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The Tully-Fisher relation (TFR) is a relation between the luminosity of a spiral galaxy and the width of its emission lines. The luminosity is related to the galaxy's stellar mass and the line broadening is a measure of its rotation velocity, ascribed to varying Doppler shifts. An estimated luminosity might be calculated and/or the relationship can be calibrated using such galaxies of known distance, making such galaxies standard candles, whose distance can be estimated.
The baryonic Tully-Fisher relation (BTFR) is a variant, specifically using K band measurements, which more closely relate to the stellar mass. The baryonic mass is related to the velocity to the power of 3.5 to 4.