At the top of Ophiuchus, the Serpent Bearer, representing his head, is the star Rasalhague (Alpha (α) Ophiuchi).
Close by is Rasalgethi, the head of another giant in the stars, located in the constellation 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% of the Sun’s mass.
The orbital period is 8.62 years and at the periastron (where both stars were at minimum separation) in 2011, they appeared separated by just 50 milli-arcseconds.
Observing Rasalhague
Rasalhague appears to shine at mag. +2.08 and has a spectral classification 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.
The star 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% 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˚.
If you've observed or photographed Rasalhague, share your observations and images with us by emailing contactus@skyatnightmagazine.com.
This article appeared in the May 2019 issue of BBC Sky at Night Magazine.
