An OH/IR star is an asymptotic giant branch (AGB), a red supergiant (RSG), or a red hypergiant (RHG) star that shows strong OH maser emission and is unusually bright at near-infrared wavelengths.
In the very late stages of AGB evolution, a star develops a super-wind with extreme mass loss. The gas in the stellar wind condenses as it cools away from the star, forming molecules such as water (H2O) and silicon monoxide (SiO). This can form grains of dust, mostly silicates, which obscure the star at shorter wavelengths, leading to a strong infrared source. Hydroxyl (OH) radicals can be produced by photodissociation or collisional dissociation.
