How do stellar populations differ between the Sagittarius Arm and the Orion Spur?

Sagittarius has intense, ongoing formation of short-lived, massive blue stars, while the Spur has a mixed population.

The age and activity of the stellar populations serve as a key indicator for tracing spiral density waves. Primary arms like the Sagittarius Arm are dynamic environments where intense gravitational compression triggers continuous star formation. This results in a population heavily weighted toward intense, short-lived, massive blue stars that are actively being born and dying. In contrast, the Orion Spur, being a less organized, minor feature, hosts a more mixed stellar population. This mixed population includes many older stars that may have migrated inward or outward from their original sites of birth, indicating a less sustained or less dominant rate of intense, recent star birth compared to the main arms.