interstellar dust

Small particles in the interstellar medium. Interstellar dust particles range in size between 0.005 and 1 micrometre and are generally mixed in with gas in the interstellar medium. Though accounting for less than one per cent of the mass of the typical interstellar medium, the dust absorbs far more light and emits far more infrared radiation than the gas. It causes both interstellar extinction and interstellar reddening. Starlight scattered from dust particles creates reflection nebulae. The absorption of energy from starlight by dust raises its temperature to a few tens of degrees above absolute zero. At such temperatures, the dust emits thermal radiation peaking in intensity in the infrared. Dust heated to temperatures higher than about 1,500 K is destroyed. It is unlikely that all interstellar dust is composed of the same material. Graphite (a common form of carbon) and silicates of iron, aluminium, calcium and magnesium are thought to be among the commonest, though the broad spectral features produced by the dust are difficult to identify with certainty. Polarization effects suggest that at least some of the particles are not spherical. Most of the dust is thought to originate in the outflow of material from cool red giant stars. As the gas cools with increasing distance from the star, solid materials condense out. Infrared emission detected from such stars shows that they are indeed surrounded by dust shells. Grains of material may also condense within molecular clouds.