Alcyone

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

Ptolemy assigns the Pleiades as a group the nature of the Moon and Mars, and Alcyone inherits that attribution as the cluster's brightest member. Robson and later compilers write it up under Alcyone's own name, which slightly obscures the fact that the assignment was originally to the cluster.

The delineation is unusually consistent and unusually unpleasant: blindness and eye trouble, weeping, sorrow, disgrace, violence, and eminence achieved through difficulty. The eye connection is worth explaining because it is systematic rather than arbitrary. Ptolemy and the tradition after him treat nebulous objects, clusters and what were taken to be cloudy patches, as affecting the eyesight, on a straightforward analogy: a blurred object in the sky signifies blurred vision. The same reasoning attaches eye trouble to Praesepe in Cancer and to other nebulous positions. It is a consistent rule, applied across the catalogue, and it is a signature argument of exactly the kind found throughout this material.

The weeping association has a separate and older root. The Pleiades are the seven daughters of Atlas, and in several tellings they are grieving: for their father's burden, or for their sisters the Hyades, whose name was connected in antiquity with rain. Rain, tears and the cluster's seasonal rising are bound together in Greek material and independently in other traditions.

Alcyone is one of Agrippa's Behenian fifteen, paired with rock crystal and with fennel.

The Pleiades' heliacal rising and setting were agricultural markers across the ancient world, Hesiod's Works and Days uses them to time ploughing and harvest, and the cluster is significant in Japanese, Maori, Aboriginal Australian and Mesoamerican traditions independently.