Jansky

Jansky
Name	Karl Guthe Jansky
Birth date	October 22, 1905
Birth place	Medford, Kansas
Death date	February 14, 1950
Death place	Redwood City, California
Nationality	United States
Fields	Electrical engineering, Radio astronomy
Known for	discovery of radio emission from the Milky Way
Institutions	Bell Labs, Bell Telephone Laboratories
Alma mater	University of Wisconsin–Madison, University of Wisconsin

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Jansky

The jansky is a non-SI unit of spectral flux density widely used in radio astronomy, astrophysics, and observational astronomy. It quantifies received power per unit area per unit frequency from astronomical sources and underpins measurements made by facilities such as the Very Large Array, Atacama Large Millimeter Array, and Square Kilometre Array. The unit facilitates comparison among observations from instruments including the Arecibo Observatory, Green Bank Telescope, and James Clerk Maxwell Telescope.

Definition and unit

A jansky is defined as 10^−26 watts per square metre per hertz (W·m^−2·Hz^−1), linking it to SI quantities employed by instruments like the Karl G. Jansky Very Large Array and used in publications from organizations such as NASA, European Southern Observatory, National Radio Astronomy Observatory, Max Planck Institute for Radio Astronomy, and Jet Propulsion Laboratory. It is commonly applied alongside units such as the kelvin for brightness temperature, the jansky/beam notation used by interferometers like LOFAR and MeerKAT, and the decibel scale used by engineers at Bell Labs, IEEE, and International Telecommunication Union. Major surveys by teams at Harvard–Smithsonian Center for Astrophysics, California Institute of Technology, and Stanford University report flux densities in janskys for sources including Cygnus A, Cassiopeia A, Vela Pulsar, Sagittarius A*, and M87.

The unit commemorates an early 20th-century engineer at Bell Telephone Laboratories whose work at Holmdel, New Jersey led to the detection of extraterrestrial radio emission later attributed to the Milky Way. The observational program intersected with developments at institutions such as Columbia University, Princeton University, Cornell University, Yale University, and Massachusetts Institute of Technology where contemporaneous researchers advanced radio receiver and antenna technology. The discovery influenced projects and figures including Grote Reber, Sir Bernard Lovell, Ralph Baldwin, Ryle Telescope, A. H. R. McKellar, Martin Ryle, and Antony Hewish, and it precipitated the growth of observatories like Jodrell Bank Observatory, Parkes Observatory, Molonglo Observatory, Siding Spring Observatory, and Palomar Observatory.

Radio astronomers measure flux densities in janskys using single-dish telescopes and interferometers developed at University of Cambridge, University of Manchester, Caltech, Johns Hopkins University, Imperial College London, University of Toronto, and National Astronomical Observatory of Japan. Surveys such as the NRAO VLA Sky Survey, Faint Images of the Radio Sky at Twenty-Centimeters, Sydney University Molonglo Sky Survey, LOFAR Two-metre Sky Survey, and GaLactic and Extragalactic All-sky MWA report results in janskys for phenomena including pulsars, radio galaxies, quasars, supernova remnants, molecular clouds, planetary nebulae, and active galactic nuclei. Calibration routines reference standards from National Institute of Standards and Technology, International Astronomical Union, Royal Astronomical Society, American Astronomical Society, and European Space Agency to convert instrumental outputs to physical units like janskys. Observational campaigns at Submillimeter Array, Herschel Space Observatory, Spitzer Space Telescope, Planck spacecraft, and Fermi Gamma-ray Space Telescope often correlate radio flux densities in janskys with multiwavelength counterparts such as Chandra X-ray Observatory, Hubble Space Telescope, and Keck Observatory.

The jansky relates directly to SI units: 1 Jy = 10^−26 W·m^−2·Hz^−1, facilitating conversions to units used by laboratories like Los Alamos National Laboratory, CERN, Lawrence Berkeley National Laboratory, and Rutherford Appleton Laboratory. Related observational measures include brightness temperature (kelvins) as used in studies by National Radio Astronomy Observatory and Max Planck Society, spectral flux (W·m^−2), and magnitude systems maintained by International Astronomical Union working groups. Engineering contexts convert janskys to antenna temperature via the radiometer equation used at Jet Propulsion Laboratory, Applied Physics Laboratory, MIT Lincoln Laboratory, and European Southern Observatory, and to decibel scales employed by IEEE Standards Association and ITU.

Accurate flux density measurements in janskys require instrument characterization at facilities like Green Bank Observatory, Arecibo Observatory, Effelsberg Radio Telescope, Westerbork Synthesis Radio Telescope, Sardinia Radio Telescope, GBT, and MeerKAT. Considerations include beam solid angle, system temperature, bandwidth, and gain stability—parameters refined by engineers and scientists at Bell Labs, NRAO, CSIRO, Observatoire de Paris, Instituto de Astrofísica de Canarias, and Korea Astronomy and Space Science Institute. Observing strategies at arrays such as Very Long Baseline Array and European VLBI Network account for primary beam correction, sidelobe suppression, self-calibration, and deconvolution algorithms developed at National Radio Astronomy Observatory, CAS research groups, Harvard-Smithsonian Center for Astrophysics, and University of Cambridge.