NGC 7331 is a spiral galaxy in the constellation Pegasus.
NGC 7331 was discovered by William Herschel in 1784. Visually, NGC 7331 is a bright galaxy, highly elongated at 6' x 1.5' in size. It is seen nearly edge-on, with prominent dust lanes, and has a large, extended, core containing a stellar nucleus. Four faint companion galaxies are to the east of NGC 7331; they may be glimpsed visually with averted vision under good skies.
NGC 7331 is an Sb galaxy at a distance of 49 million light-years, and perhaps 30,000 light years across. It is the brightest and largest member of the NGC 7331 group of galaxies. The galaxy is similar in size and structure to the Milky Way. SN 1959D, a Type II supernova, is the only supernova that has been identified within NGC 7331. This supernova was discovered by Milton Humason in a survey at Palomar Observatory in 1959.
NGC 7331 was one of 18 key galaxies chosen to calibrate the galactic distance scale, using the Hubble Space Telescope to obtain an accurate distance via Cepheid variable observations.