View source for NGC 6397

{{Short description|Globular cluster of stars in the Milky Way}}
{{Infobox globular cluster
| name = NGC 6397
| image = [[File:P0321a.jpg|250px]]
|caption = A [[Hubble Space Telescope]] (HST) image of NGC 6397
| epoch = [[J2000]]
| class = IX<ref name=hcob849_11/>
| constellation = [[Ara (constellation)|Ara]]
| ra = {{RA|17|40|42.09}}<ref name=aj140_6_1830/>
| dec = {{DEC|–53|40|27.6}}<ref name=aj140_6_1830/>
| dist_ly = {{Convert|7.8|kly|kpc|abbr=on|lk=on}}<ref name="Salazar">{{cite web|last1=Salazar|first1=Doris Elen|title=Cosmic 'Yardstick' Measures Distance to One of Universe's Oldest Objects|url=https://www.space.com/40222-wobbles-ancient-stars-cosmic-yardstick.html|website=Space.com|date=5 April 2018 |access-date=5 April 2018}}</ref>
| appmag_v = +6.68<ref name="simbad" />
| size_v = 32.0{{prime}}
| mass_msol = {{Val|1.14|e=5}}<ref name="Vitral+22"/>
| radius_ly = 34 ly<ref>distance × sin( diameter_angle / 2 ) = ~34 ly. radius</ref>
| v_hb = 14.2
| metal_fe = –1.76<ref name=mnras404_3_1203/>
| age = 13.4 ± 0.8 Gyr<ref name="eso" />
| notes = Second closest [[globular cluster]]
| names = GCl 74,<ref name="simbad" /> Lacaille III.11, Dunlop 366, Bennett 98, [[Caldwell catalog|Caldwell]] 86
}}

'''NGC 6397''' (also known as '''Caldwell 86''') is a [[globular cluster]] in the [[constellation]] [[Ara (constellation)|Ara]] that was discovered by French astronomer [[Nicolas-Louis de Lacaille]] in 1752.<ref name=Jones_1969>{{cite journal
 | title=The search for the nebulae - VI
 | last=Jones | first=K. G.
 | journal=Journal of the British Astronomical Association
 | volume=79 | pages=213–222 | date=March 1969
 | bibcode=1969JBAA...79..213J }}</ref> It is located about 7,800 [[light-year]]s from [[Earth]],<ref name="Salazar" /> making it one of the two nearest globular clusters to Earth (the other one being [[Messier 4]]). The cluster contains around 400,000 stars,<ref name="eso" /> and can be seen with the [[naked eye]] under good observing conditions.<ref name="nightsky">{{cite book
 |last = Dunlop
 |first = Storm
 |author-link = Storm Dunlop
 |title = Atlas of the Night Sky
 |url = https://archive.org/details/collinsatlasofni0000dunl
 |url-access = registration
 |publisher = [[HarperCollins|Collins]]
 |year = 2005
 |isbn = 978-0-00-717223-8
}}</ref>

NGC 6397 is one of at least 20 globular clusters of the [[Milky Way Galaxy]] that have undergone a [[Core collapse (cluster)|core collapse]],<ref name="eso" /> meaning that the core has contracted to a very dense stellar [[wikt:agglomeration|agglomeration]].

==Astronomical research==

===Estimating the age of the Milky Way===
In 2004, a team of [[astronomer]]s<ref name="eso" /> focused on the cluster to estimate the age of the [[Milky Way Galaxy]]. They used the UV-Visual [[Echelle grating|Echelle Spectrograph]] of the [[Very Large Telescope]] at [[Cerro Paranal]] to measure the [[beryllium]] content of two stars in the cluster. This allowed them to deduce the time elapsed between the rise of the first generation of stars in the entire Galaxy and the first generation of stars in the cluster. This, added to the estimated age of the stars in the cluster, gives an estimate for the age for the Galaxy: about 13.6 [[1000000000 (number)|billion]] years, which is nearly as [[Age of the universe|old as the universe]] itself.  
This estimate assumes that the NGC 6397 is not older than the Milky Way.

