jy

Jy, or Jansky, is a unit of spectral flux density used in radio astronomy to quantify the amount of energy from an astronomical source received per unit area per unit frequency. One Jansky equals 10^-26 watts per square metre per hertz (W m^-2 Hz^-1). The unit is non-SI but is widely used in observatories, catalogs, and publications and is accepted for use with SI units.

The unit is named after Karl Jansky, a pioneering figure in radio astronomy who, in the 1930s, detected radio waves coming from space and helped establish the field. In practice, flux density at a given frequency ν is denoted Sν and expressed in Jy. Submultiples include the millijansky (mJy) and the microjansky (μJy). Specifically, 1 Jy equals 1000 mJy or 1,000,000 μJy, and larger values are sometimes quoted as megajanskys (MJy) or gigajanskys (GJy).

Jy is primarily used to describe the strength of radio sources, such as galaxies, quasars, pulsars, and supernova remnants. It also serves as a standard for calibrating radio and submillimeter instruments and for comparing observations across different telescopes and frequencies. Because spectral flux density can vary with frequency, Sν is often accompanied by the observing frequency and, when relevant, by a spectral index to characterize the source’s emission spectrum.