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The gloomy fate of interstellar dust.

The gloomy fate of interstellar dust

Although interstellar dust makes up only 0.5 percent by mass of all interstellar material, these tiny, solid particles are amazingly opaque, says John S. Mathis of the University of Wisconsin at Madison. An observer staring into a volume of this dilute material compressed to the density of air would penetrate the gloom by scarcely a millimeter. To account for the efficiency with which interstellar dust absorbs starlight, Mathis proposes that each grain consists of tiny silicate and carbon particles stuck together to form a loose, fluffy aggregate. Each grain, structured somewhat like a sponge, traps a substantial amount of vacuum in the spaces between the particles.

Mathis also finds that the carbon particles necessary for his theory must come in two forms: one consisting of carbon atoms in a rather disorganized array and another in which the carbon atoms are arranged at least partially in a graphite-like structure. The presence of a small proportion offree graphite accounts for a strong spectral feature characteristic of graphite observed in the interstellar medium.

Theoretical calculations show that over a wide range of wavelengths, the postulated composite grains have optical properties closely matching those observed in interstellar space. Further evidence for the theory comes in the form of interplanetary, or cometary, grains captured high above the earth, which sometimes have a similar fluffy structure.

The new theory provides an alternative to earlier theories suggesting that interstellar dust consists entirely of separate silicate and carbon particles or largely of carbon-coated silicate particles. Mathis says his theory takes into account the way interstellar grains evolve. Exploding stars generate either silicates or carbon. Occasional collisions between these particles shatter the original stellar solids, and the small fragments collect into composite grains. In dense clouds, particles are more likely to clump together and the resulting grains tend to be larger than those found outside of clouds.
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Title Annotation:theory explains why interstellar dust absorbs starlight
Author:Peterson, Ivars
Publication:Science News
Date:Jun 18, 1988
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