Star of the Month

Al­gieba – the tight dou­ble whose name means 'fore­head' but is part of the Lion's neck

Sky at Night Magazine - - CONTENTS -

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

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