What do we mean when we say that the rotation curve for a spiral galaxy is "flat"?
A)The amount of light emitted by stars at different distances is about the same throughout the galaxy.
B)Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward.
C)The disk of a spiral galaxy is quite flat rather than spherical like the halo.
D)All the galaxy's mass is concentrated in its flat,gaseous disk.

Question

Quizplus · Accepted Answer

A flat rotation curve means that objects (such as gas clouds) located at different distances from the center of the galaxy have approximately the same orbital speed. This suggests that there is a lot of mass located far from the center, which is not observed in visible matter such as stars, and is thought to be in the form of dark matter.

What Do We Mean When We Say That the Rotation