===Lower mass limit for stars===
In 2006, a study of NGC 6397 using the [[Hubble Space Telescope]] was published that showed a clear lower limit in the [[Absolute magnitude|brightness]] of the cluster's population of faint stars. The authors deduce that this indicates a lower limit for the [[mass]] necessary for stars to develop a core capable of [[nuclear fusion|fusion]]: roughly 0.083 times the mass of the [[Sun]].<ref name="HST">{{cite web
 |title=HST analysis of faint stars in NGC6397
 |work=Results for NGC 6397
 |url=http://hubblesite.org/newscenter/newsdesk/archive/releases/2006/37
 |access-date=2006-09-26
}}</ref>

===Black holes and white dwarfs===

In February 2021, the core of NGC 6397 was reported to contain a relatively dense concentration of compact objects ([[white dwarfs]], [[neutron stars]] and [[black holes]]), based on the movement of stars near the core derived from data gathered by the Hubble Space Telescope and the European Space Agency's Gaia spacecraft.<ref name="VitralMamon21">{{Citation | last1=Vitral | first1=Eduardo | title=Does NGC 6397 contain an intermediate-mass black hole or a more diffuse inner subcluster? | last2=Mamon | first2=Gary A. | doi=10.1051/0004-6361/202039650 | date=2021 | journal=Astronomy & Astrophysics | volume=646 | issue=A63 | pages=A63 | arxiv=2010.05532 |bibcode = 2021A&A...646A..63V | doi-access=free }}</ref><ref>{{cite news |date=11 February 2021|title=Hubble Uncovers Concentration of Small Black Holes | url=https://esahubble.org/news/heic2103/ |work=ESA|access-date=21 February 2021}}</ref> 
The respective study argued that black holes could dominate the mass budget of this concentration, if the black holes were not ejected by means of dynamical interactions.
Indeed, another group of scientists shortly responded, claiming that because NGC 6397 has undergone core collapse, it should have started dense enough to speed its rates of dynamical interactions, and its original black hole population should be almost entirely gone.<ref>{{Citation | last1=Rui | first1=Nicholas Z. |title=No Black Holes in NGC 6397| last2=Weatherford | first2=Newlin C. | last3=Kremer | first3=Kyle | last4=Chatterjee | first4=Sourav | last5=Fragione | first5=Giacomo | last6=Rasio | first6=Frederic A. | last7=Rodriguez | first7=Carl L. | last8=Ye | first8=Claire S. | doi=10.3847/2515-5172/abee77 | date=2021 | journal=  Research Notes of the American Astronomical Society| volume=5 | issue=3 | page=47 | bibcode = 2021RNAAS...5...47R | arxiv=2103.06273 | s2cid=232185141 | doi-access=free }}</ref> 
This group provided dynamical simulations that showed that a concentration of white dwarfs could explain the first measurement.<ref>{{Citation | last1=Kremer | first1=Kyle |title=White Dwarf Subsystems in Core-Collapsed Globular Clusters| last2=Rui | first2=Nicholas Z. | last3=Weatherford | first3=Newlin C. | last4=Chatterjee | first4=Sourav | last5=Fragione | first5=Giacomo | last6=Rasio | first6=Frederic A. | last7=Rodriguez | first7=Carl L. | last8=Ye | first8=Claire S. | doi=10.3847/1538-4357/ac06d4 | date=2021 | journal=  The Astrophysical Journal| volume=917 | issue=1 | page=28 | bibcode = 2021ApJ...917...28K | arxiv=2104.11751 | s2cid=233394452 | doi-access=free }}</ref>

Finally, in 2022, a work was published by the former scientists, along with one of the leaders of the latter group, and other experts of Hubble and Gaia data.<ref name="Vitral+22">{{Citation | last1=Vitral | first1=Eduardo |title=Stellar graveyards: clustering of compact objects in globular clusters NGC 3201 and NGC 6397| last2=Kremer | first2=Kyle | last3=Libralato | first3=Mattia | last4=Mamon | first4=Gary A. | last5=Bellini | first5=Andrea | doi=10.1093/mnras/stac1337 | date=2022 | journal=  Monthly Notices of the Royal Astronomical Society| volume=514 | issue=1 | page=806 | doi-access=free | bibcode = 2022MNRAS.514..806V | arxiv=2202.01599 }}</ref>
This new work showed that the fits of the central mass excess in NGC 6397 from observed data agreed remarkably well with numerical simulations accounting for a population of hundreds of massive white dwarfs, and essentially no black holes.<ref>{{cite news |date=10 June 2022|title=Chasser des trous noirs dans un cimetière d'étoiles | url=https://www.insu.cnrs.fr/fr/cnrsinfo/chasser-des-trous-noirs-dans-un-cimetiere-detoiles}}</ref>

=== Distant globular clusters ===
Hubble pictures revealed many distant globular clusters in galaxies far behind NGC 6397.

