STAR OF THE MONTH

Rho Cassiopeiae, a yellow hypergiant

Rho (r) Cassiopeiae is a mag. +4.5 star located 2.5˚ southwest of Caph (beta (b) Cassiopeiae). This verging-ondim star appears similar to the multitude of stars around it, but it is different. All of the stars we can see in the night sky belong to our Milky Way. This gravitationally bound system contains several hundred billion suns but only those in a ‘bubble’ around the Sun, about 10,000 lightyears in diameter with the Sun at the centre, can be seen individually with the naked eye as points of light. At 8,100 lightyears distant, Rho Cas pushes that envelope.

To appear this bright at this distance Rho Cas has to be around 550,000 times more luminous than the Sun, making it one of the most luminous stars known. As it’s so distant, Rho Cas’s light does battle with material along its line of sight, which is estimated to dim the star by up to two magnitudes.

Rho Cas is classified as a G2 or yellow hypergiant, with a spectral classification of G2Iae. G2 indicates it’s a star of similar colour and temperature to our Sun (spectral type G2V), ‘Ia’ shows it’s a luminous supergiant and ‘e’ indicates its spectrum has emission lines present.

Rho Cas is estimated to be around 450 times larger than the Sun; about 4.3 AU across. It rotates at a speed of 25km per second but being so large even at this speed, it takes it two years to complete one rotation. It’s classed as a semi-regular variable with an apparent magnitude range of +4.1 to +6.2. In 1946 it underwent a deep

minimum, attributed to the star expelling a shell of material.

This dimming seems to occur every 50 years; the last such event occurred in 2000–01.