Ay 123 Lecture 10 - Pulsating Stars
Cepheid Period-Luminosity Relation The Cepheid period-luminosity relation has been central in establishing the cosmic distance scale since: - Cepheids are luminous Pop I variables - the -L relation can be calibrated via examples in the Milky Way - they can be found in rotating spirals and hence used to establish the rate of the Hubble expansion
Instability Strip As star cools from left to right (post-MS for Cepheids, post-He burning for RR Lyrae), the ionization zones (where drops) shift from the outer envelope (little mass) to the inner adiabatic core. The transition zone where pulsations are allowed by the -mechanism is the so-called `instability strip’ The period-luminosity relation (vertical up the instability strip) follows from ~-1/2