millijanskys

The millijansky (symbol mJy) is a unit of spectral flux density used in radio astronomy. It equals one thousandth of a jansky, with 1 Jy defined as 10^-26 watts per square meter per hertz (W m^-2 Hz^-1). Therefore 1 mJy equals 10^-29 W m^-2 Hz^-1. The jansky is a conventional unit rather than an SI unit, chosen to express the faint radio fluxes observed from astronomical sources.

Spectral flux density, Sν, describes how much energy from a source arrives at a telescope per unit area per unit frequency. In radio astronomy, source flux densities are commonly reported in mJy or Jy, depending on strength. The relationship among common scales is simple: 1 Jy = 1000 mJy and 1 mJy = 1000 µJy (microjansky). This range makes it convenient to express the diversity of sources, from faint distant galaxies to bright quasars.

Measurements are made with radio telescopes and calibrated against standard sources to account for instrument response and atmospheric or instrumental effects. Reported values include uncertainties and are often given at a specific observing frequency or bandwidth, since flux can vary with frequency.

Origin and usage: The unit is named after Karl G. Jansky, who identified cosmic radio emission in the 1930s. The millijansky remains a common unit in radio surveys and imaging, reflecting typical flux densities of many extragalactic and Galactic radio sources observed with modern facilities.

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