How do young, massive stars within the spiral arms affect the overall perceived age of light emitted by a spiral galaxy?

Answer

They effectively dilute the overall average age of the galaxy's light

The constant addition of bright, young, hot blue stars in the arms offsets the presence of older stars, resulting in an average light profile that appears younger than systems where star formation has ceased.

How do young, massive stars within the spiral arms affect the overall perceived age of light emitted by a spiral galaxy?
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