Dust Formation in Stellar Environments
Interstellar dust cannot be formed in the interstellar medium: the interstellar gas is of such a low density that the nucleation of solids from gaseous atoms is far too slow to account for the abundance of dust that is observed in the interstellar medium of the Milky Way galaxy. However, much denser regions are found in the close environments of stars, and some of these locations are observed to be sources of dust. Evidently, dust is formed in these special locations and is ejected from them into the general interstellar medium. The most important contributors to dust formation in the Milky Way are stars of intermediate mass, say, one solar mass up to nearly ten solar masses, when these stars are near the ends of their lives. In this phase, an evolved star may generate atmospheric abundances that are sometimes oxygen-rich, sometimes carbon-rich. The stellar atmospheres extend outwards in large envelopes that are observed to be drifting away from the star. The physical conditions in these envelopes can be suitable for direct dust formation from gaseous atomic and molecular constituents. Where the stellar atmosphere is oxygen-rich, solid silicates and oxides will form with minor amounts of other materials, while carbon-rich envelopes generate carbon solids. Dust formed in these outward-drifting envelopes eventually mixes with the interstellar gas. These evolved stars are not the only sources of new dust; rather surprisingly, formation of dust can also occur in the ejecta from enormous explosions that occur at the ends of the lives of rather massive stars, the so-called supernovae. In spite of this apparently hostile environment, significant amounts of dust can form, and indeed have been seen to form in the ejecta from supernovae.