In the center of this bull’s-eye- shaped false color image is a bright radio source known as Sagittarius A. Astronomers use Sag A, as the source is commonly called, to define the precise center of our galaxy. Sag A is the brightest radio source in the portion of the sky where the constellation Sagittarius is located. Although it behaves like nuclear radio sources in other spiral galaxies (see M81), Sag A is a comparatively weak source.
    Sag A is compact; it is roughly 3 billion kilometers in diameter, or about half the diameter of the solar system. Compact sources are often variable, and Sag A is no exception. The amount of radio light it emits varies significantly over the course of a day. 
    Radio waves make visible what