Stars that burn hydrogen are called main sequence (MS) stars - these are by far the most common objects in the night sky. When the hydrogen fuel is exhausted and temperatures begin to fall, the object undergoes various transformations and a white dwarf star is eventually born, the ember of the expired MS star. Temperatures of a new-born white dwarf may be in the hundreds of thousand kelvin, but if the mass of the white dwarf is less than just a few solar masses, burning of He to C and O is not possible and the star will slowly cool down forever. The coolest white dwarfs observed have temperatures of roughly 4000 K, which must mean that the universe is not old enough so that lower temperature stars cannot be found. The emission spectra of "cool" white dwarfs does not at all look like a Planck blackbody spectrum. Instead, nearly the whole infrared is attenuated or missing altogether from the star's emission, owing to CIA in the hydrogen-helium atmospheres surrounding their cores. The impact of CIA on the observed spectral energy distribution is well understood and accurately modeled for most cool white dwarfs. For white dwarfs with a mix H/He atmosphere, the intensity of the H 2 -He CIA can be used to infer the hydrogen abundance at the white dwarf photosphere. However, predicting CIA in the atmospheres of the coolest white dwarfs is more challenging, in part because of the formation of many-body collisional complexes.