Overview
Stellar dynamos support flux-blocking star spots on the turbulent surface convection zones of cool stars. These star spots significantly perturb stellar structure models, resulting in systematically cooler temperatures and inflated radii for active stars. In this talk, I describe our recent work measuring star spot filling fractions for open clusters using an two-temperature ensemble fit on APOGEE high-resolution infrared spectra. These precise measurements of stellar magnetism reveal a precise mean spot-Rossby relationship in the Pleiades, and trends of spot evolution in M67. In the Pleiades, I demonstrate that these measurements provide a means to resolve tensions which have been identified in standard models, such as in anomalies identified in temperatures, radii, and lithium. I talk about spot measurements in regimes where stalled spin down is observed in rotation, and discuss its impact on the activity environments about their companion planets. I introduce the full set of ~135,000 star spot filling fractions derived from this technique, and propose its use as a precise and independent age indicator with the potential to calibrate isochronal and gyrochronal relationships. Finally, I discuss the application of this work to SDSS-V Milky Way Mapper and its applications to exoplanetary activity data sets and RV follow-up efforts.