Star of the month

Rasal­h­ague, the bright­est star in Ophi­uchus

Sky at Night Magazine - - CONTENTS -

At the top of Ophi­uchus, the Ser­pent Bearer, rep­re­sent­ing his head, is Rasal­h­ague (Al­pha ( α) Ophi­uchi). Close by is Rasal­gethi, the head of an­other gi­ant in the stars, Her­cules, the Strong Man.

Rasal­h­ague is a bi­nary star sys­tem, 48 lightyears from Earth. It has a faint com­pan­ion, too close to its pri­mary for am­a­teur in­stru­ments to see. The pri­mary is es­ti­mated to be 2.4 times as mas­sive as the Sun, while the sec­ondary has about 85 per cent of the Sun’s mass. The or­bital pe­riod is 8.62 years and at the last pe­ri­as­tron (where both stars were at min­i­mum sep­a­ra­tion) in 2011, they ap­peared sep­a­rated by just 50 milli- arc­sec­onds.

Rasal­h­ague ap­pears to shine at mag. +2.08 and has a spec­tral clas­sif­ca­tion of A5 III – a gi­ant star (the ‘III’ part) that has ex­hausted the hy­dro­gen fuel at its core. Its com­pan­ion is red­der, with an es­ti­mated spec­tral class of K5 V.

Rasal­h­ague is a fast ro­ta­tor too, spin­ning at 240km/s. For com­par­i­son the Sun’s ro­ta­tional ve­loc­ity is a rather leisurely 2km/s. At such a speed Rasal­h­ague is close to its break-up speed of 270 km/s and will be bulging no­tice­ably at its equa­tor. An ef­fect known as ‘grav­ity dark­en­ing’ will also be caus­ing the star’s poles to be hot­ter than its equa­tor. Cur­rent es­ti­mates sug­gest the equa­to­rial ra­dius is 20 per cent larger than the po­lar ra­dius. We get to see the star al­most side­ways on, its ro­ta­tional axis be­ing in­clined to our line of sight by around 88˚.

Rasal­h­ague ro­tates ex­tremely quickly, 120 times faster than our Sun

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