Star of the month

Sheliak, a multiple star system in Lyra

Lyra, the Lyre, is a distinctive summer constellation: its alpha star Vega is the fifth brightest in the night sky. Southeast of Vega is a distinct squashed diamond pattern formed from Sheliak (Beta (b)), Sulafat (Gamma (g)), Delta (d) and Zeta (z) Lyrae. Sheliak is the mag. +3.3 star marking the diamond’s southwest corner.

Sheliak is actually a multiple star system that is around 960 lightyears away. The system has three main components: Beta Lyrae A (variable magnitude, +3.3 to +4.3), B (mag. +7.2, 46 arcseconds from A) and C (mag. +9.9, 86 arcseconds away). Stars B and C are singles, while A is another multiple.

Beta Lyrae A comprises an eclipsing binary pair identified as Aa and a single star Ab. The former, labelled Aa1 and Aa2 consists of a B7 giant and a B0.5 dwarf star. The giant is 8,300 times more luminous and 14.4 times larger than the Sun. The dwarf is 27,700 times more luminous and six times larger than the Sun.

Their mutual separation distance is around a quarter of an Astronomical Unit (the average Sun-Earth distance). At this proximity, tidal forces result in mass transfer from the giant to the dwarf, and the giant is distorted towards the smaller but heavier star. Its shape will resemble a teardrop, the pointed end drawn out towards the dwarf star. The transferred material passes through the teardrop’s point ending in a spiralling accretion disc around the dwarf star.

The orbital period of Beta Lyrae Aa is 12.9 days. Their line of sight is such that the stars exhibit a brightness variation over 6.5 days, each star mutually eclipsing the other. The hot accretion disc around the dwarf causes variation in the depth of alternating minima. The result is one normal dip to mag. +3.8, then a deeper one to mag. +4.3.