:NGC 6231

{{Short description| Open Cluster in the constellation of Scorpius }}{{More citations needed|date=June 2024}}{{Infobox open cluster

| name = NGC 6231

| image = NGC 6231 and ζ Sco AOFPK.jpg

| caption = NGC 6231 (top) with Zeta2 and Zeta1 Scorpii (bottom)

| epoch = J2000.0

| class =

| constellation = Scorpius

| ra = 16^h 54^m

| dec = −41° 48{{prime}}

| dist_ly = 5,600±400 ly

| dist_pc = 1,700±130 parsec

{{cite journal

| title= Kinematics in Young Star Clusters and Associations with Gaia DR2

| journal=The Astrophysical Journal

| volume=870 |issue=1 |pages=32|year=2018

|arxiv = 1807.02115 |bibcode = 2019ApJ...870...32K |doi = 10.3847/1538-4357/aaef8c | last1=Kuhn

| first1=Michael A.

| last2=Hillenbrand

| first2=Lynne A.

| last3=Sills

| first3=Alison

| last4=Feigelson

| first4=Eric D.

| last5=Getman

| first5=Konstantin V.

| s2cid=119328315

| doi-access=free

}}

| appmag_v = 2.6

| size_v = 15.0{{prime}}

| mass_kg =

| mass_msol =

| radius_ly =

| v_hb =

| age = 2–7 million years

| notes =

| names = NGC 6171, Caldwell 76, Collinder 315, Melotte 153, De Cheseaux 9, Dunlop 499, Ha. I.7, Lacaille II.13}}

NGC 6231 (also known as Caldwell 76 or the Baby Scorpion Cluster{{cite web | last=Goldstein | first=Alan | title=NGC 6231 | website=Astronomy Magazine | date=2024-01-01 | url=https://www.astronomy.com/science/ngc-6231/ | access-date=2024-06-10}}{{cite book|last=Stoyan|first=Ronald|last2=Schurig|first2=Stephan|title=interstellarum Deep Sky Atlas|publisher=Cambridge University Press; Oculum-Verlag GmbH|publication-place=Erlangen|date=2014|isbn=978-1-107-50338-0|oclc=920437579|url=http://www.deep-sky-atlas.com/}}) is an open cluster in the southern sky located half a degrees north of Zeta Scorpii. NGC 6231 is part of a swath of young, bluish stars in the constellation Scorpius known as the Scorpius OB1 association.{{cite book|last = Reipurth| first=B. |editor-last=Reipurth| editor-first=B. | chapter=Young Stars in NGC 6231 and the Sco OB1 Association |bibcode=2008hsf2.book..401R|title=Handbook of Star Forming Regions, Volume II: The Southern Sky ASP Monograph Publications|volume=5|page=401|date=2008| publisher=Astronomical Society of the Pacific | isbn=978-1-58381-670-7}} The star Zeta¹ (HR 6262) is a member of this association, while its brighter apparent partner, Zeta² (HR 6271), is only 150 ly from Earth and so is not a member.{{Citation needed|reason=needs reference.|date=August 2019}}

This cluster is estimated to be about 2–7 million years old,{{cite journal|display-authors=3|last1=Kuhn|first1=M. A.|last2=Medina|first2=N.|last3=Getman|first3=K. V.|last4=Feigelson|first4=E. D.|last5=Gromadzki|first5=M.|last6=Borissova|first6=J.|last7=Kurtev|first7=R.|title= The Structure of the Young Star Cluster NGC 6231. I. Stellar Population|journal= The Astronomical Journal|volume=154|pages=87|number=3|year=2017|doi=10.3847/1538-3881/aa76e8|arxiv = 1706.00017 |bibcode = 2017AJ....154...87K |s2cid=119435797 |doi-access=free }} and is approaching the Solar System at 22 km/s. The cluster and association lie in the neighboring Sagittarius Arm of the Milky Way. Zeta¹ Scorpii (spectral type O8 and magnitude 4.71.Sky Catalogue 2000.0) is the brightest star in the association, and one of the most radiant stars known in the galaxy.Crossen & Tirion, Binocular Astronomy, p. 119. NGC 6231 was used to measure the binary fraction of B-type stars: 52 ± 8%, indicating that B-type stars are commonly found in binary systems, but not as commonly as in O-type stars.{{cite journal|doi=10.1051/0004-6361/202141037|title=The observed multiplicity properties of B-type stars in the Galactic young open cluster NGC 6231|year=2022|last1=Banyard|first1=G.|last2=Sana|first2=H.|last3=Mahy|first3=L.|last4=Bodensteiner|first4=J.|last5=Villaseñor|first5=J. I.|last6=Evans|first6=C. J.|journal=Astronomy & Astrophysics|volume=658|pages=A69|arxiv=2108.07814|bibcode=2022A&A...658A..69B|s2cid=237194742}}

NGC 6231 also includes three Wolf-Rayet stars: HD 151932, HD 152270,{{Cite journal | last1 = Shylaja | first1= B. S| year=1988 | title = Study of the Wolf-Rayet members of the cluster NGC 6231 | journal = Journal of Astrophysics and Astronomy | volume = 9 | issue= 3| pages = 161–172 | doi=10.1007/BF02715061 | bibcode=1988JApA....9..161S| s2cid= 121125488}} and HD 152408.[http://adsabs.harvard.edu/abs/1997A%26A...317..532C The distinction between OIafpe and WNLha stars. A spectral analysis of HD 151804, HD 152408 and HDE 313846.]

== Discovery ==

The cluster was discovered by Giovanni Batista Hodierna before 1654. Hodierna listed it as LuminosaeStars visible to the naked eye. It meanings "luminous" in Latin. in his catalogue of deep sky observations. This catalogue was included in his book De Admirandis Coeli Characteribuse published in 1654 at Palermo. It was independently observed by other astronomers after Hodierna, including Edmond Halley (1678), Jean-Philippe de Cheseaux (1745–46), and Abbe Lacaille (1751–52).