QSO J1819+3845 is a quasar notable for being the most variable known extragalactic radio source. This quasar shows variations of factors of four or more on a timescale of hours.

If the standard theory of diffractive scintillation is assumed to apply, groundbased radio traces would seem to require that J1819+3845 contains a component that must be less than about 5 microarcseconds across; this would require its temperature to be in excess of 10¹⁵ K.

However, the scintellation may be more easily interpreted as being due to scattering in the interstellar medium (ISM). The ISM focusses and defocusses the radio waves from the source, producing a pattern of dark and bright patches. As the Earth moves through these patches we observe a temporal variation in the radio intensity.