What is the most basic difference between elliptical galaxies and spiral galaxies?

The most basic difference between elliptical galaxies and spiral galaxies lies in their structural composition, particularly regarding the presence of a disk. Elliptical galaxies are characterized by their smooth, featureless shapes that are generally spheroidal and lack the distinct disk and spiral arms observable in spiral galaxies. This absence of a disk means that elliptical galaxies do not have the same kind of organization of stars, gas, and dust seen in spirals; instead, they consist predominantly of older stars and have very little interstellar material.

Spiral galaxies, on the other hand, have a well-defined, rotating disk structure where stars are organized into spiral arms, along with substantial amounts of gas and dust that allow for ongoing star formation. This structural distinction is fundamental to their classification and highlights the varied evolutionary paths and characteristics of these two types of galaxies.

The other options do not accurately capture the most fundamental structural difference. For instance, the number of stars in galaxies can vary widely regardless of their type, and both elliptical and spiral galaxies can have significant amounts of dark matter. Additionally, the colorfulness of a galaxy is influenced by its star population and ongoing star formation rather than being a simple characteristic differentiating between the two types.