Final answer:
The primary difference between a highly elliptical orbit and a non-elliptical orbit is their eccentricity, with the former being more elongated. Elliptical galaxies, which have stars orbiting in various directions and exhibit high degrees of flattening, can be difficult to study in terms of dark matter content due to their non-systematic rotation.
Step-by-step explanation:
The difference between a highly elliptical orbit and a non-elliptical orbit, which is typically more circular, lies in their eccentricity. An elliptical orbit has a shape that can be quite stretched out along one axis, while a non-elliptical (circular) orbit is even in all directions and does not have the major elongation that is characteristic of an elliptical orbit.
The eccentricity of an orbit quantifies the degree of its elongation. For example, if the eccentricity is close to 0, the orbit is nearly circular; however, as the eccentricity approaches 1, the orbit becomes more elongated - hence, more elliptical.
In the context of galaxies, elliptical galaxies have stars that orbit the center in various directions, as opposed to spiral galaxies where stars tend to orbit in a common direction. This makes elliptical galaxies appear to lack a systematic rotation, which complicates the estimation of dark matter they might contain. The evolution of elliptical galaxies is often due to mergers, which may contribute to their shape and structure, contrasting with spiral galaxies that often evolve through the accretion of gas and are less likely to have undergone recent mergers.