99% of galaxies are either red or blue. Blue light-emitting galaxies are younger and hotter where as the red are older and cooler. The interesting thing is that there are relatively few “in-between” so called green valley(green light resides exactly in the middle of the light spectrum). This means that on a galactic time frame something really quickly quenches galaxies from blue to red.
What quenches it has been identified as a feedback loop from a massive black holes who’s powerful gravitational forces strips the nearby galaxy of available fuel thus quenching it and preventing the creation of additional stars within that galaxy.
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