Interstellar dust grains have their origin in the material
ejected by stars. They form in dense, relatively cool
environments such as the atmospheres of red giant stars, and are
released into the interstellar medium by radiation pressure,
stellar winds or in material thrown off in stellar explosions.
They generally start off as carbon or silicate grains, which
later accumulate additional atoms of the most abundant elements
(hydrogen, oxygen, carbon, nitrogen) to form icy mantles of
water ice, methane, carbon monoxide, and ammonia. All of this is
encased in a sticky outer layer of molecules and simple organic
compounds created through interaction of the mantle with
incoming UV light. Typical interstellar dust grains are much
smaller than the dust we are familiar with here on Earth and
resemble more closely the soot from a candle flame. They are
about the same size as the wavelength of blue light meaning that
they absorb and scatter UV and blue light much more efficiently
than red light. The result is that ligh