An astronomer is interested in a galaxy called M31, the nearest galaxy that resembles our Milky Way. It is about 2 million lightyears away. Which technique would be able to give us a distance to this galaxy?
A) parallax
B) radar reflections
C) period-luminosity relation for Cepheid variables
D) Kepler's laws
E) you can't fool me: there is no way at present to get a distance to an object so far away

Question

Quizplus · Accepted Answer

The period-luminosity relation for Cepheid variables is a reliable technique for measuring distances to galaxies. Cepheid variables are stars whose brightness varies periodically in a well-known way, and by measuring their apparent brightness and period of variation, astronomers can determine their absolute brightness and thus their distance. This technique was used by Edwin Hubble to determine that the galaxy M31 was located 2 million lightyears away from us.

An Astronomer Is Interested in a Galaxy Called M31, the Nearest