Find Altarf, at the end of the Crab’s leg

As a constellation, it has to be said that Cancer isn’t one of the most impressive. Yes, it contains the open cluster M44, also known as the Beehive Cluster, and there’s another fainter open cluster near Acubens (Alpha (α) Cancri) called M67, but the stars that form the Zodiacal Crab of Cancer are pretty faint.

The brightest isn’t Acubens as you might think, but Altarf (Beta (β) Cancri) which marks the southwest point of the constellation’s inverted Y-shaped pattern. The name is from the Arabic ‘Al Tarf ’ meaning ‘the End’, and it represents one of the Crab’s legs.

Altarf has a spectral type of K4III Ba1, a cool orange K4 class giant with abundances of barium (chemical symbol Ba). Its effective temperature is 3,717˚C, which is much cooler than our Sun at 5,505˚C.

Located at a distance of 290 lightyears, Altarf shines at mag. +3.5. Despite its diminutive status, it should be borne in mind that Altarf is still 61 times larger and 870 times more luminous than the Sun.

A faint red-dwarf companion accompanies the main star. Shining at 14th magnitude, it is estimated to lie around 2,600 AU from Altarf; that’s around 65 times the Sun-Pluto distance. The orbital period is estimated to be 76,000 years, but no evidence for any mutual orbital motion has been confirmed yet.

Altarf is also believed to have a planet. Discovered in 2014, this body is estimated to be nearly eight times the mass of Jupiter, orbiting the star once every 605 days at an average distance of 1.7 AU.