Galaxy main sequence (or galactic main sequence or star formation main sequence or SF main sequence) is a term for the relationship between a galaxy's star formation and its stellar mass. The term galaxy diagram can refer to a graph showing the relationship, much like the H-R diagram does for main sequence stars. When plotted on a log-log graph, galaxies fall close to a straight line, indicating star formation rate (SFR) = constant × (stellar mass)constant for two constants. The plot can also be color versus galaxy magnitude, a galaxy color magnitude diagram.
The term main sequence galaxy (MS galaxy) indicates a galaxy that fits the relationship, which generally holds. Starburst galaxies are an exception: they have a higher SFR and their plot forms another straight line indicating a similar function with different constants. Some gas-poor galaxies such as such as some ellipticals form another exception with a lower rate, and are known as quenched galaxies. Galaxies in the (star forming) main sequence are bluer and the quenched galaxies are redder (blue galaxies and red galaxies). There is some correlation between disk (spiral) and elliptical galaxies being blue and red respectively, but there are exceptions.
The area of the plot between main sequence galaxies and quenched galaxies has fewer galaxies, and has been named the green valley, the few galaxies there presumed to have less than normal star formation but more than that of quenched galaxies. The Milky Way and Andromeda are thought be green valley galaxies.
An assumption is that starbursts are caused by mergers or other occasions when gas is pushed together, and galaxies with low star formation rates are those that have lost their gas.