Why do the bright blue OB stars associated with the arms fade relatively quickly?

Answer

They burn out quickly before they can orbit far out of the wave structure

The visibility of the spiral arm is inherently tied to the lifespan of the massive stars born there. The most luminous stars, such as the blue OB stars, form right at or just behind the shock front where gas compression is maximized. However, these massive stars have very short lifespans relative to the galaxy's age. They exhaust their fuel and expire before they have time to complete a full orbit and drift significantly outside the organized density wave structure. This rapid burnout means that the brightest markers of the spiral arm are fleeting features localized precisely within the arm boundaries, whereas less massive, redder stars live long enough to leave the structure and populate the regions between the arms.

Why do the bright blue OB stars associated with the arms fade relatively quickly?

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