Star of the month
Rasalhague, the brightest star in Ophiuchus
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