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A shell galaxy is a galaxy with borders that don't merely fade into surroundings, but at some angular distance beyond the brightness consisting of the bulk of the stars, shows a discernible edge beyond which it is noticeably dimmer, possibly with concentric visible edges, each shifting to less brightness. The levels of brightness are due to the density of stars in the galactic halo. These distinct regions of high stellar density are termed tidal shells, thought to be due to bursts of star formation, likely due to a galaxy merger or other galaxy interaction, in a similar manner to the generation of galaxy tidal tails. An example shell galaxy is NGC 3923. Such shells were noticed around some elliptical galaxies, which were termed shell galaxies.
Recently, it has been announced that such a shell has been discerned around Andromeda. This is unique because M31 is a spiral galaxy.