What do we mean when we say that the rotation curve for a spiral galaxy is "flat"?

Question

Quizplus · Accepted Answer

When we say that the rotation curve for a spiral galaxy is "flat," it means that the speed of stars or gas clouds orbiting the galaxy does not significantly decrease with distance from the galactic center. This indicates that there is a significant amount of invisible matter (dark matter) distributed throughout the galaxy, which exerts a gravitational influence on the orbiting stars and gas clouds to maintain a flat rotation curve. Option A is incorrect because it refers to the shape of the galaxy rather than its rotation curve. Option B is incorrect because it does not necessarily follow from a flat rotation curve, and the amount of light emitted by stars may vary depending on their mass and age. Option D is incorrect because there is also a significant amount of dark matter in the galaxy's halo, not just in its disk.

What Do We Mean When We Say That the Rotation