NGC 7331

Lore & Background

NGC 7331 was discovered by William Herschel on 6 September 1784. The galaxy appears similar in size and structure to the Milky Way, but the Milky Way is now believed to be a barred spiral, while NGC 7331 is unbarred, potentially undermining the comparison. In NGC 7331, the central bulge rotates in the opposite direction to the rest of the disk, unlike typical spiral galaxies where they co-rotate. In both visible light and infrared photos, the core appears slightly off-center, with one side of the disk extending further from the core than the opposite side.

NGC 7331 is the brightest galaxy in a visual grouping known as the NGC 7331 Group. Other members—NGC 7335, NGC 7336, NGC 7337, and NGC 7340—lie far in the background at distances of approximately 300–350 million light years. All members of the NGC 7331 Group, along with NGC 7325, NGC 7326, NGC 7327, NGC 7333, and NGC 7338, are listed together as Holm 795 in Erik Holmberg's 1937 study.

Four supernovae have been observed in NGC 7331: SN 1959D (Type II-L, mag. 13.4, discovered 28 June 1959), SN 2013bu (Type II, mag. 16.6, discovered 21 April 2013), SN 2014C (discovered 5 January 2014, which underwent an unusual metamorphosis from Type Ib to Type IIn over a year), and SN 2025rbs (Type Ia, mag. 17.07 at discovery, brightening to mag. 11.9, the brightest supernova of 2025). A 1903 photographic plate from Yerkes Observatory shows a magnitude 16.6 candidate transient that may have also been a supernova.

Reader's Guide

NGC 7331 holds significance as a nearby spiral galaxy that has been compared to the Milky Way, though its unbarred structure and the Milky Way's barred status may weaken that analogy. Its unusual counter-rotating bulge and off-center core provide insights into galactic dynamics and formation. The galaxy is the brightest member of the NGC 7331 Group, a visual grouping that includes several background galaxies at much greater distances. The group is cataloged as Holm 795 in Erik Holmberg's 1937 study of double and multiple galaxies. The galaxy has hosted four confirmed supernovae, including SN 2014C, which underwent a rare metamorphosis from a hydrogen-poor Type Ib to a hydrogen-rich Type IIn over a year, and SN 2025rbs, which became the brightest supernova of 2025. A possible additional supernova candidate was recorded on a 1903 photographic plate. These events make NGC 7331 a valuable target for studying supernova evolution and diversity. Its legacy includes being a well-observed galaxy for amateur and professional astronomers, with numerous images and studies available.

Discovery & Celestial Identity

On 6 September 1784, William Herschel recorded a new deep-sky object in the constellation Pegasus, an entry that would eventually be assigned the catalogue number NGC 7331 and the secondary designation Caldwell 30. The galaxy sits at a distance of approximately 13.427 megaparsecs, or roughly 43.79 million light-years, placing it firmly within the local extragalactic neighborhood. Morphologically, NGC 7331 is classified as an unbarred spiral, meaning its luminous arms sweep outward from a central bulge without the elongated bar structure seen in many other spiral systems. Its placement within Pegasus, one of the most familiar figures in the northern sky, has kept the galaxy on the observing lists of both professional astronomers and dedicated amateur stargazers for well over two centuries. From Herschel's original 1784 observation through modern catalogues and astrophotography projects, the object has remained a steady fixture in the Pegasus sky, referenced in surveys, discussed in popular astronomy guides, and imaged by successive generations of observers who have added their own measurements and interpretations to the growing record.

The Milky Way Twin & Structural Anomalies

NGC 7331 has long been popularly dubbed 'the Milky Way's twin' because, at a glance, the two galaxies appear remarkably similar in overall size and spiral structure. For decades this comparison served as a convenient shorthand for understanding our own galaxy's morphology. However, discoveries made in the 2000s regarding the true structure of the Milky Way have complicated that neat parallel. The Milky Way is now widely believed to be a barred spiral, whereas NGC 7331 retains its unbarred classification, meaning the two systems differ in a fundamental architectural detail. Beyond that, the galaxy harbors a striking internal anomaly: in most spiral galaxies the central bulge co-rotates with the surrounding disk, but in NGC 7331 the bulge spins in the reverse direction relative to the rest of the disk. This counter-rotation is unusual and has drawn attention from researchers studying galactic dynamics. Adding to the oddity, both visible-light and infrared images reveal that the galactic core appears slightly off-center, with one side of the disk extending noticeably farther from the nucleus than the opposite side. Together, these features make NGC 7331 a far more complex object than its 'twin' nickname suggests.

Group Membership & Catalog History

Despite its status as the brightest galaxy in a visual grouping known as the NGC 7331 Group, the relationship is largely one of line-of-sight coincidence rather than true physical association. The four other named members—NGC 7335, NGC 7336, NGC 7337, and NGC 7340—lie far in the background at distances of approximately 300 to 350 million light-years, making them many times more distant than NGC 7331 itself. This grouping first appeared together in Erik Holmberg's 1937 monograph, A Study of Double and Multiple Galaxies Together with Inquiries into some General Metagalactic Problems, where all the members along with NGC 7325, NGC 7326, NGC 7327, NGC 7333, and NGC 7338 were listed under the designation Holm 795. In a separate cataloguing effort, NGC 7331 is also identified as the main member of the LGG 459 Group in the Lyons Groups of Galaxies, a collection of five objects that includes UGC 12082, NGC 7320, NGC 7363, and UGC 12060. The fact that the same galaxy anchors both of these groupings underscores its prominence in the Pegasus field, even if the physical connections among the listed members remain tenuous at best.

A Supernova-Rich History

Over the past seven decades, NGC 7331 has hosted five confirmed supernovae, making it an unusually active host for a galaxy at its distance. The first, SN 1959D, was a Type II-L event of magnitude 13.4, spotted by Milton Humason and Howard S. Gates during a Palomar Observatory survey on 28 June 1959. Nearly half a century later, Kōichi Itagaki discovered SN 2013bu, a Type II of magnitude 16.6, on 21 April 2013. The most scientifically remarkable of the group is SN 2014C, found by the Lick Observatory Supernova Search on 5 January 2014; over the course of a year it underwent a bizarre metamorphosis, shifting from a hydrogen-poor Type Ib to a hydrogen-rich Type IIn. In July 2025, the GOTO survey detected SN 2025rbs, a Type Ia that peaked at magnitude 11.9 to become the brightest supernova of that year, still shining near magnitude 14.4 by late August. ATLAS then recorded SN 2026aaiv, another Type Ia of magnitude 17.325, on 1 September 2026. A 1903 Yerkes Observatory plate also preserves a magnitude 16.6 candidate transient that may represent a sixth, unconfirmed event.