View source for Messier 73

{{short description|Asterism of four stars in the constellation Aquarius}}
{{Infobox open cluster
| name = [[Messier object|Messier]] 73
| image = [[Image:Messier 73 wikisky.jpg|300px]]
| caption= Asterism Messier 73 in Aquarius
| epoch = [[J2000]]
| ra = {{RA|20|58|54}}<ref name="simbad">
{{cite simbad
 | title=M 73
 | access-date=2006-12-21
}}</ref>
<ref name="seds">
{{cite web
 | work=SEDS Messier pages
 | title=Messier 73
 | url=http://messier.seds.org/m/m073.html
 | access-date=2010-02-28
}}</ref>
| dec = {{DEC|-12|38}}<ref name="simbad" /><ref name="seds" />
| dist_ly = Mean distance star approx. 1685 [[light-year|ly]]<ref>{{cite Gaia EDR3}}</ref>
| dist_pc = approx. 516.6 [[parsec|pc]]
| appmag_v = 9.0<ref name="simbad" /><ref name="seds" />
| size_v = 2.8′<ref name="seds" />
| mass_kg =
| mass_msol =
| radius_ly =  ly
| v_hb =
| age =  million yrs{{Citation needed|date=February 2007}}
| constellation = [[Aquarius (constellation)|Aquarius]]
| notes =
| names = M 73, [[New General Catalogue|NGC]] 6994,<ref name="simbad" /><ref name="seds" /> [[Collinder catalog|Cr]] 426
}}
'''Messier 73''' ('''M73''', also known as  '''NGC 6994''') is an [[asterism (astronomy)|asterism]] of four [[star]]s in the constellation [[Aquarius (constellation)|Aquarius]] which was long thought to be a small [[open cluster]]. It lies several arcminutes east of [[globular cluster]] [[Messier 72|M72]]. According to [[Gaia (spacecraft)|Gaia EDR3]], the stars are {{val|1030|9}}, {{val|1249|10}}, {{val|2170|22}}, and {{val|2290|24}} light-years from the Sun, with the second being a binary star.<ref>{{cite Gaia EDR3}}</ref>

==History==
M73 was discovered by [[Charles Messier]] in 1780{{efn|on October 4}} who originally described the object as a [[star cluster|cluster]] of four stars with some nebulosity.  Much later observations by [[John Herschel]] could not find any nebulosity.  Moreover, Herschel noted that the designation of M73 as a cluster was questionable.  Nonetheless, Herschel included M73 in his [[General Catalogue of Nebulae and Clusters|General Catalogue]] of clusters, [[nebulae]], and [[galaxies]], and [[John Louis Emil Dreyer|John Dreyer]] included M73 when he compiled the [[New General Catalogue]].<ref name="jonesbook1991">
{{cite book
 | author=K. G. Jones
 | date=1991 | edition= 2nd
 | title=Messier's Nebulae and Star Clusters
 | publisher=[[Cambridge University Press]]
 | isbn=978-0-521-37079-0
}}</ref>

==Relation between the stars==
M73 was once treated as a potential sparsely populated [[open cluster]], which consists of stars that are physically associated in space as well as on the sky. The question of whether the stars were an asterism or an open cluster was a matter of debate in the early 2000s.

