Life cycle of a dust grain.
of processes. Sometimes molecules can simply sublime from the surface. (Sublimation is a phase change directly from the solid phase to the gas phase.) Collisions with atoms in the gas can break up grains. Collisions between grains can also destroy the grains.
14.4 I Interstellar gas
Formation determine its temperature directly from infrared observations.
Early studies of cold interstellar gas utilized optical absorption lines. When light from a star passes through a cloud, as shown in Fig. 14.10, some energy is removed at wavelengths corresponding to transitions in the atoms and molecules in the cloud. These studies revealed the existence of trace elements such as sodium or calcium. (These elements happen to have convenient spectral lines to study.) In addition to these atoms, some simple molecules were discovered: CH (in 1937), CN (in 1940), and CH+ (in 1941).
We still know very little about the evolution of dust grains. Fig. 14.9 indicates, schematically, the life cycle of a typical dust grain. The densities in interstellar clouds are probably too low for the grains to be formed directly where we see them. We think that most dust grains are formed in the envelopes of red giants undergoing mass loss. As material leaves the surface, it is hot enough to be gaseous. However, as it gets farther from the surface, it cools. When the temperature is low enough, about 1000-2000 K, many of the materials, such as silicates, can no longer exist as a gas. They form small solid particles. These particles are blown into the interstellar medium either via the effects of stellar winds, or as part of a planetary nebula. This is another example of the cyclical processing of material between stars and the interstellar medium.
Once the grains are in clouds, they can collect particles from the gas and grow. There are some limits. For example, once a layer of molecular hydrogen (H2) one molecule thick forms on the grains, no more hydrogen will stick. Grains will be destroyed, or diminished in size by a number
Starlight passing through cloud o-
Interstellar Cloud with Atoms & Molecules
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