In this talk, I will discuss the rich interplay of light particles with density. In particular, I will show that the couplings of the QCD axion get modified in high-density conditions. This can be calculated order by order in a systematic expansion within chiral perturbation theory. I will discuss the implications on astrophysical constraints of the QCD axion, such as supernova and neutron star bounds. I will then look at the impact that axion-like particles can have on the structure of stellar remnants. Even though their coupling is very weak, collective effects can modify the structure of white dwarfs as well as neutron stars. We can use this to place novel bounds on some of these models.