When stellar aging causes a galaxy to appear red, what kind of stellar population dominates the system?

Answer

A population dominated by middle-aged and old stars, which are redder and cooler.

When a galaxy exhausts the cold gas needed for new blue star formation, the existing stellar population ages. This leaves behind a system dominated by intrinsically redder and cooler middle-aged and old stars.

When stellar aging causes a galaxy to appear red, what kind of stellar population dominates the system?

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