Star of the Month
A dominant multiple star in the Pleiades cluster
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 (d) 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.
Alcyone is a hot, young star of spectral type B5IIIe. ‘B5’ gives its position within the MorganKeenan 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.
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.
Long exposure photographs of the Pleiades reveal blue swirls of nebulous material. This is the result of the cluster stars passing through a cloud of fine dust in space. 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.