Messier 23

Messier 23, also known as NGC 6494, is an open cluster of stars in the northwest of the southern constellation of Sagittarius.^[7] It was discovered by Charles Messier in 1764.^{[lower-alpha 1][3]} It can be found in good conditions with binoculars or a modestly sized telescope.^[3] It is in front of "an extensive gas and dust network", which there may be no inter-association.^[4] It is within 5° the sun's position (namely in mid-December) so can be occulted by the moon.^[8]

Messier 23
Open cluster Messier 23 in Sagittarius
Observation data (J2000 epoch)
Right ascension	17h 57m 04s[1]
Declination	−18° 59′ 06″
Distance	2,050 ly (628 pc)[2]
Apparent magnitude (V)	5.5[3]
Apparent dimensions (V)	35′[2]
Physical characteristics
Mass	1,206[4] M☉
Radius	8 ly
Estimated age	330±100 myr[5]
Other designations	M23, NGC 6494,[6] Cr 356, C 1753-190
Associations
Constellation	Sagittarius
See also: Open cluster, List of open clusters

The cluster is centered about 2,050 light years away. Estimates for the number of its members range from 169^[9] up to 414, with a directly-counted mass of 1,206 M_☉; 1,332 M_☉ by application of the virial theorem.^[4] The cluster is around 330 million years old with a near-solar metallicity of [Fe/H] = −0.04.^[5] The brightest component (lucida) is of magnitude 9.3.^[10] Five of the cluster members are candidate red giants,^[11] while orange variable VV Sgr in the far south,^{[lower-alpha 2]} is a candidate asymptotic giant branch star.^[12]

A 6th-magnitude star, shown in the top-right corner, figures in the far north-west as a foreground star – HD 163245 (HR 6679). Its parallax shift is 9.8912±0.0518 mas, having taken into account proper motion, which means it is about 101 parsecs (330 ly) away.^[13]

Gallery

• 
  Map showing the location of M23

See also

• List of Messier objects

Footnotes and References

Footnotes

1. On June 20
2. east of blue-white 7th magnitude fellow member HD 163427

References

1. Wu, Zhen-Yu; et al. (November 2009), "The orbits of open clusters in the Galaxy", Monthly Notices of the Royal Astronomical Society, 399 (4): 2146–2164, arXiv:0909.3737, Bibcode:2009MNRAS.399.2146W, doi:10.1111/j.1365-2966.2009.15416.x, S2CID 6066790.
2. Kharchenko, N. V.; et al. (2005), "Astrophysical parameters of Galactic open clusters", Astronomy and Astrophysics, 438 (3): 1163–1173, arXiv:astro-ph/0501674, Bibcode:2005A&A...438.1163K, doi:10.1051/0004-6361:20042523, S2CID 9079873.
3. Thompson, Robert; Thompson, Barbara (2007), Illustrated Guide to Astronomical Wonders: From Novice to Master Observer, DIY science, O'Reilly Media, Inc., p. 408, ISBN 978-0596526856
4. McNamara, B. J.; Sanders, W. L. (February 1983), "A virial mass determination of the open cluster NGC 6494", Astronomy and Astrophysics, 118 (2): 361–362, Bibcode:1983A&A...118..361M.
5. Netopil, M.; Paunzen, E.; Heiter, U.; Soubiran, C. (2016), "On the metallicity of open clusters. III. Homogenised sample", Astronomy & Astrophysics, 585: A150, arXiv:1511.08884, Bibcode:2016A&A...585A.150N, doi:10.1051/0004-6361/201526370, S2CID 118382876.
6. "M 23". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved November 12, 2018.
7. Gilmour, Jess K. (2003), The Practical Astronomer's Deep-sky Companion, The Patrick Moore Practical Astronomy Series, Springer Science & Business Media, p. 121, ISBN 978-1852334741.
8. Ridgway, S. T.; et al. (February 1979), "Twenty-eight angular diameters for late-type stars by the lunar occultation technique", Astronomical Journal, 84: 247–256, Bibcode:1979AJ.....84..247R, doi:10.1086/112414. See VV Sgr.
9. Sanders, W. L.; Schroeder, R. (August 1980), "Membership in the open cluster NGC 6494", Astronomy and Astrophysics, 88: 102–107, Bibcode:1980A&A....88..102S.
10. Sanders, W. L. (September 1990), "UBV photometry of NGC 6494 and metallicity considerations", Astronomy and Astrophysics Supplement Series, 84 (3): 615–618, Bibcode:1990A&AS...84..615S.
11. Claria, J. J.; Lapasset, E. (November 15, 1989), "Multicolour photometry of red giants in three southern open clusters", Monthly Notices of the Royal Astronomical Society, 241 (2): 301–310, Bibcode:1989MNRAS.241..301C, doi:10.1093/mnras/241.2.301
12. Jura, M. (February 1987), "Mass-losing red giants in open clusters", Astrophysical Journal, Part 1, 313: 743–749, Bibcode:1987ApJ...313..743J, doi:10.1086/165012.
13. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.

External links

• Messier 23, SEDS Messier pages
• Messier 23 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images