Interstellar Clouds (Clouds ,Nebulae Star Births And Deaths)

The unevenness of the interstellar medium
The previous chapter described the ways in which the interstellar medium can be observed, and summarized the major properties of the dust, the gas, and the cosmic rays which comprise it. This chapter is concerned with the way in which the interstellar medium is generally concentrated into clouds, particularly clouds associated with the births and deaths of stars.
There are many mechanisms which have the effect of stirring up the interstellar medium and generating clouds, nebulae, and other structural features. These include:

(a) Density waves associated with rotating galactic spiral arms.
(b) Thermal instabilities which tend to separate interstellar gas into regions which are either cool and dense or hot and rare.
(c) Gravitational collapse of clouds to form new stars.
(d) lonization of certain parts of space (H+ regions) by the ultraviolet radiation from 0- and B-type stars.
(e) Expulsion of matter from old stars in the form of planetary nebulae or supernovae.
(f) Magnetic fields.

The various types of cloud produced by these processes are transient objects that are generally in a state of either expansion or contraction. Most evolve with timescales of thousands or millions of years which, though long by human standards, are short comĀ¬pared with the lifetime of the Galaxy. The study of clouds is there-lore primarily the study of the evolution of the interstellar medium and of its interaction with the stars.

Filed under: Clouds


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