RS Persei
RS Persei is a red supergiant variable star located in the Double Cluster in Perseus. The star's apparent magnitude varies from 7.82 to 10.0, meaning it is never visible to the naked eye. Location. RS Persei is a member of the cluster NGC 884, χ Persei, one half of the famous Double Cluster. Variability. RS Persei is classified as a semiregular variable star, with its brightness varying from magnitude 7.82 to 10.0 over 245 days, Detailed studies show that it also pulsates with a long secondary period of days. Properties. RS Persei is a large cool star  with a temperature of 3,500 K. This makes it luminous, although much of its radiation is emitted in the infrared. In 2005, RS Per was calculated to have a bolometric luminosity of and a radius around . More recently, 2014 calculations across all wavelengths gives the star a lower luminosity of based on an assumed distance, and a radius of based on the measured angular diameter and luminosity. Even more recent measurements based on its Gaia Data Release 2 parallax gives a luminosity below with a corresponding radius of . It is surrounded by dust that has condensed from material lost by  the star. RS Persei has sometimes been considered to be a highly evolved low mass Asymptotic Giant Branch (AGB) star, but calculations of its current mass suggest that it is a low mass supergiant. NGC 244 is also too young to host AGB stars.