S Cassiopeiae

S Cassiopeiae (S Cas, HD 7769) is a Mira variable and S-type star in the constellation Cassiopeia. It is an unusually cool star, rapidly losing mass and surrounded by dense gas and dust producing masers.

Distance

In the absence of a measure of its parallax by the Hipparcos satellite, its distance from the solar system was estimated between 1,860 and 2,770 light-years. Gaia Data Release 2 published a parallax of 0.8585±0.1626 mas, indicating a distance around 1200 pc, but the observations have a very high noise level and are considered unreliable. A distance of 460 pc is preferred.

Spectral type

With a spectral type of S3,4e-S5,8e, S Cassiopeiae is an S-type star similar to χ Cygni; these are asymptotic giant branch (AGB)stars similar to those of class M except that the dominant spectral bands of metal oxides are formed by metals of the fifth period of the periodic table as zirconium or yttrium. Another feature of this class of stars is the high mass loss; in the case of S Cassiopeiae it is estimated at 3.5×10⁻⁶ M_☉ per year.

Characteristics

S Cassiopeiae has a radius of 930 solar radii; if placed at the center of the Solar System, it would extend past the orbit of Mars and the Asteroid Belt. Its effective temperature is 1,800 K, which is exceptionally cool for any star, and its bolometric luminosity is 8,000 times that of the sun.

S Cassiopeiae is a variable Mira, a pulsating variable star whose visual brightness varies over several magnitudes with a somewhat regular period and amplitude. Its visual magnitude varies between +7.9 and +16.1 over an average period of 612.43 days. Mira variables are stars in the last stages of evolution whose instability comes from pulsations in its surface, causing changes in color and brightness. Some of them, including S Cassiopeiae show SiO maser emission.