Why do some galaxies look red while others look blue?
Quick Answer
Blue galaxies usually have lots of young, hot stars and active star formation, while red galaxies are dominated by older stars with little new star birth. Dust can also make galaxies look redder.
The Full Story
Hot, massive stars emit more blue and ultraviolet light, so galaxies with ongoing star formation look bluer. Galaxies that have used up or lost much of their gas no longer form many new stars, leaving older, cooler stars to dominate the light and giving them a red appearance. Dust can redden the color further by absorbing blue light more strongly. This color difference helps astronomers classify galaxies and trace how they evolve over cosmic time. It is one of the main clues that galaxies change as their gas supply changes.
Key Facts
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