Arp 150 (The Guitar). Image credit: Canada-France-Hawaii Telescope.
Ambartsumian's Knot is a dwarf elliptical galaxy in the constellation Ursa Major located at the end of a what appears to be a bridge of matter extending from the elliptical galaxy NGC 3561B (RA 11h 11m, Dec +28° 42'). The latter is interacting with the nearby spiral galaxy NGC 3651A giving rise to a number of tidal tails and tidal dwarf galaxies.
The entire system is called Arp 105 or, colloquially, The Guitar, because of its overall shape. The enormously long tidal tail visible to the left (north) of NGC 3651A in the accompanying photo, Arp 105N, stretches out for 100,000 parsecs (325,000 light-years) from the parent galaxy. Ambartsumian's Knot lies at the end of a shorter tail called Arp 105S.
Arp 150 lies close to the center of the rich cluster Abell 1185, which lies about 400 million light-years away.