NGC 7331

NGC 7331, also known as Caldwell 30, is an unbarred spiral galaxy about away in the constellation Pegasus. It was discovered by William Herschel on 6 September 1784.
The galaxy appears similar almost in size and structure to the Milky Way, and is sometimes referred to as "the Milky Way's twin". However, discoveries in the 2000s regarding the structure of the Milky Way may call this similarity into doubt, particularly because the latter is now believed to be a barred spiral, compared to the unbarred status of NGC 7331. In spiral galaxies the central bulge typically co-rotates with the disk but the bulge in the galaxy NGC 7331 is rotating in the opposite direction to the rest of the disk. In both visible light and infrared photos of the NGC 7331, the core of the galaxy appears to be slightly off-center, with one side of the disk appearing to extend further away from the core than the opposite side.

Galaxy Groups

NGC 7331 is the brightest galaxy in the field of a visual grouping known as the NGC 7331 Group of galaxies. In fact, the other members of the group, NGC 7335, NGC 7336, NGC 7337 and NGC 7340, lie far in the background at distances of approximately 300–350 million light years.
All of the members of the NGC 7331 Group, along with NGC 7325, NGC 7326, NGC 7327, NGC 7333, NGC 7338, are listed together as Holm 795 in Erik Holmberg's A Study of Double and Multiple Galaxies Together with Inquiries into some General Metagalactic Problems, published in 1937.

Supernovae

Four supernovae have been observed in NGC 7331:

SN 1959D was discovered by Milton Humason and Howard S. Gates in a survey at Palomar Observatory on 28 June 1959.
SN 2013bu was discovered by Kōichi Itagaki on 21 April 2013.
SN 2014C was discovered by the Lick Observatory Supernova Search on 5 January 2014. The supernova underwent an unusual "metamorphosis" from a hydrogen-poor Type Ib to a hydrogen-rich Type IIn over the course of a year.
SN 2025rbs was discovered by GOTO on 14 July 2025. It got as bright as magnitude 11.9, making it the brightest supernova of 2025. By the end of August 2025, the supernova was still around magnitude 14.4.
In addition to the confirmed supernovae, a 1903 photographic plate from Yerkes Observatory shows a magnitude 16.6 candidate transient that may have also been a supernova.