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NGC 6256

NGC 6256
NGC 6256 as seen through the Hubble Space Telescope
Observation data (J2000 epoch)
ConstellationScorpius
Right ascension16h 59m 32.68s[1]
Declination−37° 07′ 17.1″[1]
Distance22 kly (6.8 kpc)[2]
Apparent magnitude (V)11.3[3]
Apparent dimensions (V)7′[4]
Physical characteristics
Absolute magnitude−7.15[citation needed]
Estimated age13.0±0.5 Gyr[2]
Other designationsNGC 6256, GCL 49.1, ESO 391-SC6 and vdB-Hagen 208[5]
See also: Globular cluster, List of globular clusters

NGC 6256 is a globular cluster of stars in the southern constellation of Scorpius. It was discovered by the Scottish astronomer James Dunlop on Aug 2, 1826.[6] In J. L. E. Dreyer's New General Catalogue annotation it is described as, "very faint, very large, very gradually bright in the middle, well resolved clearly consisting of stars."[7] The cluster is located at a distance of 22 thousand light-years (6.8 kpc) from the Sun.[2]

This is an ancient cluster with an estimated age of about 13 billion years; it was formed during the very early stages of assembly of the Milky Way galaxy. The cluster is orbiting within the galactic bulge with a low orbital eccentricity. It is heavily reddened by extinction due to interstellar gas and dust.[2] The structure is very concentrated at the center, showing a post core collapse morphology.[8]

The HR diagram for this cluster displays two sequences of blue straggler stars, which are the products of stellar mergers. The bluer sequence is well-defined and narrow, most likely being generated over a short time span. The redder sequence is more sparse, being the result of a continuous process of formation. It is hypothesized that the bluer sequence was formed around the time of the cluster's core collapse about one billion years ago; an event that made stellar collisions more likely.[9]

The cluster contains a millisecond X-ray pulsar, designated IGR J16597-3704.[10]

References

  1. ^ a b 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. arXiv:astro-ph/0511128. Bibcode:2006MNRAS.365.1357D. doi:10.1111/j.1365-2966.2005.09819.x. S2CID 17838243.
  2. ^ a b c d Cadelano, M.; et al. (May 2020). "Digging for Relics of the Past: The Ancient and Obscured Bulge Globular Cluster NGC 6256". The Astrophysical Journal. 895 (1). id.54. arXiv:2004.06131. Bibcode:2020ApJ...895...54C. doi:10.3847/1538-4357/ab88b3.
  3. ^ O'Meara, Stephen James (2013). Deep-Sky Companions: Southern Gems. Cambridge University Press. p. 331. ISBN 9781107015012.
  4. ^ Frommert, Hartmut. "NGC 6256". Students for the Exploration and Development of Space. Retrieved 2024-05-23.
  5. ^ "NGC 6256". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2024-05-23.
  6. ^ Seligman, Courtney. "NGC Objects: NGC 6250 - 6299". Celestial Atlas. Retrieved 2024-05-23.
  7. ^ Terzan, A.; Lelievre, G. (December 1973). "The Cluster NGC 6256". Astronomy and Astrophysics. 29: 431. Bibcode:1973A&A....29..431T.
  8. ^ Ortolani, S.; et al. (April 1999). "Colour-magnitude diagrams of the post-core collapse globular clusters NGC 6256 and NGC 6717 (Palomar 9)". Astronomy and Astrophysics Supplement. 136 (2): 237–243. Bibcode:1999A&AS..136..237O. doi:10.1051/aas:1999210.
  9. ^ Cadelano, Mario; et al. (December 2022). "Discovery of a Double Sequence of Blue Straggler Stars in the Core-collapsed Globular Cluster NGC 6256". The Astrophysical Journal. 941 (1). id. 69. arXiv:2211.02671. Bibcode:2022ApJ...941...69C. doi:10.3847/1538-4357/aca016.
  10. ^ Tetarenko, A. J.; et al. (February 2018). "A Radio Frequency Study of the Accreting Millisecond X-ray Pulsar, IGR J16597-3704, in the Globular Cluster NGC 6256". The Astrophysical Journal. 854 (2). id. 125. arXiv:1801.05778. Bibcode:2018ApJ...854..125T. doi:10.3847/1538-4357/aaa95a.
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