Jansky

Our galaxy, the Milky Way, is a colossal bunch of energy and matter. Karl Jansky is credited with first discovering that it emits strong radio emissions. The measurement of electromagnetic flux density is named after him, the Jansky, with symbol Jy. It is most naturally defined as an integration over the whole solid angle subtended by the source. For extended surfaces/sources (like the whole galaxy), a per-solid-angle measurement is used (IRAS mission products use MJy/sr). Check this page for more discussion of what the Jansky means over the spectrum.

Jansky's Rotating Antenna (source linked below)

A wonderful historical note by John Kennewell and Malcolm Wilkinson is well worth reading: "The First 50 Years of Radio Astronomy". In other media, the excitement in Sagan's novel and the derivative movie "Contact" (1997) is about a strong encoded ('intelligent') radio source of well over 100 Janskys.