  Stellar energy loss is a sensitive probe of light, weakly coupled dark
sectors, including ones containing millicharged particles (MCPs). The emission
of MCPs can affect stellar evolution, and therefore can alter the observed
properties of stellar populations. In this work, we improve upon the accuracy
of existing stellar limits on MCPs by self-consistently modelling (1) the MCP
emission rate, accounting for all relevant in-medium effects and production
channels, and (2) the evolution of stellar interiors (including backreactions
from MCP emission) using the MESA stellar evolution code. We find MCP emission
leads to significant brightening of the tip of the red-giant branch. Based on
photometric observations of 15 globular clusters whose bolometric magnitudes
are inferred using parallaxes from Gaia astrometry, we obtain robust bounds on
the existence of MCPs with masses below 100 keV.
