Why should spiral arms in the Milky Way not be considered solid physical structures?
Answer
They are dynamic regions of higher density where gas and dust accumulate
Spiral arms are not rigid, physical spokes on a wheel. Instead, they are dynamic regions where higher concentrations of gas and dust accumulate. As these materials enter these high-density regions, they are compressed, triggering the birth of new, bright stars. It is these active star-forming regions that make the arms appear prominent to observers, even though the arms are evolving, moving features rather than fixed, solid objects.
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