|Observation data (J2000 epoch)|
|Right ascension||22h 37m 04.1s|
|Declination||+34° 24′ 56″|
|Redshift||816 ± 1 km/s|
|Distance||39.8 ± 3.3 Mly (12.2 ± 1.0 Mpc) |
|Apparent magnitude (V)||10.4|
|Size||120,000 ly (diameter)|
|Apparent size (V)||10.5′ × 3.7′|
|UGC 12113, PGC 69327, Caldwell 30|
NGC 7331, also known as Caldwell 30, is an unbarred spiral galaxy about 40 million light-years (12 Mpc) away in the constellation Pegasus. It was discovered by William Herschel in 1784. 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, 7336, 7337 and 7340, lie far in the background at distances of approximately 300-350 million light years.
The galaxy appears similar 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.
Multiple supernova events have been observed in this galaxy. SN 1959D, a Type IIL supernova, was the first supernova identified within NGC 7331. The supernova was discovered by Milton Humason and H. S. Gates in a survey at Palomar Observatory. More recent supernovae are SN 2013bu and SN 2014C, the latter of which underwent an unusual "metamorphosis" from a hydrogen-poor Type Ib to a hydrogen-rich Type IIn over the course of a year .  A 1903 photographic plate from Yerkes Observatory shows a magnitude 16.6 candidate transient that may have also been a supernova.
- M94 – another galaxy with a prominent starburst ring
- NGC 1512 – another galaxy with a prominent starburst ring
- Flocculent spiral galaxy
- "NASA/IPAC Extragalactic Database". Results for NGC 7331. Retrieved 2006-11-10.
- Jensen, Joseph B.; Tonry, John L.; Barris, Brian J.; Thompson, Rodger I.; et al. (February 2003). "Measuring Distances and Probing the Unresolved Stellar Populations of Galaxies Using Infrared Surface Brightness Fluctuations". Astrophysical Journal. 583 (2): 712–726. arXiv:astro-ph/0210129. Bibcode:2003ApJ...583..712J. doi:10.1086/345430. S2CID 551714.
- The NGC/IC Project Archived 2011-02-28 at the Wayback Machine : NGC Discoverers List by Bob Erdmann.
- "Spiral Galaxy NGC 7331, Galaxy Group (NGC 7335, 7336, 7337)".
- "Seeing Double: Spitzer Captures Our Galaxy's Twin". Spitzer Space Telescope Newsroom. Spitzer Science Center. 2004-06-28. Archived from the original on May 17, 2007. Retrieved 2008-04-24.
- "The Milky Way Has Only Two Spiral Arms". 2008-06-03. Retrieved 2008-06-04.
- A Counter-rotating Bulge in the Sb Galaxy NGC 7331 , F. Prada, C. Gutierrez, R.F. Peletier, C.D. McKeith, the Astrophysical Journal, 463 :L9–L12, 20/5/1996
- "NASA/IPAC Extragalactic Database". Results for SN 1959D. Retrieved 2006-11-10.
- M. L. Humason; H. S. Gates (1960). "The 1959 Palomar Supernova Search". Publications of the Astronomical Society of the Pacific. 72 (426): 208–209. Bibcode:1960PASP...72..208H. doi:10.1086/127513.
- D. Milisavljevic; et al. (2015). "Metamorphosis of SN 2014C: Delayed Interaction Between a Hydrogen Poor Core-collapse Supernova and a Nearby Circumstellar Shell". The Astrophysical Journal. 815 (2): 120. arXiv:1511.01907. Bibcode:2015ApJ...815..120M. doi:10.1088/0004-637X/815/2/120. S2CID 31773513.
- Cerny; Chapman; Glusman; Kron; Liang; et al. (2021). "Precise Photometric Measurements from a 1903 Photographic Plate Using a Commercial Scanner". Publications of the Astronomical Society of the Pacific. 133 (1022): 044501. arXiv:2101.03699. Bibcode:2021PASP..133d4501C. doi:10.1088/1538-3873/abec20. S2CID 231573185.
- Media related to NGC 7331 at Wikimedia Commons
- Calar Alto Observatory – NGC 7331
- APOD (2004-07-01) – "A Galaxy So Inclined"
- SST – "Morphology of Our Galaxy's 'Twin'"
- NGC 7331 at the astro-photography site of Mr. T. Yoshida
- NGC7331 at W. Kloehr Astrophotography
- NGC 7331 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images
- SEDS – NGC 7331