The Andromeda spiral galaxy is almost edge-on to us. Astronomers have detected the orbital motion of this and other such galaxies' disk material by
A) using Hubble's law.
B) the big bang theory of 3-Kelvin radiation.
C) the steady state theory.
D) measuring Doppler shifts of different parts of the disk.
E) watching the stars travel around the nucleus back to where they started.

Question

Quizplus · Accepted Answer

By measuring the Doppler shifts of different parts of the disk, astronomers can determine the orbital motion of the disk material in a spiral galaxy such as Andromeda. This is because the Doppler shift of light emitted by a star can be used to calculate its radial velocity, or how fast it is moving towards or away from us. By studying the radial velocities of stars in different parts of the galaxy's disk, astronomers can infer the motion of the disk material as a whole.

The Andromeda Spiral Galaxy Is Almost Edge-On to Us