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| Vega magnitude system | |
|---|---|
| Name | Vega magnitude system |
| Type | Photometric magnitude system |
| Established | 19th century (formalized 20th century) |
| Zero point | Vega-based |
| Usage | Optical and near-infrared photometry |
Vega magnitude system The Vega magnitude system is a photometric scale that sets the flux of the star Vega as the reference zero point for many optical and near-infrared bands. It underpins large portions of observational photometry used by instruments at observatories such as Palomar Observatory, Mauna Kea Observatories, and missions like Two Micron All-Sky Survey. The system connects historical photometry from observers such as Norman Pogson to modern surveys and calibration chains used by facilities including European Southern Observatory and Space Telescope Science Institute.
The origins trace to the 19th century work of Norman Pogson who defined the modern magnitude logarithm, and the early choice of bright stars—most prominently Vega at Lyra (constellation)—as practical standards used by observatories like Royal Greenwich Observatory. During the 20th century, institutions such as Yerkes Observatory and Mount Wilson Observatory adopted Vega-based calibrations for photographic and photoelectric systems. The advent of infrared detectors and projects including Two Micron All-Sky Survey and instruments on Keck Observatory extended Vega-based conventions into the near-infrared, while space observatories like Hubble Space Telescope and missions from European Space Agency maintained Vega-referenced calibrations for legacy continuity.
In the Vega system the magnitude m in a given photometric band is defined by m = -2.5 log10(F/F0), where F0 is the flux density assigned to Vega in that band. The practical zero point F0 historically derives from spectrophotometry of Vega observed with instruments at Cerro Tololo Inter-American Observatory, Kitt Peak National Observatory, and Calar Alto Observatory. Absolute spectrophotometric efforts involving standards tied to works by Arlo U. Landolt and spectra measured with International Ultraviolet Explorer set band-specific F0 values. Because Vega is a real star—catalogued in resources like the Henry Draper Catalogue and located at Alpha Lyrae—its nonzero color indices and rotationally distorted atmosphere make the assignment of F0 dependent on the adopted Vega spectrum from groups such as the Cohen et al. calibration team.
The Vega system contrasts with the AB magnitude system developed by David Hogg and others, which defines zero magnitude by a constant flux per unit frequency, and with the ST (Space Telescope) system used at Space Telescope Science Institute that uses constant flux per unit wavelength. Conversions among Vega, AB, and ST require the Vega spectral energy distribution as observed by standards from Caltech and flux tables referenced to photometric systems like Johnson–Cousins and Sloan Digital Sky Survey. Large surveys such as Sloan Digital Sky Survey and instruments on Very Large Telescope often report AB magnitudes to simplify comparison across wide wavelength ranges, while legacy datasets from Two Micron All-Sky Survey remain Vega-based.
Calibrating a Vega-based system uses primary and secondary standard stars established by observers such as Arlo U. Landolt and teams at European Southern Observatory. Primary spectrophotometric standards include Vega and other bright stars catalogued by Mount Wilson Observatory and measurements from International Astronomical Union working groups. Secondary networks anchored to Vega involve fields used by Hubble Space Telescope calibration programs and ground-based sequences observed at Cerro Tololo Inter-American Observatory. Modern calibrations incorporate model atmospheres computed by groups associated with Kurucz and stellar parameter determinations from projects like Gaia to account for Vega’s peculiarities.
In practice photometrists convert instrumental magnitudes to the Vega system using observations of Landolt or other standards at sites like Kitt Peak National Observatory and Cerro Tololo. Transformations between Vega and AB for bands such as Johnson, 2MASS, and SDSS require synthetic photometry using spectra from the Pickles Library or model grids produced by Kurucz and validation against datasets from Two Micron All-Sky Survey and Sloan Digital Sky Survey. Software packages developed at institutions like Space Telescope Science Institute and European Southern Observatory implement these conversions and apply color terms measured with photometers at Las Campanas Observatory.
Systematic errors arise because Vega is not a perfect blackbody: it is a rapid rotator with gravity darkening as revealed by interferometry at Palomar Observatory and model work by Gareth Collins and others. Atmospheric extinction measured at Mauna Kea Observatories and Cerro Tololo Inter-American Observatory introduces site-dependent color terms, while instrumental passbands defined for instruments at Very Large Telescope or Subaru Telescope differ from ideal filters, producing zeropoint shifts. Absolute flux calibration uncertainties propagate from reference spectra produced by teams at National Institute of Standards and Technology and spectrophotometric comparisons involving Hubble Space Telescope standards. For precision cosmology and spectrophotometry, these effects motivate preference for AB or ST calibrations in projects led by Dark Energy Survey and European Space Agency missions.
Despite limitations, the Vega system remains widely used in near-infrared photometry from projects like Two Micron All-Sky Survey and ground-based observations at Mauna Kea Observatories and Cerro Tololo Inter-American Observatory. Vega-referenced magnitudes appear in catalogs including the Henry Draper Catalogue cross-matched with surveys such as Sloan Digital Sky Survey, and in calibration work for instruments on Gemini Observatory and Keck Observatory. Studies of stellar populations in clusters observed at European Southern Observatory telescopes, brown dwarf photometry from facilities like United Kingdom Infrared Telescope, and variable star monitoring in programs at AAVSO often report Vega magnitudes for historical consistency.
Category:Astronomical photometry