What primary characteristics do astronomers use to classify major spiral arms in the Milky Way?
Answer
High density, long, continuous structure, and very high star formation
In current astronomical models, structures within the Milky Way are categorized by their density and star-forming activity. Major spiral arms are characterized by their high density, their long and continuous nature, and very high rates of star formation. This contrasts with minor arms, which are generally of lower density and more fragmented, or local features like the Orion Spur, which have moderate star-forming activity.
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