NGC 523, also known as Arp 158, from the ARP catalog is a spiral galaxy located in the constellation Andromeda. It was discovered separately by William Herschel on 13 September 1784, and by Heinrich d'Arrest on 13 August 1862. d'Arrest's discovery was listed as NGC 523, while Herschel's was listed as NGC 537; the two are one and the same. John Dreyer noted in the New General Catalogue that NGC 523 is a double nebula.In September 2001 a type Ia supernova, SN 2001en was discovered in NGC 523.

300/350mm - 13.1" (12/22/84): moderately bright, thin edge-on 4:1 ~E-W. Located north of the NGC 483-517 cluster.

600/800mm - 24" (11/24/14): fairly bright, moderately bright, very elongated 7:2 ~E-W, ~1.1'x0.3'. This disrupted galaxy (or merger) appeared very asymmetric, widening a bit at the east end and tapering slightly to the west. A small, relatively bright knot (HII complex or core of companion?), ~10" diameter, is at the east end. An easily visible mag 14-14.5 star is embedded at the west end. The main body, which extends ~40" from the knot to the star, is fairly thin and only very weakly brighter in the center. With careful viewing, a very faint narrow plume extends west of the main glow.