Pulsation Modal Behavior of Classical Cepheids in the HR Diagram

Mapping of the Cepheid region of the Hertzsprung-Russell diagram has been difficult to do from a theoretical viewpoint because the masses and compositions of the Cepheids have always been very uncertain. It now appears that the various mass anomalies have been solved by distance scale changes and by the realization of very helium rich convection zones. This helium enrichment is caused by a Cepheid wind which blows away more hydrogen than helium, just as in the solar wind. Now with the evolutionary theory masses, radii, luminosities, and our new composition structures, the predicted blue edges of the instability strip and periods throughout the strip agree very well with observations.