Star of the Month
Algieba – the tight double whose name means 'forehead' but is part of the Lion's neck
The Sickle asterism, representing the neck and head of the Lion of Leo, is one of the most prominent patterns of the spring sky. It looks like a backward question mark, with the bright mag. +1.4 star Regulus (Alpha (_) Leonis) providing the punctuation dot. Located 4.8º north of Regulus is mag. +3.5 Eta (d) Leonis. Our target star is the next one along the Sickle, Algieba (Gamma (a) Leonis). Shining at mag. +2.0, it is the most prominent member of the asterism after Regulus.
Algieba is a lovely double star formed from an orange-red primary and yellow secondary. A common trap for the unwitting observer is the mag. +4.8 star 40 Leonis, which is 22.5 arcminutes to the south of Algieba, an easy naked eye star. This is sometimes misidentified as the companion. The actual binary system is much tighter, separated by just 4 arcseconds. We’d recommend a 3-inch or larger scope at a magnification of 120x or more to split Algieba. Both stars are giants, taking 510 years to complete one orbit. The brighter star is 180 times as luminous as the Sun, the companion 50 times more luminous.
The distance to the system is estimated to be 126 lightyears, which means that the 4 arcsecond separation equates to a physical separation of around 170 AU – in other words, approximately four times the distance between Pluto and the Sun. The duo's position in the Sickle is odd considering Algieba’s Arabic meaning of ‘forehead’. If anything it appears located within the Lion’s neck.
In 2009 it was announced that a planetary system had been discovered around the primary star. To date, it’s believed that there are two planets in orbit, one 8.8 times the mass of Jupiter at a distance of 1.2 AU with a period of 429 days. The presence of the other planet remains uncertain but, if it is there, it is estimated to be 2.1x the mass of Jupiter, orbiting 2.6 AU from the star, and taking 3.7 years to complete one orbit.
Don't mistake 40 Leonis as being the companion – it's a decoy