In 2000, L. P. Bassino, S. Waldhausen, and R. E. Martinez published an analysis of the colors and luminosities of the stars in and around M73. They concluded that the four bright central stars and some other nearby stars followed the color-luminosity relation that is also followed by stars in open clusters (as seen in a [[Hertzsprung–Russell diagram]]). Their conclusion was that M73 was an old open cluster that was 9&nbsp;[[arcminute|′]] wide.<ref name="bassinoetal2000">
{{cite journal
 | author=L. P. Bassino | author2=S. Waldhausen | author3=R. E. Martinez
 | name-list-style=amp
 | date=2000
 | title=CCD photometry in the region of NGC 6994: The remains of an old open cluster
 | journal=[[Astronomy & Astrophysics]]
 | volume=355 | pages=138–144
 | bibcode=2000A&A...355..138B
 |arxiv = astro-ph/0001238 }}</ref>  However, G. Carraro, published results in 2000 based on a similar analysis and concluded that the stars did not follow any color-luminosity relation.  Carraro's conclusion was that M73 was an asterism.<ref name="carraro2000">
{{cite journal
 | author=G. Carraro
 | date=2000
 | title=NGC 6994: An open cluster which is not an open cluster
 | journal=[[Astronomy & Astrophysics]]
 | volume=357 | pages=145–148
 | bibcode=2000A&A...357..145C
|arxiv = astro-ph/0003372 }}</ref>  Adding to the controversy, E. Bica and collaborators concluded that the chance alignment of the four bright stars seen in the center of M73 as well as one other nearby star was highly unlikely, so M73 was probably a sparse open cluster.<ref name="bicaetal2001">
{{cite journal
 | author=E. Bica | author2=B. X. Santiago | author3=C. M. Dutra | author4=H. Dottori | author5=M. R. de Oliveira | author6=D. Pavani
 | name-list-style=amp
 | date=2001
 | title=Dissolving star cluster candidates
 | journal=[[Astronomy & Astrophysics]]
 | volume=366
 | issue=3 | pages=827–833
 | bibcode=2001A&A...366..827B
 | doi=10.1051/0004-6361:20000248
|arxiv = astro-ph/0011280 | s2cid=10979809 }}</ref> The controversy was resolved in 2002, when M. Odenkirchen and C. Soubiran published an analysis of the high resolution [[Astronomical spectroscopy|spectra]] of the six brightest stars within 6&nbsp;[[arcminute|′]] of the centre point.  They demonstrated that the distances from the Earth to the six stars were very different from each other, and the stars were moving in different directions.  Therefore, they concluded that the stars were only an asterism.<ref name="odenkirchecnsoubiran2002">
{{cite journal
 | author=M. Odenkirchen | author2=C. Soubiran
 | name-list-style=amp
 | date=2002
 | title=NGC 6994: Clearly not a physical stellar ensemble
 | journal=[[Astronomy & Astrophysics]]
 | volume=383
 | issue=1 | pages=163–170
 | bibcode=2002A&A...383..163O
 | doi=10.1051/0004-6361:20011730
|arxiv = astro-ph/0111601 | s2cid=15545816
 }}</ref>

Although M73 was determined to be only a chance alignment of stars, further analysis of asterisms is still important for the identification of sparsely populated open clusters.  A full study of very many such clusters would demonstrate how, how often, and to what degree open clusters are ripped apart by the [[gravitation|gravitational forces]] in the [[Milky Way]] and reveal more of the sources of [[dark matter|these forces]].{{cn|date=October 2024}}

==Location==
[[File:Aquarius constellation map.svg|thumb|none|250px|M73 is in [[Aquarius (constellation)|Aquarius]], just to the east (left) of [[Messier 72|M72]].]]

==See also==
* [[List of Messier objects]]
* [[Messier 40]] - ''a double star included in the [[Messier catalogue]] that was also mistakenly identified as having nebulosity''

== Notes and references ==
{{notelist}}
{{reflist}}

==External links==
{{commonscat}}
* [http://messier.seds.org/m/m073.html SEDS: Messier Object 73]
* [https://www.perseus.gr/Astro-DSO-NGC-6994.htm Messier 73, LRGB CCD image based on two-hours total exposure]{{WikiSky}}

{{Portal bar|Astronomy|Stars|Outer space}}
{{Catalogs|M=73|NGC=6994|Cr=426}}
{{Messier objects}}
{{Ngc70}}
{{Aquarius (constellation)}}
{{Sky|20|58|54|-|12|38|00|2500}}

{{DEFAULTSORT:Messier 73}}
[[Category:Asterisms (astronomy)|Messier 073]]
[[Category:Aquarius (constellation)|Messier 073]]
[[Category:Messier objects|073]]
[[Category:NGC objects|Messier 073]]
[[Category:Orion–Cygnus Arm]]
[[Category:Astronomical objects discovered in 1780]]
[[Category:Discoveries by Charles Messier]]