Volume 48 number 1 (2020)
(Abstract only) Cepheid variable stars are core helium-burning stars of around 4 to 15 solar masses that show radial pulsations with typical periods of a few days to a few weeks, and magnitude variations of a few tenths to up to 2 magnitudes per pulsation cycle. Cepheids are well within the reach of amateur observers, with over 200 Galactic Cepheids brighter than 10th magnitude, and have been the target of numerous AAVSO observations. Cepheids show a period-luminosity relation, discovered by Henrietta Leavitt in 1908, that has been used to determine distances within the Galaxy and to galaxies beyond the Milky Way. Cepheids are also a laboratory to test stellar interior physics, such as nuclear reaction rates for helium burning, turbulence models, and opacities, under conditions not easily accessible in laboratories on Earth. Current problems in Cepheid research include the discrepancy between the Hubble constant derived from the Cepheid period-luminosity relation and that derived from cosmic microwave background observations, and the discrepancy between Cepheid masses derived from pulsation periods or binary dynamics and that derived using stellar evolution models. Here we will show how the open-source MESA (Modules for Experiments in Stellar Astrophysics) code (Paxton et al. 2011, 2013, 2015, 2018, 2019, http://mesa.sourceforge.net/) can be used to explore Cepheid evolution. We will also show results using the new radial stellar pulsation (RSP) capability in MESA to model the hydrodynamics of Cepheid envelopes during their pulsations, and simulate light curve and radial velocity variations. We will compare models with observations of Cepheids with well-known properties such as δ Cep, α UMi (Polaris), and V1334 Cyg. These stellar modeling capabilities are accessible to anyone with a laptop computer, following the directions in the MESA tutorial for installation, and starting with the examples in the MESA test suite.