Though we know much more about the processes of galaxy formation and evolution than we did even a decade ago, many key points still elude us.
One particular puzzle is that of how star formation ends in a galaxy. We think that galaxies eventually transition from bright, blue, star-forming disks into red and quiescent ellipticals — but what causes star formation in a galaxy to shut down during this transition?
Since galaxies form stars out of cold gas, we could assume that star formation stops only when the cold gas supply is depleted.
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