At mag. +2.9, Alcyone is the brightest star in the Pleiades open cluster, M45, and the third brightest in Taurus.
It’s designated as Eta (η) Tauri, a multiple star system that is a genuine member of the Pleiades cluster.
The Pleiades is about 100 million years old and is located 444 lightyears from Earth.
Discover more winter star clusters and star clusters visible with the naked eye.
Alcyone facts
Alcyone is a hot, young star of spectral type B5IIIe. ‘B5’ gives its position within the Morgan- Keenan spectral classification system, which classifies stars by temperature.
The ‘III’ means it’s a normal giant-type star; the ‘e’ indicates that its spectrum has emission lines present.
Alcyone is a little over 2,000 times more luminous than the Sun and 9.3 times as big. Its rotational velocity is high at around 149km/s.
For comparison, the Sun rotates at 2km/s. As a consequence, material has spun off Alcyone’s equator into a light-emitting disc, the source of the emission lines indicated by ‘e’ in the star’s spectral type.
Alcyone the multiple star
The main star – Alcyone A – is a triple.
A low-mass companion orbits the primary star over a four-day period with a heavier companion, approximately half as massive as the primary, orbiting at a distance similar to that of Jupiter from the Sun.
This system has three orbital companions.
Alcyone B and C are 8th magnitude, white A-type stars separated from Alcyone A by 117 and 181 arcseconds respectively.
Alcyone C is a Delta Scuti-type variable with a small brightness variation from mag. +8.25 to mag. +8.30 over a period of 73 minutes.
Alcyone D is a yellow-white, F-type, mag. +8.7 star located 191 arcseconds from Alcyone A.
The Pleiades’ blue nebulosity
The Pleiades is well-known for being a wonderfully blue star cluster easily visible with the naked eye and great for exploring with even a modest pair of binoculars.
Long exposure photographs of the Pleiades really bring out the blue swirls of nebulous material.
This is the result of the cluster stars passing through a cloud of fine dust in space.
To see this for yourself, read our guide on how to photograph the Pleiades.
The light from the brighter stars reflects off this material – typically carbon compounds (eg, diamond dust) with iron and nickel.
The particles’ size makes them particularly good at scattering blue light.
Another star worth seeking out in the Pleiades is Electra, the furthest from the ‘handle’ of the star cluster’s saucepan shape.
Are you a Pleiades fan? What are your favourite among its stars? Let us know by emailing contactus@skyatnightmagazine.com.
This article appeared in the November 2018 issue of BBC Sky at Night Magazine.