Interstellar dust

Only about 0.7 percent of the mass of the interstellar space is within the sort of solid grains, but these grains have a profound effect on the physical conditions within the gas. Their main effect is to soak up stellar radiation; for photons unable to ionize hydrogen and for wavelengths outside absorption lines or bands, the dust grains are far more opaque than the gas. The dust absorption increases with photon energy, so long-wavelength radiation (radio and far-infrared) can penetrate dust freely, near-infrared very well , and ultraviolet relatively poorly. Dark, cold molecular clouds, within which all star formation takes place, owe their existence to dust. Besides absorbing starlight, the dust acts to heat the gas under some conditions (by ejecting electrons produced by the photoelectric effect, following the absorption of a stellar photon) and to chill the gas under other conditions (because the dust can radiate energy more efficiently than the gas then generally is colder). the most important chemical effect of dust is to supply the sole site of molecular hydrogen formation on grain surfaces. It also removes some heavy elements (especially iron and silicon) that might act as coolants to the gas. The optical appearance of most nebulae is significantly modified by the obscuring effects of the dust.

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