View source for NGC 2808

{{short description|Globular cluster in the constellation Carina}}
{{ Infobox globular cluster
| name = [[New General Catalogue|NGC]] 2808
| image = [[Image:NGC 2808 HST.jpg|250px]]
|caption= A [[Hubble Space Telescope]] (HST) image of NGC 2808
| epoch = [[J2000]]
| class = I<ref name=hcob849_11/>
| constellation = [[Carina (constellation)|Carina]]
| ra = {{RA|09|12|03.10}}<ref name=aj140_6_1830/>
| dec = {{DEC|–64|51|48.6}}<ref name=aj140_6_1830/>
| dist_ly = {{Convert|31.3|kly|kpc|abbr=on|lk=on}}<ref name=apj742_1_51/>
| appmag_v = 6.2<ref name="ngcProj">{{cite web |url=http://www.ngcicproject.org |title=Data for NGC 2808 |website=NGC/IC Project |access-date=19 November 2013 |archive-url=https://web.archive.org/web/20130115035321/http://ngcicproject.org/ |archive-date=15 January 2013 |url-status=dead }}</ref>
| size_v=13.8{{prime}}<ref name="ngcProj" />
| mass_msol = {{Val|1.42|e=6}}<ref name=apj742_1_51/>
| metal_fe = –1.14<ref name=apj742_1_51/>
| radius_ly = 
| v_hb = 
| age = 10.2&nbsp;[[Gigayear|Gyr]]<ref name=mnras385_4_1998/>
| notes = 
| names = GCl 13, [[Melotte catalogue|Melotte]] 95<ref name="simbad" />
}}

'''NGC 2808''' is a [[globular cluster]]<ref name="simbad" />  in the [[constellation]] [[Carina (constellation)|Carina]]. The cluster currently belongs to the [[Milky Way]], although it was likely stolen from [[Gaia Sausage|a dwarf galaxy]] that collided with the Milky Way. NGC 2808 is one of the Milky Way's most massive clusters, containing more than a million stars. It is estimated to be 12.5 billion years old.

The cluster is being disrupted by the [[galactic tide]], trailing a long [[tidal tail]].<ref>{{citation|arxiv=1710.08927|year=2017|title=Tails and streams around the Galactic globular clusters NGC 1851, NGC 1904, NGC 2298 and NGC 2808|doi=10.1093/mnras/stx2767 |last1=Carballo-Bello |first1=Julio A. |last2=Martínez-Delgado |first2=David |last3=Navarrete |first3=Camila |last4=Catelan |first4=Márcio |last5=Muñoz |first5=Ricardo R. |last6=Antoja |first6=Teresa |last7=Sollima |first7=Antonio |journal=Monthly Notices of the Royal Astronomical Society |volume=474 |pages=683–695 |doi-access=free }}</ref>
==Star generations==
It had been thought that NGC 2808, like typical [[globular cluster]]s, contains only one generation of [[star]]s formed simultaneously from the same material. In 2007, a team of astronomers led by [[Giampaolo Piotto]] of the [[University of Padua]] in Italy investigated [[Hubble Space Telescope]] images of NGC 2808 taken in 2005 and 2006 with Hubble's [[Advanced Camera for Surveys]]. Unexpectedly, they found that this cluster is composed of three generations of stars, all born within 200 million years of the formation of the cluster.<ref>{{cite journal
 | author=Piotto, G. |display-authors=etal  | title=A Triple Main Sequence in the Globular Cluster NGC 2808
 | journal=The Astrophysical Journal | volume=661| issue=1
 | pages=L53–L56 |date=May 2007
 | doi=10.1086/518503 | bibcode=2007ApJ...661L..53P |arxiv = astro-ph/0703767 |s2cid=18691514 }}</ref>

Astronomers have argued that globular clusters can produce only one generation of stars, because the radiation from first generation stars would drive the residual gas not consumed in the first star generation phase out of the cluster. However, the great mass of a cluster such as NGC 2808 may suffice to gravitationally counteract the loss of gaseous matter. Thus, a second and a third generation of stars may form.

An alternative explanation for the three star generations of NGC 2808 is that it may actually be the remnant core of a [[dwarf galaxy]] that collided with the Milky Way, called the [[Sausage Galaxy]].<ref>{{cite journal |last1=C. |first1=Myeong, G. |last2=W. |first2=Evans, N. |last3=V. |first3=Belokurov |last4=L. |first4=Sanders, J. |last5=E. |first5=Koposov, S. |title=The Sausage Globular Clusters |journal=The Astrophysical Journal |date=2018 |volume=863 |issue=2 |pages=L28 |doi=10.3847/2041-8213/aad7f7 |arxiv=1805.00453 |language=en|bibcode=2018ApJ...863L..28M |s2cid=67791285 |doi-access=free }}</ref>

==See also==
*[[Messier 54]] and [[Messier 79]], two other possibly extragalactic globular clusters
*[[Omega Centauri]]

==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 | postscript=. |arxiv = 1008.2755 | s2cid=119183070 }}</ref>

<ref name="simbad">{{cite simbad | title=NGC 2808 | access-date=2007-05-07 }}</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=mnras385_4_1998>{{citation | display-authors=1 | last1=Koleva | first1=M. | last2=Prugniel | first2=Ph. | last3=Ocvirk | first3=P. | last4=Le Borgne | first4=D. | last5=Soubiran | first5=C. | title=Spectroscopic ages and metallicities of stellar populations: validation of full spectrum fitting | journal=[[Monthly Notices of the Royal Astronomical Society]] | volume=385 | issue=4 | pages=1998–2010 |date=April 2008 | doi=10.1111/j.1365-2966.2008.12908.x | doi-access=free | bibcode=2008MNRAS.385.1998K |arxiv = 0801.0871 | s2cid=17571531 }}</ref>

}}

==External links==
* {{commonscat-inline}}


{{Ngc30}}
{{Carina (constellation)}}

{{DEFAULTSORT:NGC 2808}}
[[Category:Globular clusters]]
[[Category:Carina (constellation)]]
[[Category:NGC objects|2808]]
[[Category:Canis Major Overdensity]]