Star of the month
Cebalrai, at the northeast of Ophiuchus
Cebalrai, Beta (b) Ophiuchi, marks the northeast ‘shoulder’ of Ophiuchus, the Serpent Bearer. The star is an easy naked-eye object, shining at mag. +2.8 and appearing orange-hued. Its spectral classification is K2III; a giant star with a temperature of 4,194˚C. It’s located 82 lightyears away and is 1.13 times as massive, 12.4 times larger and 63 times more luminous than the Sun. Having entered a late stage of evolution, it has become swollen in size and is probably fusing helium into carbon deep within its core. Consequently, although it’s only 13 per cent larger than the Sun, its increased size is what boosts its overall luminosity.
Detailed Doppler analysis of the star’s spectrum reveals three variable periods for Cebalrai, periods over which the size of the star also shows variability. The variability periods are 0.26, 13.1 and 142 days. The longer period aligns with the star’s rotation period and it can be surmised that this variability is due to the passage of darker regions across its disc. The 13.1-day pulse origin is less understood.
Cebalrai sits 8.2˚ southsoutheast of Rasalhague (Alpha (a) Ophiuchi). Gamma (γ) Ophiuchi lies 2.2˚ to the southeast and the open cluster IC 4665, 1.3˚ to the north, while 5.2˚ to the east is the asterism known as Poniatowski’s Bull.
An important star is located between Cebalrai and mag. +4.8, 66 Ophiuchi, marking the end of the western arm of the ‘V’ of Poniatowski’s Bull. Located 0.7˚ to the westnorthwest of 66 Ophiuchi is mag. +9.5 Barnard’s Star, the star with the largest apparent proper motion. Over a human lifetime, it appears to move by a quarter of a degree in the sky.