Wendy Freedman used infrared measurements to constrain these stars’ Wesenheit (“VESS-en-hite”) function. Observing that an equation concerning these stars depended on “population” prompted Walter Baade to recalculate the size of the known universe to double its previous estimate. Arthur Eddington proposed that compressing these stars’ outer layers causes an increase in opacity, an effect later known as the (-5[1])“kappa mechanism.” The Hubble constant was initially derived from observations of these stars in spiral galaxies, utilizing a linear relation between “M-sub-V” and “log P.” These standard candles are commonly used to calibrate distances to type Ia (“one-A”) supernovae and satisfy a period-luminosity relation discovered by Henrietta Swan Leavitt. For 10 points, name these variable stars. ■END■ (10[1])