Why are the spiral arms of a galaxy the brightest parts when viewed in visible light?
Answer
They are predominantly populated by young, hot, blue stars.
The spiral arms appear brightest in visible light because they are the primary sites of ongoing stellar birth. As gas and dust are compressed when entering these density waves, new stars are ignited. These newly formed stars are massive, extremely hot, and short-lived, emitting copious amounts of bright blue and white light. This intense luminosity from these young stellar populations completely dominates the galaxy's visible light output, creating the striking contrast against the older, often yellowish or reddish populations found in the bulge and inner disk areas.
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