Final answer:
The Tully-Fisher relation is limited to spiral galaxies, linking their luminosity to rotational velocity measured by the width of the 21-cm hydrogen line, but it cannot be used for elliptical galaxies.
Step-by-step explanation:
The limit to the Tully-Fisher relation for measuring galactic distance is that it can only be applied to determine the distance to spiral galaxies. This empirical relationship connects the luminosity of a spiral galaxy to its rotational velocity, which is indicated by the breadth of the 21-cm line produced by cold hydrogen gas within the galaxy. The broader the 21-cm line, the faster the gas is orbiting in the galaxy, implying a more massive and luminous galaxy. For other galaxy types such as elliptical galaxies, astronomers must employ different methods to estimate distances.