Pleiades, in astronomy, famous open star cluster in the constellation Taurus; cataloged as M45. The cluster consists of some 500 stars, has a diameter of 35 light-years, and is 400 light-years distant from the earth. Six stars are easily visible to the naked eye—Alcyone (the brightest), Electra, Celaeno, Sterope, Maia, and Taygete. Known as the Seven Sisters, this group was named by the Greeks for the seven daughters of Atlas and Pleione; the seventh Pleiad was, according to legend, lost or in hiding. Many faint stars associated with the other six are visible with the telescope; one of these stars may have been much brighter and visible to the naked eye in ancient times, thus accounting for the many early references to seven stars. The Pleiades cluster is 150 million years old, making it a young star cluster.