Several types of highly evolved star develop vigorous stellar winds that eject a substantial fraction of the original stellar mass into the interstellar medium. These stars are often classified as either oxygen-rich (most carbon is bound is CO and there is abundant oxygen to combine with other elements) or carbon rich. The oxygen-rich stars typically generate silicate dust in their stellar winds.
Oxygen-rich evolved stars may host up to three types of maser from the molecules SiO, H2O and OH. The SiO masers from closest to the star, in a zone that is repeatedly heated by shock-waves resulting from stellar pulsations. The H2O masers form further out, in a zone where the pulsations transform into a steady outward wind. This zone is shared with masers from the 1665 and 1667-MHz transitions of OH. Much further out again, we find masers from the 1612-MHz OH transition.
Recent work has replaced a radiation transfer model based on the large-velocity-gradient approximation with more exact spherical accelerated lambda iteration. The model is able to calculate, for the SiO masers, the projected radii of maser zones for many lines as a function of stellar phase, and these can be compared with observations.