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Reference.org
Tully–Fisher relation
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<p>In <a href="/facts/Astronomy/0ArJVM1p">astronomy</a>, the Tully–Fisher relation (TFR) is a widely verified <a href="/facts/Empirical_relationship/WSxeL4St">empirical relationship</a> between the mass or intrinsic <a href="/facts/Luminosity/1tdhpk9l">luminosity</a> of a <a href="/facts/Spiral_galaxy/hRT2dtxJ">spiral galaxy</a> and its <a href="/facts/Galaxy_rotation_curve/YJyb1Jy3">asymptotic rotation velocity</a> or <a href="/facts/Emission_line/HuI4Bj5f">emission line</a> width. Since the observed brightness of a galaxy is distance-dependent, the relationship can be used to estimate distances to galaxies from measurements of their rotational velocity. </p>