These cosmic objects generally have a mass several times that of the sun, and their distinctive features result from their location in the instability strip on the Hertzsprung-Russel diagram. Stars with a 2  lower mass than the sun that fall into this category are called W Virginis. Their distinguishing feature is caused by the opacity of ionized helium, which then holds in radiation, causing the star to expand. As the star expands, it cools, becomes less luminous, and the helium ions recombines with free electrons, increasing luminosity. The source of calibration for 
These cosmic objects generally have a mass several times that of the sun, and their distinctive features result from their location in the instability strip on the Hertzsprung-Russel diagram. Stars with a 2  lower mass than the sun that fall into this category are called W Virginis. Their distinguishing feature is caused by the opacity of ionized helium, which then holds in radiation, causing the star to expand. As the star expands, it cools, becomes less luminous, and the helium ions recombines with free electrons, increasing luminosity. The source of calibration for the standard candela, FTP, identify this class of variable stars, named after the delta star of the King constellation.
These cosmic objects generally have a mass several times that of the sun, and their distinctive features result from their location in the instability strip on the Hertzsprung-Russel diagram.
Cepheid variables