galaxy main sequence

Galaxy main sequence is a term for a common relationship between a galaxy's star formation and the combined mass of its stars (its stellar mass). The term galaxy diagram can refer to a graph that demonstrates the relationship, much like the H-R diagram does for main sequence stars: when plotted on a log-log graph, galaxies fall somewhat close to a straight line, indicating star formation rate (SFR) = constant × (stellar mass)^constant for two constants (i.e., a power law). The relationship also shows on a plot of the color (color index) versus galaxy magnitude, a galaxy color-magnitude diagram. The term main sequence galaxy (MS galaxy or star-forming galaxy, SFG) indicates a galaxy that fits the relationship, which generally holds. Starburst galaxies are an exception: they have a higher SFR and their plot roughly forms another straight line indicating a similar function with different constants. Some gas-poor galaxies (quenched galaxies) such as some ellipticals form another exception with a lower rate. Galaxies in the (star formation) main sequence are bluer and the quenched galaxies are redder (blue galaxies and red galaxies, the latter sometimes described as red and dead). 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 to be green valley galaxies (occasionally termed green galaxies). An assumption is that starbursts are caused by galaxy mergers or other occasions when gas is pushed together, and galaxies with low star formation rates are those that have lost their gas. The term post-starburst galaxy (PSB galaxy) refers to galaxy that is presumed to have undergone this, generally among those with reduced star formation (green valley galaxies).