NGC 6397

NGC 6397 (also known as Caldwell 86) is a globular cluster in the constellation Ara. It is located about 7,800 light-years from Earth,[3] making it one of the two nearest globular clusters to Earth (the other one being Messier 4). The cluster contains around 400,000 stars,[8] and can be seen with the naked eye under good observing conditions.[9]

NGC 6397
A Hubble Space Telescope (HST) image of NGC 6397
Observation data (J2000 epoch)
ClassIX[1]
ConstellationAra
Right ascension17h 40m 42.09s[2]
Declination–53° 40′ 27.6″[2]
Distance7.8 kly (2.4 kpc)[3]
Apparent magnitude (V)+6.68[4]
Apparent dimensions (V)32.0′
Physical characteristics
Mass1.14×105[5] M☉
Radius34 ly[6]
VHB14.2
Metallicity = –1.76[7] dex
Estimated age13.4 ± 0.8 Gyr[8]
Notable featuresSecond closest globular cluster
Other designationsGCl 74,[4] Lacaille III.11, Dunlop 366, Bennett 98, Caldwell 86

NGC 6397 is one of at least 20 globular clusters of the Milky Way Galaxy that have undergone a core collapse,[8] 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[8] 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.[10]

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.[11][12] 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.[13] This group provided dynamical simulations that showed that a concentration of white dwarfs could explain the first measurement.[14]

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.[5] 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.[15]

Distant globular clusters

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

  • Hubble's view of globular cluster NGC 6397
    Hubble's view of globular cluster NGC 6397[16]

See also

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. Dunlop, Storm (2005). Atlas of the Night Sky. Collins. ISBN 978-0-00-717223-8.
  10. "HST analysis of faint stars in NGC6397". Results for NGC 6397. Retrieved 2006-09-26.
  11. Vitral, Eduardo; Mamon, Gary A. (2021), "Does NGC 6397 contain an intermediate-mass black hole or a more diffuse inner subcluster?", A&A, 646 (A63): A63, arXiv:2010.05532, Bibcode:2021A&A...646A..63V, doi:10.1051/0004-6361/202039650
  12. "Hubble Uncovers Concentration of Small Black Holes". ESA. 11 February 2021. Retrieved 21 February 2021.
  13. 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
  14. 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
  15. "Chasser des trous noirs dans un cimetière d'étoiles". 10 June 2022.
  16. "Hubble's view of dazzling globular cluster NGC 6397". www.spacetelescope.org. Retrieved 13 April 2018.
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