| NGC 6401 | |
| Appmag V: | 7.4 |
NGC 6401 is a globular cluster in the equatorial constellation of Ophiuchus. 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. It is visible in a small telescope, with an apparent visual magnitude of 7.4 and an angular diameter of .
This cluster is located at a distance of from the Sun and is orbiting in the galactic bulge, in the inner part of the Milky Way galaxy. 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. NGC 6401 is a very old cluster with an estimated age of, but has not undergone core collapse. It has a cluster radius of .
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.