NGC 6397

{{Infobox globular cluster

| name = NGC 6397

| image = 250px

|caption = A Hubble Space Telescope (HST) image of NGC 6397

| epoch = J2000

| class = IX

| constellation = Ara

| ra = {{RA|17|40|42.09}}

| dec = {{DEC|–53|40|27.6}}

| dist_ly = {{Convert|7.8|kly|kpc|abbr=on|lk=on}}{{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}}

| appmag_v = +6.68

| size_v = 32.0{{prime}}

| mass_msol = {{Val|1.14|e=5}}

| radius_ly = 34 lydistance × sin( diameter_angle / 2 ) = ~34 ly. radius

| v_hb = 14.2

| metal_fe = –1.76

| age = 13.4 ± 0.8 Gyr

| notes = Second closest globular cluster

| names = GCl 74, Lacaille III.11, Dunlop 366, Bennett 98, Caldwell 86

}}

NGC 6397 (also known as Caldwell 86) is a globular cluster in the constellation Ara that was discovered by French astronomer Nicolas-Louis de Lacaille in 1752.{{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 }} It is located about 7,800 light-years from Earth, making it one of the two nearest globular clusters to Earth (the other one being Messier 4). The cluster contains around 400,000 stars, and can be seen with the naked eye under good observing conditions.{{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 = Collins

|year = 2005

|isbn = 978-0-00-717223-8

}}

NGC 6397 is one of at least 20 globular clusters of the Milky Way Galaxy that have undergone a core collapse, meaning that the core has contracted to a very dense stellar agglomeration.

Astronomical research

=Estimating the age of the Milky Way=

In 2004, a team of astronomers focused on the cluster to estimate the age of the Milky Way Galaxy. They used the UV-Visual 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 billion years, which is nearly as 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 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 fusion: roughly 0.083 times the mass of the Sun.{{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

}}

=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.{{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 }}{{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}}

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.{{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 }}

This group provided dynamical simulations that showed that a concentration of white dwarfs could explain the first measurement.{{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 }}

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.{{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 }}

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.{{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}}

= Distant globular clusters =

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

Gallery

File:Hubble's view of dazzling globular cluster NGC 6397.jpg|Hubble's view of globular cluster NGC 6397{{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}}

File:L R.BedinNGC6397.gif|alt=NGC 6397 region double observation HST& JWST|NGC 6397 region double observation HST and JWST{{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 }}

File:Euclid’s view of globular cluster NGC 6397 ESA25170813.jpg|NGC 6397 imaged by Euclid

References

{{Reflist|refs=

{{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 }}

{{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 }}

{{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=. }}

{{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 }}

}}