The measurement of cosmic distances was helped tremendously by the discovery, in the early part of the 20th century, that in Cepheid variable stars, the average luminosity was related to:
A) their radial velocity
B) the abundance of hydrogen in their atmospheres
C) their distance from the Sun
D) the length of time they took to vary
E) their parallax

Question

Quizplus · Accepted Answer

The average luminosity of Cepheid variable stars was found to be related to the length of time they took to vary, allowing astronomers to use them as standard candles to measure cosmic distances. This relationship is known as the period-luminosity relationship.

The Measurement of Cosmic Distances Was Helped Tremendously by the Discovery