NGC 6540

NGC 6540
Hubble Space Telescope image of NGC 6540
Observation data (J2000 epoch)
ConstellationSagittarius
Right ascension18h 06m 08.60s[1]
Declination−27° 45′ 55.0″[1]
Distance12.07 ± 0.98 kly (3.7 ± 0.3 kpc)[2]
Apparent magnitude (V)9.30[3]
Physical characteristics
Absolute magnitude−5.38[1]
Radius4.75′ × 4.75′[3]
Metallicity = −1.20[1] dex
Other designationsC 1803-278, NGC 6540, Cr 364[4]
See also: Globular cluster, List of globular clusters

NGC 6540 is a globular cluster of stars in the souther constellation Sagittarius, positioned about 4.66° away from the galactic center.[5] It was discovered by German-British astronomer Wilhelm Herschel on May 24, 1784, with an 18.7-inch mirror telescope, who described the cluster as "pretty faint, not large, crookedly extended, easily resolvable". It has an apparent visual magnitude of 9.3[3] with an angular diameter of about 9.5 arcminutes.

The cluster is located at a distance of 12 kly (3.7 kpc) from the Sun,[2] and 14 kly (4.4 kpc) from the galactic center. It was originally thought to be an open cluster before being designated a globular.[1] The cluster includes a peculiar X-ray source of uncertain type.[6]

  1. ^ a b c d e Cite error: The named reference Di_Criscienzo_et_al_2006 was invoked but never defined (see the help page).
  2. ^ a b Cite error: The named reference Pérez-Villegas_et_al_2018 was invoked but never defined (see the help page).
  3. ^ a b c Cite error: The named reference seds was invoked but never defined (see the help page).
  4. ^ Cite error: The named reference simbad was invoked but never defined (see the help page).
  5. ^ Cite error: The named reference Bica_et_al_1994 was invoked but never defined (see the help page).
  6. ^ Cite error: The named reference Mereghetti_et_al_2018 was invoked but never defined (see the help page).