Color index

A color index is the difference between an object’s measured magnitudes in two wavelength bands. It quantifies color in astronomical photometry, with smaller values generally indicating bluer sources.

Apparent magnitude: A logarithmic measure of an astronomical object's brightness as observed from Earth. Color indices subtract magnitudes measured through separate bands.
Stellar classification: The grouping of stars by observable properties such as spectra, temperature, and luminosity. Color indices help estimate stellar temperatures and distinguish broad stellar types.
Pogson's equation: The logarithmic relation defining astronomical magnitude differences from ratios of measured fluxes. Subtracting magnitudes converts a two-band flux ratio into a color index.
Color temperature: The temperature of a blackbody whose color best matches the observed spectrum of an object. It interprets a spectrum as a temperature, while a color index is a measured band difference.
Photometric system: A standardized set of wavelength filters and calibration conventions for measuring astronomical brightness. Its filters determine which magnitudes form a particular color index.
Effective temperature: The temperature of a blackbody that would emit the same total energy per unit area as an object. A star’s color index can serve as an observational proxy for this temperature.
Zero point (photometry): A calibration constant that sets the reference flux for magnitudes in a photometric band. The band zero points affect the numerical value assigned to a color index.
Spectral energy distribution: The amount of energy emitted or received across a range of wavelengths. A color index compresses information from a full spectrum into two band measurements.
Astronomical filter: An optical element that transmits a selected range of wavelengths for astronomical measurement. A color index compares measurements made through two such wavelength bands.
Interstellar reddening: The change in an object's observed color when dust preferentially absorbs and scatters shorter-wavelength light. Dust shifts measured color indices away from their intrinsic values.

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