1. Ay 123 Lecture 10 - Pulsating Stars

2. 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

3. 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