View source for Astronomy:Messier 68

{{Short description|Globular cluster in the constellation Hydra}}
{{Infobox globular cluster
| name = [[Astronomy:Messier object|Messier]] 68
| image = [[Image:Messier 68 Hubble WikiSky.jpg|300px]]
| caption = M68 from [[Astronomy:Hubble Space Telescope|Hubble Space Telescope]] in 2008; 3.32{{prime}} view
| credit = 
| epoch = [[Astronomy:J2000|J2000]]
| class = X<ref name=hcob849_11/>
| constellation = Hydra
| ra = {{RA|12|39|27.98}}<ref name=aj140_6_1830/>
| dec = {{DEC|–26|44|38.6}}<ref name=aj140_6_1830/>
| dist_ly = {{Convert|33.6|kly|kpc|abbr=on|lk=on}}<ref name=apj742_1_51/>
| appmag_v = 7.8<ref>{{cite web |url=https://messier.seds.org/m/m068.html |title=Messier 68 |access-date=30 April 2022 |website=SEDS Messier Catalog}}</ref>
| size_v = 11.0{{prime}}
| mass_msol = {{Val|2.23|e=5}}<ref name=apj742_1_51/>
| metal_fe = –2.23<ref name=apj742_1_51/>
| radius_ly = 53&nbsp;[[Astronomy:Light-year|ly]]<ref>distance × sin( diameter_angle / 2 ) = 53 ly. radius</ref>
| v_hb = 
| age = 11.2&nbsp;Gyr<ref name=aaa481_3_701/>
| notes = Relatively metal poor.<ref name=apj742_1_51/>
| names = M68, NGC 4590, GCl 20<ref name="simbad" />
}}

'''Messier 68''' (also known as '''M68''' or '''NGC 4590''') is a [[Astronomy:Globular cluster|globular cluster]] found in the east south-east of [[Astronomy:Hydra (constellation)|Hydra]], away from its precisely [[Celestial equator|equatorial]] part. It was discovered by [[Biography:Charles Messier|Charles Messier]] in 1780. William Herschel described it as "a beautiful cluster of stars, extremely rich, and so compressed that most of the stars are blended together". His son [[Biography:John Herschel|John]] noted that it was "all clearly resolved into stars of 12th magnitude, very loose and ragged at the borders".<ref name=stoyan2008/>

M68 is centred about 33,600 [[Astronomy:Light-year|light-year]]s away from [[Astronomy:Earth|Earth]]. It is orbiting our galaxy's [[Astronomy:Galactic bulge|galactic bulge]] with a great [[Astronomy:Orbital eccentricity|eccentricity]] of 0.5. This takes it to 100,000 light years from the center.<ref name=stoyan2008/> It is one of the most metal-poor globular clusters, which means it has a paucity of elements other than hydrogen and helium. The cluster may be undergoing core-collapse, and it displays signs of being in rotation. The cluster may have been acquired in its gravitational tie to the [[Astronomy:Milky Way|Milky Way]] through accretion from a satellite galaxy.<ref name=aa578_A128_23/>

As of 2015, 50 [[Astronomy:Variable star|variable star]]s have been identified in this cluster; the first 28 being identified as early as 1919&ndash;20 by American astronomer Harlow Shapley. Most of the variables are of type [[Astronomy:RR Lyrae variable|RR Lyrae]], or periodic variables. Six of the variables are of the [[Astronomy:SX Phoenicis variable|SX Phoenicis]] variety, which display short pulsating behavior.<ref name=aa578_A128_23/>

==Gallery==
<gallery>File:A_Ten_Billion_Year_Stellar_Dance.jpg|A 2012 view of M68 from the Wide Field Camera of Hubble's Advanced Camera for Surveys</gallery>

==See also==
* [[Astronomy:List of Messier objects|List of Messier objects]]

==References==
{{Reflist|refs=

<ref name=hcob849_11>{{citation
 | last1=Shapley | first1=Harlow | last2=Sawyer | first2=Helen B.
 | title=A Classification of Globular Clusters
 | journal=Harvard College Observatory Bulletin | volume=849 | issue=849 | pages=11–14
 |date=August 1927
 | bibcode=1927BHarO.849...11S }}</ref>

