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{{Infobox open cluster
| name = Messier 23
| image =
| caption = 2MASS Atlas image
| epoch = J2000
| class = I,2,r
| ra = {{RA|17|57|04}}[1]
| dec = {{Dec|−18|59|06}}
| dist_ly = {{Convert|628|pc|ly|order=flip|abbr=on|lk=on}}[2]
| appmag_v = 5.5[3]
| size_v = {{Val|35|ul=arcminute}}[2]
| mass_msol = 1,206[5]
| radius_ly = 8 ly
| age = {{Val|330|100|u=myr}}[6]
| constellation = Sagittarius
| notes =
| names = C 1753-190, Messier 23, NGC 6494
}}Messier 23, also known as NGC 6494, is an open cluster of stars in the southern constellation of Sagittarius.[8] It was discovered by Charles Messier on June 20, 1764.[3] M23 can be found with binoculars or a modestly sized telescope.[3] The cluster is positioned in front of "an extensive gas and dust network", although it is unclear if there is an association between the two.[5] It is located near the ecliptic and thus is subject to lunar occultations.[12]The cluster is located 2,050 light years away. Estimates for the number of cluster members range from 169[13] up to 414, with a directly-counted mass of {{val|1206|fmt=commas|ul=solar mass}}; {{Val|1332|fmt=commas|u=solar mass}} by application of the virial theorem.[5] The cluster is around 330 million years old with a near-solar metallicity of [Fe/H] = −0.04.[6] The brightest component (lucida) is of magnitude 9.3.[16] Five of the cluster members are candidate red giants,[17] while the variable VV Sgr is a candidate asymptotic giant branch star.[18] GalleryReferences1. ^1 {{Citation | display-authors=1 | last1=Wu | first1=Zhen-Yu | last2=Zhou | first2=Xu | last3=Ma | first3=Jun | last4=Du | first4=Cui-Hua | title=The orbits of open clusters in the Galaxy | journal=Monthly Notices of the Royal Astronomical Society | volume=399 | issue=4 | pages=2146–2164 | date=November 2009 | arxiv=0909.3737 | bibcode=2009MNRAS.399.2146W | doi=10.1111/j.1365-2966.2009.15416.x | postscript=. }}
2. ^1 2 {{Citation | last1=Kharchenko | first1=N. V. | last2=Piskunov | first2=A. E. | last3=Röser | first3=S. | last4=Schilbach | first4=E. | last5=Scholz | first5=R.-D. | display-authors=1 | title=Astrophysical parameters of Galactic open clusters | journal=Astronomy and Astrophysics | volume=438 | issue=3 | pages=1163–1173 | year=2005 | bibcode=2005A&A...438.1163K | arxiv=astro-ph/0501674 | doi=10.1051/0004-6361:20042523 | postscript=. }}
3. ^1 2 {{Citation | title=On the metallicity of open clusters. III. Homogenised sample | journal=Astronomy & Astrophysics | last1=Netopil | first1=M. | last2=Paunzen | first2=E. | last3=Heiter | first3=U. | last4=Soubiran | first4=C. | volume=585 | pages=A150 | year=2016 | doi=10.1051/0004-6361/201526370 | postscript=. | bibcode=2016A&A...585A.150N | arxiv=1511.08884 }}
4. ^1 2 3 {{citation | title=Illustrated Guide to Astronomical Wonders: From Novice to Master Observer | first1=Robert | last1=Thompson | first2=Barbara | last2=Thompson | series=DIY science | publisher=O'Reilly Media, Inc. | year=2007 | isbn=978-0596526856 | page=408 | url=https://books.google.com/books?id=ymt9nj_uPhwC&pg=PA408 }}
5. ^1 {{citation | title=The Practical Astronomer's Deep-sky Companion | series=The Patrick Moore Practical Astronomy Series | first1=Jess K. | last1=Gilmour | postscript=. | publisher=Springer Science & Business Media | year=2003 | isbn=978-1852334741 | page=121 | url=https://books.google.com/books?id=3IpZrC0BeewC&pg=PP121 }}
6. ^1 2 3 {{citation | title=A virial mass determination of the open cluster NGC 6494 | last1=McNamara | first1=B. J. | last2=Sanders | first2=W. L. | journal=Astronomy and Astrophysics | volume=118 | issue=2 | pages=361–362 | date=February 1983 | bibcode=1983A&A...118..361M | postscript=. }}
7. ^1 {{citation | title=Membership in the open cluster NGC 6494 | last1=Sanders | first1=W. L. | last2=Schroeder | first2=R. | journal=Astronomy and Astrophysics | volume=88 | pages=102–107 | date=August 1980 | bibcode=1980A&A....88..102S | postscript=. }}
8. ^1 {{citation | title=Mass-losing red giants in open clusters | last1=Jura | first1=M. | postscript=. | journal=Astrophysical Journal, Part 1 | volume=313 | pages=743–749 | date=February 1987 | doi=10.1086/165012 | bibcode=1987ApJ...313..743J }}
9. ^1 {{citation | title=Twenty-eight angular diameters for late-type stars by the lunar occultation technique | last1=Ridgway | first1=S. T. | last2=Wells | first2=D. C. | last3=Joyce | first3=R. R. | last4=Allen | first4=R. G. | journal=Astronomical Journal | display-authors=1 | volume=84 | pages=247–256 | date=February 1979 | postscript=. | doi=10.1086/112414 | bibcode=1979AJ.....84..247R }} See VV Sgr.
10. ^1 {{citation | title=Multicolour photometry of red giants in three southern open clusters | last1=Claria | first1=J. J. | last2=Lapasset | first2=E. | journal=Monthly Notices of the Royal Astronomical Society | volume=241 | issue=2 | date=November 15, 1989 | pages=301–310 | doi=10.1093/mnras/241.2.301 | bibcode=1989MNRAS.241..301C }}
11. ^1 {{citation | title=UBV photometry of NGC 6494 and metallicity considerations | last1=Sanders | first1=W. L. | journal=Astronomy and Astrophysics Supplement Series | volume=84 | issue=3 | date=September 1990 | page=615–618 | bibcode=1990A&AS...84..615S | postscript=. }}
}}External links- Messier 23, SEDS Messier pages
- {{WikiSky}}
{{Sky|17|56|48|-|19|01|00|2150}}{{Messier objects}}{{Ngc65}} 6 : Open clusters|Orion–Cygnus Arm|Sagittarius (constellation)|Messier objects|NGC objects|Astronomical objects discovered in 1764 |