Small solid particles, around 10−7 or 10−6 m across, found mixed with interstellar gas. Dust grains are predominantly composed of carbonaceous material and silicates, but may be surrounded by an icy mantle. Dust is very effective at absorbing and scattering ultraviolet and visible light. It consists of tiny lumps of solid compounds of carbon, oxygen, silicon and other metals. When mixed in with the interstellar gas, dust causes the light from stars to be blocked and/or reddened, as you saw in [Topic 2 Section 1.3.2](https://learn2.open.ac.uk/mod/oucontent/olink.php?id=2158714&targetdoc=Topic+2+Part+1+The+spectral+continuum&targetptr=1.3.2). Understanding the locations and properties of dust is therefore important for interpreting astronomical observations. Dust is also a vital part of the physics and chemistry of the ISM gas. It controls the temperature of the gas by re-radiating absorbed starlight in the far-infrared part of the spectrum, and the surfaces of dust grains are important sites for the molecular chemistry necessary to initiate star formation. [[Docent/S284 Astronomy]]