Interstellar Dust Clouds

    Dust particles make up a small percentage of the mass of the Interstellar Medium (~1%), however they are important in the scattering of starlight. On average there is one dust particle in every 10⁶ m³. From the observed scattering we can begin to approximate the nature of the interstellar grains. Much work has been done in order to decipher the data obtained, but as yet no complete solution has been found. There are a number of possibilities, some of which are listed below:

1. Elongated dirty-ice grains
2. Grains of graphite (carbon)
3. Particles with small cores and large icy mantles
4. Large, complex molecules called polycyclic aromatic hydrocarbons (PAHs)
5. Silicate particles

    Recently there has been a lot of progress determining the structure of complex silicate particles with carbon atoms involved in the structure. Below is a theoretical image of a fractal dust particle, which are usually less than 1/1000 of a millimetre across.