Rasalmothallah, at Triangulum’s point

Rasalmothallah (Alpha (α) Tranguli) is located in the small isosceles triangle of stars known as Triangulum, the Triangle. It is a curious constellation, because it could be argued that any three stars form a triangle, unless they’re in a line! There’s a southern equivalent too, Triangulum Australe, the Southern Triangle. In mythology the northern version represents the island of Sicily, but sadly the southern variant has no mythology of its own. Rasalmothallah marks the western end of Triangulum.

The ‘Rasal’ portion of the name may sound familiar. The two most common examples of its use are in the star names Rasalgethi and Rasalhague, two stars representing the heads of Hercules and Ophiuchus respectively, so you may have guessed that this means ‘head’. The name Rasalmothallah means ‘head of the triangle’, something echoed by its now defunct name Caput Trianguli. Caput means ‘head’ in Latin, another example being Serpens Caput, the Serpent’s Head. It’s worth noting that the International Astronomical Union Working Group on Star Names (WGSN) set up in 2016, approved the revised name Mothallah for Alpha Trianguli – robbing the triangle of its head!

The star itself shines at mag. +3.4, lies at a distance of 63.3 lightyears from the Sun and is a spectroscopic binary. The components rotate around their common centre of mass once every 1.736 days. Analysis gives the system a spectral classification ranging from F5 III to F6 IV. F5-F6 puts it in the middle of the yellow-white colour temperature range of 5,720–7,220˚C. III-IV indicates a giant or sub-giant. The primary rotates quickly at 81.6 km/s and its shape is most likely to be ellipsoidal as a consequence.

The gap between both stars is likely to be rather small, in the order of 0.04 AU.