Fr.: relation Tully-Fisher
An observed correlation between the luminosity of a spiral galaxy and its rate of rotation (measured from its 21 cm hydrogen line). This means that more luminous galaxies have stars that are moving faster. Knowing the rotational velocity of a spiral galaxy, this relation provides its absolute magnitude and then its distance. → Faber-Jackson relation.
Named after R. B. Tully and J. R. Fisher who first derived this relationship (1977, A&A 54, 661); → relation.