NGC 7331 is a superb spiral galaxy lying in northern Pegasus, one of the brightest visible in the autumn sky, which can be found in apertures as small as 60–80mm (~three inches). It is a fine sight through a moderate-aperture telescope, looking like a miniature version of mighty Messier 31 in Andromeda, and makes an irresistible imaging target, with its dusty spiral arms giving up a huge amount of detail.
There are quite a number of fantastic galaxies in the New General Catalogue (NGC) that have not been afforded ‘Messier’ status. NGC 7331 is one of the very few vying for the title of the best non-Messier galaxy in the more northerly reaches of the night sky that Charles Messier, along with William Herschel, made his own over 250 years ago.
NGC 7331 shines at magnitude +10.3 and its inclined form spans a substantial 10.5’ x 3.7’. It’s a superb spiral galaxy with a morphological classification of SA(s)b), in line with what we see as moderately tightly-wound spiral arms and a lack of a central bar. It is highly inclined by 77 degrees to our line of sight, which gives us an oblique view of its majestic spiral arms. It’s believed that NGC 7331 spans around 100,000 light years and lies around 40 million light years away.
There’s no simple star-hop to find NGC 7331; try starting at Scheat (beta [β] Pegasi), the magnitude +2.4 star that marks the north-western corner of the famous ‘Square of Pegasus’. From here, move five degrees west-north-west to land on magnitude +2.9 Matar (eta [η] Pegasi) and then sweep just over four degrees north-north-west. On early October nights NGC 7331 culminates at about 11pm BST, high overhead at the lofty heights of 70 degrees or so.
NGC 7331 is probably out of range of a pair of 10 x 50 binoculars from UK shores, unless you are lucky enough to be under an extremely transparent sky at a very dark site. However, only marginally more light grasp would bag it, as an 80mm telescope will show it as a small, fuzzy oval of light spanning perhaps five arcminutes.
A 150–200mm (six–eight-inch) telescope operating at low to medium power will reveal much more. Study the galaxy for some time and a hint of its brighter core and an asymmetry – the core seems to lie off to one side – should become apparent. Owners of larger telescopes, say in the 250–300mm (10- to 12-inch) range, should be able to see a faint halo of nebulosity – the galaxy’s superb spiral arms – and perhaps a hint of a dark lane running down its western flank.
Images and light-bucket Dobsonians reveal a number of smaller galaxies surrounding NGC 7331. The brighter examples all have NGC designations – 7333, 7335, 7337, 7338, 7336, 7325, 7327 and 7326. Along with NGC 7331 they are called the ‘Deer Lick Group’ or just the ‘NGC 7331 group’. They are not physically associated with NGC 7331, as they lie six times further away than the relatively nearby distance of 50 million light years attributed to the main galaxy.