Alcyone is Eta Tauri, the brightest star of the Pleiades, apparent magnitude 2.87, at about 440 light years, spectral type B7 IIIe.
The e in that classification matters: Alcyone is a Be star, a rapidly rotating hot star that has flung off a gaseous disc from its equator. Such discs produce emission lines and variability. A 2021 interferometric study concluded that Alcyone is a single star, despite the several optical companions listed in older catalogues, which are line-of-sight neighbours rather than physical partners.
The Pleiades are a genuine open cluster, gravitationally associated stars formed together from one cloud, at roughly 440 light years, with an age usually given around 130 million years, which is young. Alcyone appears somewhat younger than the cluster average, which may make it a blue straggler. The nebulosity visible around the cluster in photographs is not the gas the stars formed from; that has long dispersed. It is an unrelated dust cloud the cluster is currently passing through, and the distinction was established early in the twentieth century.
The cluster contains well over a thousand stars. Most people see six with the naked eye, and the traditional count is seven, which has generated the widespread lost Pleiad stories found in Greek and in many unrelated traditions. Whether one of the stars has genuinely faded, or whether seven is a conventional number that never matched what was visible, is not resolvable; the naked-eye count varies with eyesight and sky conditions, and several people can see more than seven under good skies.
Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with Almagest (Mathematike Syntaxis)Claudius Ptolemy