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cosmic dustSmall particles or grains of matter found in many regions of space. Their size ranges from about 10 μm to less than 0.01 μm. They are thought to be composed primarily of carbon and silicate material, which may in some cases have mantles of water and ammonia ice or of solid carbon dioxide. Within the Solar System they are associated with the zodiacal light. In the interstellar medium they are found in molecular clouds and dark nebulae, causing interstellar extinction. The grains are also found in circumstellar shells causing the infrared excess seen in the spectrum of many stars.
particles of matter in interstellar and inter-planetary space. Light-absorbing aggregations of cosmic dust are visible as dark spots in photographs of the Milky Way. Attenuation of light by cosmic dust—interstellar absorption, or extinction—is not the same for electromagnetic waves of different wavelengths λ, which leads to the observed reddening of stars. The extinction in the visible range is approximately proportional to λ-1, whereas in the near ultraviolet region it is almost independent of the wavelength; an additional absorption maximum, however, is observed at 1400 Å. Much of the extinction is explained by the scattering of light rather than by the absorption of light. This follows from the observations of reflection nebulae, which contain cosmic dust and may be observed around the stars of spectral class B and some other stars that are sufficiently bright to illuminate the dust. A comparison of the brightness of the nebulae and of the surrounding stars shows that the albedo of the dust is large.
The observed extinction and albedo lead to the conclusion that cosmic dust consists of dielectric particles with an admixture of metal particles somewhat less than 1 jbtm in size. The ultraviolet extinction maximum may be explained by the fact that the interior of the dust particles contains graphite flakes measuring about 0.05 X 0.05 X 0.01 ju, m. Diffraction of light by the particles, which are of comparable dimensions to the wavelength, causes the light to scatter primarily in the forward direction. Interstellar absorption frequently leads to light polarization, which may be explained by the anisotropy of the properties of the dust particles (elongated shape of the dielectric particles, or anisotropy of the conductivity of graphite) and by the ordered orientation of the particles in space. The latter may be attributed to the action of the weak interstellar field, which orients the dust particles with their long axis at right angles to the lines of force. Thus, the orientation of the field in interstellar space may be deduced from the observations of the polarized light from remote celestial objects.
The relative quantity of dust may be determined from the magnitude of mean light absorption in the galactic plane of the Milky Way Galaxy, which ranges from 0.5 to several stellar magnitudes per kiloparsec in the visible region of the spectrum. The mass of the dust is about 1 percent of the mass of interstellar matter. Dust, like gas, is distributed nonuniformly, forming clouds and denser formations called globules. Dust acts as a cooling factor in globules, screening stellar light and emitting the energy acquired from inelastic collisions with gaseous atoms in the infrared region. Recombination of atoms into molecules occurs at the surface of the dust particles, which act as catalysts.
Dust is apparently formed by the condensation of gaseous molecules on the nuclei—particles of graphite, SiO2, and other materials—in interstellar space. The nuclei themselves are formed in the atmosphere of cold giant stars and in the expanding shells of supernovas; the expansion of supernovas leads to cooling and to the condensation of molecules. Some of the dust may condense into planets during the formation of stars in a dense cloud.
REFERENCESBakulin, P. L, E. V. Kononovich, and V. I. Moroz. Kurs obshchei astronomii, 2nd ed. Moscow, 1970.
Greenberg, J. M. Mezhzvezdnaia pyl’. Moscow, 1970. (Translated from English.)
S. B. PIKEL’NER