<ref name=aj140_6_1830>{{citation | display-authors=1 | last1=Goldsbury | first1=Ryan | last2=Richer | first2=Harvey B. | last3=Anderson | first3=Jay | last4=Dotter | first4=Aaron | last5=Sarajedini | first5=Ata | last6=Woodley | first6=Kristin | title=The ACS Survey of Galactic Globular Clusters. X. New Determinations of Centers for 65 Clusters | journal=The Astronomical Journal | volume=140 | issue=6 | pages=1830–1837 |date=December 2010 | doi=10.1088/0004-6256/140/6/1830 | bibcode=2010AJ....140.1830G  |arxiv = 1008.2755 | s2cid=119183070 }}</ref>

<ref name="simbad">{{cite simbad | title=M 68 | access-date=2006-11-17 }}</ref>

<ref name=apj742_1_51>{{citation | display-authors=1 | last1=Boyles | first1=J. | last2=Lorimer | first2=D. R. | last3=Turk | first3=P. J. | last4=Mnatsakanov | first4=R. | last5=Lynch | first5=R. S. | last6=Ransom | first6=S. M. | last7=Freire | first7=P. C. | last8=Belczynski | first8=K. | title=Young Radio Pulsars in Galactic Globular Clusters | journal=The Astrophysical Journal | volume=742 | issue=1 | page=51 |date=November 2011 | doi=10.1088/0004-637X/742/1/51 | bibcode=2011ApJ...742...51B  |arxiv = 1108.4402 | s2cid=118649860 }}</ref>

<ref name=aaa481_3_701>{{citation | display-authors=1 | last1=Sollima | first1=A. | last2=Lanzoni | first2=B. | last3=Beccari | first3=G. | last4=Ferraro | first4=F. R. | last5=Fusi Pecci | first5=F. | title=The correlation between blue straggler and binary fractions in the core of Galactic globular clusters | journal=Astronomy and Astrophysics | volume=481 | issue=3 | pages=701–704 |date=April 2008 | doi=10.1051/0004-6361:20079082 | bibcode=2008A&A...481..701S |arxiv = 0801.4511 | s2cid=3088769 }}</ref>

<ref name=aa578_A128_23>{{citation
 |author=Kains, N. |author2=Alsubai, K. A. |collaboration=LCOGT/Robonet and MINDSTEp Consortiums
 | title=A census of variability in globular cluster M 68 (NGC 4590) 
 | journal=Astronomy & Astrophysics
 | volume=578 | id=A128 | page=23 | date=June 2015
 | doi=10.1051/0004-6361/201424600 | bibcode=2015A&A...578A.128K 
 | arxiv=1502.07345 | s2cid=14132566 
}}</ref>

<ref name=stoyan2008>{{citation
 | title=Atlas of the Messier Objects: Highlights of the Deep Sky 
 | first1=Ronald | last1=Stoyan
 | first2=Stefan | last2=Binnewies
 | first3=Susanne | last3=Friedrich 
 | translator1-first=Klaus-Peter | translator-last=Schroeder 
 | publisher=Publisher Cambridge University Press
 | year=2008 | isbn=978-1-139-47311-8 | page=826
 | url=https://books.google.com/books?id=x2VuBAAAQBAJ&pg=PT826
}}</ref>

}}

==External links==
* [http://messier.seds.org/m/m068.html Globular Cluster M68 @ SEDS Messier pages]
* [http://gclusters.altervista.org/cluster_4.php?ggc=M+68 Messier 68, Galactic Globular Clusters Database page]
* {{WikiSky}}

{{Sky|12|39|28.01|-|26|44|34.9|33300}}
{{Messier objects}}

{{DEFAULTSORT:Messier 068}}
[[Category:Globular clusters]]
[[Category:Hydra (constellation)]]
[[Category:Messier objects|068]]
[[Category:NGC objects]]

{{Sourceattribution|Messier 68}}