Jansky

jansky
jansky
Unit system	non-SI metric unit
Unit of	spectral flux density
Symbol	Jy
Named after	Karl Guthe Jansky
Conversions
1 Jy in ...	... is equal to ...
SI units	10−26 W⋅m−2⋅Hz−1
CGS units	10−23 erg⋅s−1⋅cm−2⋅Hz−1

The jansky (symbol Jy, plural janskys) is a non-SI unit of spectral flux density, or spectral irradiance, used especially in radio astronomy. It is equivalent to 10⁻²⁶ watts per square metre per hertz.

The spectral flux density or monochromatic flux, $S$ , of a source is the integral of the spectral radiance, $B$ , over the source solid angle: $S=\iint \limits _{\text{source}}B(\theta ,\phi )\,\mathrm {d} \Omega .$

The unit is named after pioneering US radio astronomer Karl Guthe Jansky and is defined as

$1~\mathrm {Jy} =10^{-26}~\mathrm {W} {\cdot }\mathrm {m^{-2}} {\cdot }\mathrm {Hz^{-1}}$ (SI)
$1~\mathrm {Jy} =10^{-23}~\mathrm {erg} {\cdot }\mathrm {s^{-1}} {\cdot }\mathrm {cm^{-2}} {\cdot }\mathrm {Hz^{-1}}$ (CGS).

Since the jansky is obtained by integrating over the whole source solid angle, it is most simply used to describe point sources; for example, the Third Cambridge Catalogue of Radio Sources (3C) reports results in janskys.

For extended sources, the surface brightness is often described with units of janskys per solid angle; for example, far-infrared (FIR) maps from the IRAS satellite are in megajanskys per steradian (MJy⋅sr⁻¹).
Although extended sources at all wavelengths can be reported with these units, for radio-frequency maps, extended sources have traditionally been described in terms of a brightness temperature; for example the Haslam et al. 408 MHz all-sky continuum survey is reported in terms of a brightness temperature in kelvin.