Institute for Advanced Study/Princeton University Joint Astrophysics Colloquium

Until recently the number of "benchmark stars" --- stars having empirically measured radii, masses, and other properties with the few-percent accuracy needed to stringently test stellar models --- was of order 10^1. These crucial stellar benchmarks have been, almost without exception, the result of painstaking analyses of eclipsing binary star systems. We give examples from this work which has elucidated some important physics, especially the role of strong stellar magnetism in altering the stellar properties, due to tidally induced rotationally driven dynamos. But the ability to test standard models of single stars has remained a challenge. Now, combining data from current and upcoming all-sky surveys such as Gaia, TESS, and the fifth Sloan Digital Sky Survey (SDSS-V), will enable accurate, empirical measurements of fundamental properties for ~10^7 stars throughout the Milky Way and across the Hertzsprung-Russell diagram. We demonstrate a methodology by which individual stellar radii can now be measured with precisions of order 1%, and how these radii then can be leveraged to determine accurate stellar masses for these singleton stars. We give examples of the advances that such precise stellar radii and masses, combined with precise spectroscopic temperatures and detailed chemical abundances, promise for stellar astrophysics and exoplanet science, including studies of unusual stellar evolution pathways, and forensic analyses of planet formation and evolution. Finally, we discuss the opportunity represented by combining the above with ultraprecise light curves such as will be delivered by TESS, specifically as tests of theories of stellar interiors.