The Jansky ( Jy ) is a common in radio astronomy non- SI unit for the spectral flux density, ie for the incoming from a source at the Observers energy per time per area and per frequency interval. It was named after the radio astronomer Karl Guthe Jansky. The Jansky was originally defined for the radio region of the electromagnetic spectrum, but is occasionally used at shorter wavelengths to the ultraviolet.

The unit is defined as hundertquadrillionster part of a watt per square meter per hertz. The choice of such a small unit was made because most astronomical objects are very far away from us, so you have to carry the flux densities of small account. Moreover, most of the heavenly bodies emit most of their energy emitted at shorter frequency ranges than those of radio waves.

In figures, the definition of " Jansky " means:

This radio astronomers avoid to cut seconds against Hertz ( ) in the right part of the equation. Namely, one keeps the above notation for the dependence of the flux density is to immediately recognize both the integration time (in s ) and the bandwidth of the instruments used (in Hz).


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

In visible V-band (wavelength 550 nm), a star of magnitude 0 with 3640 Jy lights.

  • Astronomical unit
  • Radio astronomy