NGC 663 is an open cluster in Cassiopeia. This fine cluster should have made Messier's list, and is the 10th object in the Caldwell catalog.
NGC 663 is easy to see in the smallest of backyard telescopes or even large binoculars. Four 8th magnitude, and ten 9th to 10th magnitude stars are sprinkled among sixty fainter stars over a 15' area. The cluster contains the double stars Struve 151, Struve 152, and Struve 153. In a viewfinder NGC 663 appears as a rather large, hazy spot.
NGC 663, with an age of about 9 million years, contains the largest percentage of Be stars known in any young open cluster. Be stars are formally defined as non-supergiant B-type stars whose spectra have Balmer emission lines. The emission is produced from a flattened disk of gas around the star's equator. The mystery of the "Be phenomenon" is that the hydrogen emission can come and go episodically on timescales as short as a few days, and as long as several decades. Rapid rotation, nonradial pulsation, radiation-driven winds, flarelike magnetic activity, and binary interaction have all been proposed as factors that may cause the star to shed material from its surface, leading to the formation of the disk.