Jansky and AB magnitude converter
Convert the brightness of a source between janskys and AB magnitudes, and the SI and CGS units of spectral flux density.
That's 10,000× as bright as the faintest galaxies in Hubble’s deep fields, while the faintest star the eye can see is 398,107× brighter.
What it means
How bright is that source?
AB magnitudes are defined per unit frequency. Vega-system magnitudes differ by a band-dependent offset; references are rough visual values.
Every unit
That flux density in every unit
Learn
Three things worth knowing
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5 magnitudes = ×100
AB 0 is 3,631 Jy, AB 5 is 36.3 Jy, AB 10 is 0.363 Jy, AB 20 is 36 µJy.
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Jansky’s merry-go-round
Karl Jansky built a rotating antenna at Bell Labs in 1932 to find radio static and traced a hiss to the centre of the galaxy.
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Bigger magnitude, fainter
Magnitudes run backwards: AB 25 is a hundred times fainter than AB 20.
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Background
About janskys and AB magnitudes
Radio and infrared astronomers measure brightness per unit frequency in janskys: 1 Jy = 10⁻²⁶ W m⁻² Hz⁻¹, named after Karl Jansky, who first detected radio waves from the Milky Way in 1933.
The AB magnitude system ties that to the optical magnitude scale: AB = −2.5 log₁₀(F ÷ 3,631 Jy). Five magnitudes is a factor of 100; the faintest galaxies in Hubble’s deep fields are about AB 30.