ABmagnitudijärjestelmät

The AB magnitude system is a photometric scale used in astronomy to express the brightness of astronomical objects in terms of their flux density per unit frequency, fν. It is defined so that a source that has a constant fν across frequency has AB magnitude zero. The defining relation is mAB = -2.5 log10 fν - 48.60, where fν is measured in erg s^-1 cm^-2 Hz^-1. Equivalently, when fν is expressed in Jansky (Jy), mAB = -2.5 log10 fν(Jy) + 8.90. The zero point corresponds to fν = 3631 Jy, giving mAB = 0.

The AB system was introduced in the 1980s to provide a physically meaningful, filter-based magnitude scale that is independent of detector response. It has been broadly adopted by major surveys and instruments, including the Sloan Digital Sky Survey (SDSS), GALEX, and many Hubble Space Telescope programs, enabling direct comparisons of measurements across different telescopes and filters.

In practice, AB magnitudes are obtained by converting observed flux measurements through a given filter into a band-averaged fν and then applying the defining equation. Because real astronomical sources do not have perfectly flat spectra, color terms and k-corrections are often required when relating AB magnitudes between bands or to other magnitude systems, such as the Vega system.

Compared with Vega-based systems, the AB system has a fixed physical zero point tied to 3631 Jy, not to a specific star. This makes AB magnitudes particularly convenient for comparing broad-band data and for interpreting results in terms of intrinsic flux densities.