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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). The galaxy resides in the green valley, a region of graphs that relate galaxies' star formation to their stellar mass, which is a region with relatively few galaxies. Galaxy color-magnitude diagrams effectively do this. The Milky Way and Andromeda are in the green valley, the former with an estimated SF rate of on the order of two solar masses per year. The development of 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 a lot star formation at an unsustainable level that would use up the galaxy's gas (neutral atomic hydrogen and molecular hydrogen). A galaxy undergoing such an episode is termed a starburst galaxy.