NGC 6540
Globular cluster in the constellation Sagittarius
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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.
| NGC 6540 | |
|---|---|
Hubble Space Telescope image of NGC 6540 | |
| Observation data (J2000 epoch) | |
| Constellation | Sagittarius |
| Right ascension | 18h 06m 08.60s[1] |
| Declination | −27° 45′ 55.0″[1] |
| Distance | 12.07 ± 0.98 kly (3.7 ± 0.3 kpc)[2] |
| Apparent magnitude (V) | 9.30[3] |
| Physical characteristics | |
| Absolute magnitude | −5.38[1] |
| Radius | 4.75′ × 4.75′[3] |
| Metallicity | [Fe/H] = −1.20[1] dex |
| Other designations | C 1803-278, NGC 6540, Cr 364[4] |
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]
