Final answer:
Stars in a galaxy revolve around its center, and this motion is described by Kepler's third law, leading to differential galactic rotation.
Step-by-step explanation:
The stars in a galaxy are revolving around its center. This motion is governed by the same physical laws that describe the motion of planets around the Sun, specifically Kepler's third law.
According to this law, stars that are farther out from the center of the galaxy have longer orbital periods than those closer to the center, an effect known as differential galactic rotation. In addition, most of the galaxy's mass, including stars, gas, and dust, is concentrated in a rotating disk, which has a spiral structure when viewed from above. The mass of the interstellar matter is about 15% of the mass of the stars in this disk.