A third possibility is one of the stars used to be much brighter for whatever reason. This is an idea that's been tossed around for a while; it has the problem

A third possibility is one of the stars used to be much brighter for whatever reason. This is an idea that's been tossed around for a while; it has the problem that ancient Greek discussions agree that there were only six stars visible as long as anyone can recall -- even though the myth said there were seven. So if there was a faded star, it faded prior to 500 BC. (Also, no good candidates for a faded star from modern astronomical studies of the Pleiades.) Ancient drawings would help locate the 7th star. Rappenglück (1997) suggested that a collection of dots in the Lascaux cave paintings might represent the Pleiades, in part because it's located above the shoulder of an aurochs (i.e., like the Pleiades is located relative to the constellation of Taurus). But there are only six dots.