# Jansky

The Jansky (Jy) (after the radio astronomer Karl Guthe Jansky ), formerly also called the Flux Unit (FU) , is a non- SI unit for the spectral flux density that is commonly used in radio astronomy . H. for the energy arriving at the observer from a source per time, area and frequency interval. The Jansky was originally defined for the radio part of the electromagnetic spectrum , but is occasionally also used for shorter wavelengths down to the ultraviolet .

The unit is defined as the hundred quadrillionth part of a watt per square meter and hertz :

{\ displaystyle {\ begin {aligned} 1 \, \ mathrm {Jy} & = 10 ^ {- 26} \ {\ frac {\ mathrm {W}} {\ mathrm {Hz} \ cdot \ mathrm {m} ^ {2}}} \\ [2ex] & = 10 ^ {- 26} \ {\ frac {\ mathrm {J}} {\ mathrm {s} \ cdot \ mathrm {Hz} \ cdot \ mathrm {m} ^ {2}}} \\ [2ex] & = 10 ^ {- 23} \ {\ frac {\ mathrm {erg}} {\ mathrm {s} \ cdot \ mathrm {Hz} \ cdot \ mathrm {cm ^ { 2}}}} \ end {aligned}}}

with the unit erg from the CGS system of units .

Radio astronomers avoid shortening seconds against Hertz (Hz = s −1 ) in the right part of the equation . If you keep the above notation, the dependence of the flux density on both the integration time (in s) and the bandwidth of the instruments used (in Hz) can be seen immediately .

The choice of such a small unit was made because most astronomical objects are very far away from us, so that one has to take into account the small flux densities. In addition, most celestial bodies emit most of their radiated energy at higher frequencies than radio waves.

## Examples

The strongest radio sources have flux densities in the order of 1 - 100 Jy. The 3C catalog lists over 300 radio sources in the northern hemisphere that emit more than 9 Jy at a frequency of 159 MHz.

In the visible V-band (wavelength 550 nm) a star of the zeroth magnitude shines with 3640 Jy.