Neilson et al., JAAVSO Volume 44, 2016 179 Period Changes and Evolution in Pulsating Variable Stars Hilding R. Neilson John R. Percy Department of Astronomy and Astrophysics, University of Toronto, Toronto, ON M5S 3H4, Canada;
[email protected],
[email protected] Horace A. Smith Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824-2320;
[email protected] Invited review paper, received October 18, 2016 Abstract We review ways in which observations of the changing periods of pulsating variable stars can be used to detect and directly measure their evolution. We briefly describe the two main techniques of analysis—(O–C) analysis and wavelet analysis— and results for pulsating variable star types which are reasonably periodic: type I and II Cepheids, RR Lyrae stars, β Cephei stars, and Mira stars. We comment briefly on δ Scuti stars and pulsating white dwarfs. For some of these variable star types, observations agree approximately with the predictions of evolutionary models, but there still exist significant areas of disagreement that challenge future models of stellar evolution. There may be a need, for instance, to include processes such as rotation, mass loss, and magnetic fields. There may also be non-evolutionary processes which are contributing to the period changes. 1. Introduction known constant period P. When (O–C) is plotted against time, it produces a straight line if the period is constant, a parabola Star lives are measured in billions of years, millions of opening upward if the period is increasing linearly, a parabola years for very rare massive stars.