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| NGC 6401 | |
|---|---|
 HST image of NGC 6401 | |
| Observation data (J2000 epoch) | |
| Constellation | Ophiuchus |
| Right ascension | 17h 38m 36.93s[1] |
| Declination | −23° 54′ 31.5″[1] |
| Distance | 25.1 kly (7.70 kpc)[2] |
| Apparent magnitude (V) | 7.4[3] |
| Apparent dimensions (V) | 4.8′[3] |
| Physical characteristics | |
| Metallicity | = −1.15±0.20[4] dex |
| Estimated age | 13.2±1.2 Gyr[4] |
| Other designations | GC 4314, NGC 6301[5] |
![{\displaystyle {\begin{smallmatrix}\left[{\ce {Fe}}/{\ce {H}}\right]\end{smallmatrix}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/4c0821bd80891e071c08e7c7ee8e022baedf522c)
NGC 6401 is a globular cluster in the equatorial constellation of Ophiuchus.[3] This star cluster was discovered by German-English astronomer William Herschel in 1784, but he mistakenly classified it as a bright nebula. Later, his son John Herschel came to the same conclusion because the technology of the day did not allow the individual stars to be visually resolved.[6] It is visible in a small telescope, with an apparent visual magnitude of 7.4 and an angular diameter of 4.8′.[3]
This cluster is located at a distance of 25.1 kly (7.70 kpc) from the Sun[2] and is orbiting in the galactic bulge, in the inner part of the Milky Way galaxy.[4] Because of its high mass, it may be gravitationally trapped inside the Milky Way's bar. From the perspective of the Earth, this cluster is situated 5.3° from the Galactic Center and is strongly reddened due to extinction from interstellar dust.[7] NGC 6401 is a very old cluster with an estimated age of 13.2±1.2 Gyr,[4] but has not undergone core collapse.[8] It has a cluster radius of 2.4′.[7]
23 class ab and 11 class c RR Lyrae variables have been identified within a cluster radius of the core. From this, it is deduced that this is an Oosterhoff type I (Oo I) cluster.[7]
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