What is the Tully-Fisher relation?
A) The more distant the galaxy, the greater is the recessional velocity.
B) The brighter the Cepheid, the longer is the pulsation period.
C) The more distant the galaxy, the fainter it appears.
D) The brighter the galaxy, the wider is the 21-cm radio emission line.

Question

Quizplus · Accepted Answer

The Tully-Fisher relation is an empirical relationship between the intrinsic luminosity of a spiral galaxy and the width of its 21-cm line emission, which is related to the galaxy's rotational speed. Brighter galaxies have faster rotation speeds, which broadens the 21-cm line due to the Doppler effect.

What Is the Tully-Fisher Relation