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A green valley galaxy is a rare galaxy that forms stars at a distinct but low star formation rate (SF rate). A particular region of graphs that relate galaxies' star formation to their stellar mass (such as galaxy color-magnitude diagrams) is termed the green valley. Only a small percentage of galaxies fall in this region of such graphs, but both Milky Way and Andromeda are green valley galaxies, the former with an estimated SF rate of on the order of two solar masses per year. The means by which galaxies become green valley galaxies is of research interest.
The green valley is between two more-populated regions of such graphs. "Above" the green valley (with a higher SF rate) are those with a typical SF rate, termed main sequence galaxies; the typical rate is observed to vary exponentially with the mass of a galaxy's stars. "Below" the green valley are quenched galaxies (aka quiescent galaxies), with virtually no star formation (i.e., not star-forming galaxies). A common means by which galaxies are quenched is a starburst, an episode of extremely high star formation that necessarily quickly uses up the galaxy's gas (neutral atomic hydrogen and molecular hydrogen). A galaxy undergoing such an episode is termed a starburst galaxy.