Most of the light from a galaxy comes from the inner parts. If this means that most of the galaxy's mass is also in the inner region, then how would the galaxy's speed of rotation behave in its outer region?
A) The rotation speed should increase with increasing distance from the center.
B) The rotation speed should decrease sharply to zero at the outer edge of the visible galaxy.
C) The rotation speed should not change appreciably with increasing distance from the center (i.e., a "flat" rotation curve).
D) The rotation speed should decrease smoothly with increasing distance from the center.

Question

Quizplus · Accepted Answer

If most of a galaxy's mass is concentrated in its inner region, the gravitational pull on outer stars decreases with distance. According to Kepler's laws, objects in orbit around a mass will move slower if they are further away from the mass. Thus, the galaxy's rotation speed should decrease smoothly with increasing distance from the center.

Most of the Light from a Galaxy Comes from the Inner