Astrophysical systems can be abundant sources or sinks of new particles beyond the Standard Model, and in combination with the right observational technique one can constrain new parameter space by considering the effects of these systems on the particle content of their surroundings. I will discuss two examples of recent ideas along these lines: the production of gravitationally bound millicharged particles in the Sun and the depletion of axion dark matter via decays stimulated by the light from supernova remnants. In the former case, I will show how solar millicharged particles can be detected with an experimental setup that coherently deflects the solar MCP wind, generating an oscillating electric field in a shielded detector. In the latter case I will show how the photon resulting from stimulated axion decay that is emitted in the backward direction (“gegenschein”) from supernova remnants could be detectable with the FAST radio telescope. In both of these examples, collective effects and the particles’ phase space play a role, which highlights the increasing importance of these considerations in astrophysical searches for new particles.