Final answer:
Stars in elliptical galaxies have random, non-aligned orbits around the center, making the galaxies lack systematic rotation and challenging to measure for rotational velocity and dark matter.
Step-by-step explanation:
Stars in elliptical galaxies that appear spherical in shape have velocities that are not consistent with organized rotational movement. Unlike spiral galaxies where stars orbit in a somewhat uniform, flat plane, stars within elliptical galaxies orbit the galactic center in a more chaotic manner. These stars follow orbits that are randomly oriented and can be quite elliptical. Because these orbits are not aligned, elliptical galaxies do not show systematic rotation, making it challenging to measure their rotational velocity and assess the amount of dark matter. By analyzing the motion of stars within elliptical galaxies, researchers can infer the existence of additional unseen mass, as stars often move at speeds suggesting more mass than what is visible, hinting at the presence of dark matter.