globular Cluster Luminosity Function". Monthly Notices of the Royal Astronomical Society 365 (4): 1357–1366. doi:10.1111/j.1365-2966.2005.09819.x. Bibcode: 2006MNRAS.365.1357D. 
  3. ↑ 3.0 3.1 3.2 3.3 Koch, Andreas et al. (August 2018). "Kinematics of outer halo globular clusters: M 75 and NGC 6426". Astronomy & Astrophysics 616: id. A74. doi:10.1051/0004-6361/201833110. Bibcode: 2018A&A...616A..74K. 
  4. ↑ 4.0 4.1 4.2 O'Meara, Stephen James (2007). Herschel 400 Observing Guide. Cambridge University Press. p. 231. ISBN 9780521858939. https://books.google.com/books?id=Nyh9fAC_tpIC&pg=PA231. 
  5. ↑ 5.0 5.1 Dotter, Aaron et al. (September 2011). "Globular Clusters in the Outer Galactic Halo: New Hubble Space Telescope/Advanced Camera for Surveys Imaging of Six Globular Clusters and the Galactic Globular Cluster Age-metallicity Relation". The Astrophysical Journal 738 (1): id. 74. doi:10.1088/0004-637X/738/1/74. Bibcode: 2011ApJ...738...74D. 
  6. ↑ "NGC 6426". SIMBAD. Centre de données astronomiques de Strasbourg. http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=NGC+6426. 
  7. ↑ Seligman, Courtney. "NGC 6426 (= GCL 76)". Celestial Atlas. http://cseligman.com/text/atlas/ngc64.htm#6412. 
  8. ↑ Hanke, M. et al. (March 2017). "Insights into the chemical composition of the metal-poor Milky Way halo globular cluster NGC 6426". Astronomy & Astrophysics 599: id. A97. doi:10.1051/0004-6361/201629650. Bibcode: 2017A&A...599A..97H. 
  9. ↑ Papadakis, I. et al. (February 2000). "The NGC 6426 RR Lyrae Variables and Horizontal-Branch Morphology". The Astronomical Journal 119 (2): 851–858. doi:10.1086/301221. Bibcode: 2000AJ....119..851P. 
  10. ↑ Sharina, M. et al. (October 2012). "A CH star in the globular cluster NGC 6426". Monthly Notices of the Royal Astronomical Society: Letters 426 (1): L31–L35. doi:10.1111/j.1745-3933.2012.01317.x. Bibcode: 2012MNRAS.426L..31S. 



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