What does it mean when an emission line of helium from a galaxy is shifted toward red?

Question

A. the galaxy is moving toward the viewer
B. the galaxy contains a small percent of hydrogen
C. the galaxy contains a large percent of hydrogen
D. the galaxy is moving away from the viewer.

Answer 1

When an emission line of helium from a galaxy is shifted toward the red end of the spectrum, it indicates that the galaxy is moving away from the viewer. This phenomenon is known as redshift and is commonly observed in the context of the expanding universe. Therefore, the correct answer is:

D. the galaxy is moving away from the viewer.