At the river’s end, Cursa, 2nd-brightest star in Eridanus
The constellation Eridanus, the River is very long. It starts near the foot of Orion, which is marked by blue supergiant Rigel (Beta (β) Orionis). The River starts at the group of stars mag. +3.6 Tau (τ) Orionis and mag. +4.3 Lambda (λ), mag. +4.8 Psi (ψ) and +2.8 Beta (β) Eridani, the latter known as Cursa. This group was known in Arabic as ‘the chair (or footstool) of the Central One’. This is one of two footstools used by Orion, the other known as ‘the hindmost chair of Jawzā’, formed from stars in Lepus further to the south.
Back to the foremost footstool, and Cursa is clearly the brightest of the three Eridani stars. Located 3.5° northwest of Rigel, it stands out well in the sparse area southwest of Orion. The river meanders back and forth as it heads south. Eridanus’s brightest star is mag. +0.4 Achernar (Alpha (α) Eridani), a name that means ‘the end of the river’. This is too far south to be seen from the UK.
Cursa is a giant of spectral class A3 III. It’s a rapid rotator too, with a projected rotational velocity of 196km/s (our Sun’s rotation is 2km/s at its equator). The star has twice the mass of the Sun and shines with a luminosity 25 times larger. It is interesting that its motion through space suggests it belongs to the Ursa Major Moving Group, although physical property mismatches suggest it may just be an interloper.
Cursa has a rare stellar property in that it can exhibit bright flash events. In 1985 it brightened by a factor of 15 for a period of a couple of hours.