==Gallery==
<gallery widths="250" heights="250">
File:Hubble's view of dazzling globular cluster NGC 6397.jpg|Hubble's view of globular cluster NGC 6397<ref>{{cite web|title=Hubble's view of dazzling globular cluster NGC 6397|url=https://www.spacetelescope.org/images/opo1824a/|website=www.spacetelescope.org|access-date=13 April 2018}}</ref>
File:L R.BedinNGC6397.gif|alt=NGC 6397 region double observation HST& JWST|NGC 6397 region double observation HST and JWST<ref>{{Cite journal |title=A caccia di nane bianche e pianeti distrutti con Jwst |url=https://www.media.inaf.it/2024/06/07/nane-bianche-pianeti-distrutti/ |journal=Il Notiziario Online dell'Istituto Nazionale di Astrofisica|date=7 June 2024 }}</ref>
File:Euclid’s view of globular cluster NGC 6397 ESA25170813.jpg|NGC 6397 imaged by [[Euclid (spacecraft)|Euclid]]
</gallery>

==See also==
* [[List of brightest stars]]
* [[List of nearest bright stars]]
* [[Lists of stars]]
* [[Historical brightest stars]]

==References==
{{Reflist|refs=

<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 6397 | access-date=2006-11-16 }}</ref>

<ref name="eso">{{cite web | title=How Old is the Milky Way ? | work=Results for NGC 6397 | url=http://www.eso.org/outreach/press-rel/pr-2004/pr-20-04.html | access-date=2006-09-26 | archive-url=https://web.archive.org/web/20080905233636/http://www.eso.org/outreach/press-rel/pr-2004/pr-20-04.html | archive-date=2008-09-05 | url-status=dead }}</ref>

<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 | postscript=. }}</ref>

<ref name=mnras404_3_1203>{{citation | last1=Forbes | first1=Duncan A. | last2=Bridges | first2=Terry | title=Accreted versus in situ Milky Way globular clusters | journal=[[Monthly Notices of the Royal Astronomical Society]] | volume=404 | issue=3 | pages=1203–1214 |date=May 2010 | doi=10.1111/j.1365-2966.2010.16373.x | doi-access=free | bibcode=2010MNRAS.404.1203F | postscript=. |arxiv = 1001.4289 | s2cid=51825384 }}</ref>

<!-- ♦♦♦ not used <ref name=mnras406_3_2000>{{citation | last1=Marks | first1=Michael | last2=Kroupa | first2=Pavel | title=Initial conditions for globular clusters and assembly of the old globular cluster population of the Milky Way | journal=Monthly Notices of the Royal Astronomical Society | volume=406 | issue=3 | pages=2000–2012 |date=August 2010 | doi=10.1111/j.1365-2966.2010.16813.x | bibcode=2010MNRAS.406.2000M | postscript=. |arxiv = 1004.2255 }} Mass is from M<sub>PD</sub> on Table 1.</ref> ♦♦♦-->

}}

== External links ==
{{commons category|NGC 6397}}
* {{wikiSky}}
* {{APOD |date=8 August 2003 |title=Blue Stragglers in NGC 6397}}
* {{APOD |date=20 February 2002 |title=Oddities of Star Cluster NGC 6397}}
* [http://www.messier.seds.org/xtra/ngc/n6397.html SEDS] – NGC 6397

{{Ngc65}}
{{Caldwell catalogue}}
{{Ara (constellation)}}
{{Portal bar|Stars|Outer space}}

{{DEFAULTSORT:NGC 6397}}
[[Category:Ara (constellation)]]
[[Category:Globular clusters]]
[[Category:NGC objects|6397]]
[[Category:Caldwell objects|086b]]