Jansky

Jansky, symbol Jy, is a non-SI unit of spectral flux density used in radio astronomy. It quantifies the amount of energy from an astronomical radio source received per unit area per unit frequency. By definition, 1 Jansky equals 10^-26 watts per square meter per hertz.

Karl Guthe Jansky, an American physicist and radio engineer, is the unit’s namesake. In 1933 at Bell Telephone Laboratories he detected radio emission from the Milky Way, proving for the first time that celestial objects emit radio waves and helping inaugurate the field of radio astronomy. The unit was named in his honor and has since become a standard measure in radio astronomy.

In practice, flux densities are reported in Jy, and smaller values are expressed in millijanskys (mJy) or microjanskys (μJy), where 1 Jy = 1,000 mJy = 1,000,000 μJy. Because the Jansky is a non-SI unit, it is customary to provide the equivalent SI flux density when precision is required, though in astronomy the Jy is widely used for convenience. Typical bright radio sources may have flux densities ranging from tens to thousands of Jy, while deep surveys may reach the microjansky or even nanajansky (nJy) level.

See also Karl Jansky; radio astronomy; spectral flux density.