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, 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
A Hubble Space Telescope (HST) image of NGC 6397
Observation data (J2000 epoch)
Class	IX
Constellation	Ara
Right ascension	17h 40m 42.09s
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	1.14×105 M☉
Radius	34 ly
VHB	14.2
Metallicity	${\displaystyle {\begin{smallmatrix}\left[{\ce {Fe}}/{\ce {H}}\right]\end{smallmatrix}}}$ = –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

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.