Glossary of astronomy - Q

Quadratic field strength is a method of computing the mean strength of a varying stellar magnetic field. It is determined by taking the root mean square of a series of longitudinal magnetic field strength measurements taken at different time periods.[13]

Quasar, or "quasi-stellar radio source", is a distant, point-like energy source originating from a powerful active galactic nucleus. Its luminosity is generated by accretion of gas onto a supermassive black hole. Quasars emit radiation across the electromagnetic spectrum from radio waves to X-rays, and their ultraviolet and optical spectra are characterized by strong, broad emission lines.