Question

how does the appearance of spiral galaxies vary? select the two correct answers. (1 point)
they vary in how tightly their arms are wound.
they vary in the brightness of stars located on their arms.
some have a band of stars and materials through their middle, but others do not.
some are more elongated in shape than others.

Explanation:

Spiral galaxies can be classified based on the tightness of their spiral arms (e.g., Sa - tight arms, Sc - loose arms). Also, there are normal spiral galaxies and barred spiral galaxies. Barred spirals have a band of stars and material through their middle, while normal spirals do not. The brightness of stars on arms is more related to stellar properties rather than a fundamental way spiral galaxy appearance varies in a classification - based sense. And elongation is more a feature of elliptical galaxies rather than a key varying feature of spiral galaxies.

Answer:

They vary in how tightly their arms are wound.
Some have a band of stars and materials through their middle, but others do not.