Tully–Fisher method(tul -ee fish -er) The most accurate method for measuring the distances to distant spiral galaxies; it uses a relation between a galaxy's absolute magnitude and its rotation velocity that was discovered by R.B. Tully and J.R. Fisher in 1977. Radio observations at 21 cm wavelength give the rotation velocity; the Tully–Fisher relation then indicates the absolute magnitude, and a comparison with the apparent magnitude gives the distance. The Tully–Fisher relation must be calibrated by observations of nearby spirals, whose distances are known from Cepheid variables, etc. (see distance determination). See also Faber–Jackson relation.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006