NGC 891 is a fine edge-on spiral galaxy, located in the constellation Andromeda. A masterpiece that Messier missed, NGC 891 is also Caldwell 23 in Patrick Moore's List.
NGC 891 was discovered by William Herschel on October 6, 1784, and cataloged as H V.19. However, in the appendix to his first catalog, he confused it with his H V.18 (M110, NGC 205) when discussing the discoveries of his sister Caroline Herschel; this remark was picked up by Admiral Smyth and later authors so that its discovery was wrongly attributed to Caroline Herschel for a long time.
Visually, NGC 891 is visible in small telescopes as a faint, 10th magnitude elongated smear of light. In larger apertures, it is a fine 13.5' x 12.8' needle suspended in a rich star field, with a dust lane visible along its equator. Dreyer describes it as "moderately faint, large, elongated 10' x 2' NNE-SSW with a moderate brightening to a slightly bulged core."
At 125x there are bright patches visible along the major axis on each side of the core separated by a very faint, indistinct dark lane needing averted vision. There is a 13th magnitude star at the SSW tip. The SE field is moderately populated with faint stars near or touching the halo.
NGC 891 about 30 million light years away. It is a member of a small group of galaxies, sometimes called the NGC 1023 group, which also contains NGCs 925, 949, 959, 1003, 1023, and 1058 as well as UGCs 1807, 1865 (DDO 19), 2014 (DDO 22), 2023 (DDO 25), 2034 (DDO 24), and 2259. Supernova 1986J was discovered in NGC 891 on August 21, 1986, and reached magnitude 14.
In 1999 the Hubble Space Telescope imaged NGC 891 in infrared. From these images, astronomers suspect that this galaxy might have a bar (and thus be of Hubble type SBb) which is not seen in the visible image because of its edge-on orientation.