jansky

The jansky (Jy) is a unit of spectral flux density used in radio astronomy, to describe the brightness of sources at particular frequencies. It is defined as 10^-26 watts per square meter per hertz. It is neither a standard CGS or SI unit, but is an invention of radio astronomers for their own use, scaled closer to the order-of-magnitude encountered in radio astronomy, and is defined in terms of SI units. Example jansky values:

• 4 million - Sun at 10 GHz
• 1 million - Milky Way at 20 MHz
• 2,000 - Milky Way at 10 GHz
• 1,000 - quiet Sun at 20 MHz

Commonly seen are millijansky (mJy, also referred to as m.f.u. for milli flux unit), and megajansky (MJy).

The jansky is useful to characterize a point source: if a single source is within the beam, spectral flux density offers a measure that other radio telescopes can reproduce. For extended sources, "Jansky per solid angle" has been used in radio astronomy, as have other measures of specific intensity, such as brightness temperature.

I believe the jansky was devised by/for radio astronomy but it is also used for other bands. One use is that the definition of the AB system of magnitudes (which, for example, is used for visible light photometry) is often cited using janskys.