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The effective temperature (Teff, Teff, or what is often meant by black-body temperature, TBB) of an astronomical body such as a star or planet is the temperature that a black body of the same surface-area and bolometric luminosity would have. It is essentially a measure of the mean radiance (aka intensity or surface brightness) over the surface of the body. For a body with isotropic emission, it can be roughly the body's surface temperature (which is not perfectly defined because the surface is often ambiguous: a star's radiation reaches us from more than one depth and these layers have differing temperatures; planets with significant atmospheres have the same issue, and planets also invariably have differing surface temperatures toward and away from the host star). There is no practical way to measure an entire spectrum in detail, with good calibration, so determining the effective temperature involves sampling the spectrum and interpolating and extrapolating, or using known spectral patterns from analogous bodies, or deriving it from other parameters. The effective temperature is defined so that it has a relation with two other parameters: the size of the body's surface (for a spherical object, related to its radius), and its luminosity, using the Stefan-Boltzmann law:
Teff = ( 4 π R2 ρ )1/4
Other measures of a body's temperature that are more directly determinable include the brightness temperatures (based on a single wavelength or passband), and color temperatures (based on a color index), and spectral temperatures (based upon spectral signatures, such as indicated by a star's spectral type). Sometimes they are reasonable estimates of the effective temperature, and they also may be used as data-points for better determinations.