dust grains

Small particles of matter, typically of order 10-100 nanometres in diameter, which co-exist with atoms and molecules of gas in interstellar space. The dust grains are thought to consist mainly of silicates and/or carbon in the form of graphite. They form in the extended atmospheres of red giant stars. Dark clouds of dust are evident when they obscure the light from stars and luminous gas clouds, as happens in the plane of the Milky Way. Though tenuous, such clouds are very effective at absorbing visible light, but radiation at millimetre and longer wavelengths can pass through the dust clouds unimpeded. The presence of dust is also revealed by the emission of infrared radiation, generated when the grains are warmed by the absorption of visible and ultraviolet radiation. The temperature of dust is typically in the range 30-500 K. Dust grains are thought to play an important role in the formation of interstellar molecules, acting as host surfaces on which gas atoms can combine. The molecules so formed can then leave the surfaces of the grains. Dust clouds are also an important constituent of star-forming regions. The dust appears to shield interstellar molecules from the destructive effect of high-energy radiation and provides protostars with an efficient means of radiating away surplus energy.