  Using, in particular, the data upon Sgr A*, we propose the unified model of
compact radio sources, i.e. of pulsars, maser sources, and active galactic
nuclei. The unification is based on the wavelength dependence of radio source
size. It is shown that the compact sources are characterized by a maser
amplification of thermal radio emission. The density, temperature, and magnetic
field profiles of compact sources are discussed. The wavelength dependence of
Sgr A* size is explained by the effect of the gravitational field of a central
energy source on a gas flow.
