Janskys

Jansky (symbol Jy) is the unit of spectral flux density used in radio astronomy and related fields. One Jansky equals 10^-26 watts per square meter per hertz (W m^-2 Hz^-1). Because it is a non-SI unit, it is accepted for use with the SI system in astronomical contexts.

The unit is named after Karl G. Jansky, who in the 1930s detected radio waves from the Milky Way, laying the foundation of radio astronomy. The term was adopted later by the astronomical community to quantify faint radio signals from cosmic sources.

In practice, flux densities are measured as Sν at a given frequency; common subunits are millijansky (mJy) and microjansky (µJy). 1 Jy equals 1000 mJy or 1,000,000 µJy. Typical sources observed in radio surveys span from a few tenths of a Jy for bright radio galaxies to millijansky and microjansky levels for many faint or distant objects. The Jansky is also used, with the same definition, in infrared and submillimeter astronomy, though the spectral energy distribution of a source determines how flux density translates into physical properties across frequencies.