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NGC 6397
A Hubble Space Telescope (HST) image of NGC 6397
Observation data (J2000 epoch)
Class	IX
Constellation	Ara
Right ascension	17 40 42.09
Declination	–53° 40′ 27.6″
Distance	7.8 kly (2.4 kpc)
Apparent magnitude (V)	+6.68
Apparent dimensions (V)	32.0′
Physical characteristics
Mass	1.14×10 M☉
Radius	34 ly
VHB	14.2
Metallicity	${\displaystyle {\begin{smallmatrix}\left\end{smallmatrix}}}$ = –1.76 dex
Estimated age	13.4 ± 0.8 Gyr
Notable features	Second closest globular cluster
Other designations	GCl 74, Lacaille III.11, Dunlop 366, Bennett 98, Caldwell 86
See also: Globular cluster, List of globular clusters

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. 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.

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.

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

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. 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.

Distant globular clusters

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

Gallery

• Hubble's view of globular cluster NGC 6397
• NGC6397 region double observation HST and JWST

See also

• List of brightest stars
• List of nearest bright stars
• Lists of stars
• Historical brightest stars

References

1. Shapley, Harlow; Sawyer, Helen B. (August 1927), "A Classification of Globular Clusters", Harvard College Observatory Bulletin, 849 (849): 11–14, Bibcode:1927BHarO.849...11S.
2. ^ Goldsbury, Ryan; et al. (December 2010), "The ACS Survey of Galactic Globular Clusters. X. New Determinations of Centers for 65 Clusters", The Astronomical Journal, 140 (6): 1830–1837, arXiv:1008.2755, Bibcode:2010AJ....140.1830G, doi:10.1088/0004-6256/140/6/1830, S2CID 119183070.
3. ^ Salazar, Doris Elen (5 April 2018). "Cosmic 'Yardstick' Measures Distance to One of Universe's Oldest Objects". Space.com. Retrieved 5 April 2018.
4. ^ "NGC 6397". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2006-11-16.
5. ^ Vitral, Eduardo; Kremer, Kyle; Libralato, Mattia; Mamon, Gary A.; Bellini, Andrea (2022), "Stellar graveyards: clustering of compact objects in globular clusters NGC 3201 and NGC 6397", Monthly Notices of the Royal Astronomical Society, 514 (1): 806, arXiv:2202.01599, Bibcode:2022MNRAS.514..806V, doi:10.1093/mnras/stac1337
6. distance × sin( diameter_angle / 2 ) = ~34 ly. radius
7. Forbes, Duncan A.; Bridges, Terry (May 2010), "Accreted versus in situ Milky Way globular clusters", Monthly Notices of the Royal Astronomical Society, 404 (3): 1203–1214, arXiv:1001.4289, Bibcode:2010MNRAS.404.1203F, doi:10.1111/j.1365-2966.2010.16373.x, S2CID 51825384.
8. ^ "How Old is the Milky Way ?". Results for NGC 6397. Archived from the original on 2008-09-05. Retrieved 2006-09-26.
9. Jones, K. G. (March 1969). "The search for the nebulae - VI". Journal of the British Astronomical Association. 79: 213–222. Bibcode:1969JBAA...79..213J.
10. Dunlop, Storm (2005). Atlas of the Night Sky. Collins. ISBN 978-0-00-717223-8.
11. "HST analysis of faint stars in NGC6397". Results for NGC 6397. Retrieved 2006-09-26.
12. Vitral, Eduardo; Mamon, Gary A. (2021), "Does NGC 6397 contain an intermediate-mass black hole or a more diffuse inner subcluster?", Astronomy & Astrophysics, 646 (A63): A63, arXiv:2010.05532, Bibcode:2021A&A...646A..63V, doi:10.1051/0004-6361/202039650
13. "Hubble Uncovers Concentration of Small Black Holes". ESA. 11 February 2021. Retrieved 21 February 2021.
14. Rui, Nicholas Z.; Weatherford, Newlin C.; Kremer, Kyle; Chatterjee, Sourav; Fragione, Giacomo; Rasio, Frederic A.; Rodriguez, Carl L.; Ye, Claire S. (2021), "No Black Holes in NGC 6397", Research Notes of the American Astronomical Society, 5 (3): 47, arXiv:2103.06273, Bibcode:2021RNAAS...5...47R, doi:10.3847/2515-5172/abee77, S2CID 232185141
15. Kremer, Kyle; Rui, Nicholas Z.; Weatherford, Newlin C.; Chatterjee, Sourav; Fragione, Giacomo; Rasio, Frederic A.; Rodriguez, Carl L.; Ye, Claire S. (2021), "White Dwarf Subsystems in Core-Collapsed Globular Clusters", The Astrophysical Journal, 917 (1): 28, arXiv:2104.11751, Bibcode:2021ApJ...917...28K, doi:10.3847/1538-4357/ac06d4, S2CID 233394452
16. "Chasser des trous noirs dans un cimetière d'étoiles". 10 June 2022.
17. "Hubble's view of dazzling globular cluster NGC 6397". www.spacetelescope.org. Retrieved 13 April 2018.
18. "A caccia di nane bianche e pianeti distrutti con Jwst". Il Notiziario Online dell'Istituto Nazionale di Astrofisica. 7 June 2024.

External links

• NGC 6397 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images
• NASA Astronomy Picture of the Day: Blue Stragglers in NGC 6397 (8 August 2003)
• NASA Astronomy Picture of the Day: Oddities of Star Cluster NGC 6397 (20 February 2002)
• SEDS – NGC 6397

v t e Constellation of Ara
Ara in Chinese astronomy List of stars in Ara
Stars	Bayer α (Choo/Tchou) β γ δ ε ε ζ (Tseen Yin) η θ ι κ λ μ (Cervantes) ν (V539) ν π ρ ρ σ Variable R S OY V626 V821 V828 V829 V854 V862 V872 HR 6177 6187 6207 6236 6251 6253 6297 6300 6312 6323 6408 6416 (41 G.) 6423 6438 6477 6478 6483 6487 6513 6759 HD 152079 153201 154672 154857 156411 (Inquill) Other 2MASS J18082002−5104378 CXOU J164710.2−455216 Gliese 674 Gliese 676 IRAS 16594-4656 SAO 244567 SGR 1627−41 Westerlund 1-20 Westerlund 1 W26 Westerlund 1-75 Westerlund 1-237 Westerlund 1-243 XTE J1650−500
Exoplanets Gliese 674 b HD 154672 b HD 154857 b c HD 156411 b μ Arae b c d e
Star clusters	NGC 6193 6200 6204 6208 6250 6352 6362 6397 Other ESO 280-SC06 IC 4651 Westerlund 1
Nebulae	NGC 6188 6326 Other Stingray Nebula Water Lily Nebula
Galaxies	NGC 6215 6221 6300 6328
Category

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• Ara (constellation)
• Globular clusters
• NGC objects
• Caldwell objects