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Silicon monoxide (SiO) is a compound with molecules each consisting of one silicon and one oxygen atom. Within astronomy, SiO serves as a tracer for shock: silicon is present in dust grains and a shock front's rising temperature can break down the grains, after which some SiO can form. Spectral signatures consisting of various ratios of spectral line depths offers clues regarding the velocities and temperatures in the shock front. This is useful for investigating stellar wind, supershells, active galactic nuclei, pre-main-sequence stars including their jets and their effect on surrounding star-forming regions.
SiO also forms masers, e.g., around some post-main-sequence stars.