The diagram has three main features: the red sequence, the green valley, and the blue cloud. The red sequence includes most red galaxies, which are generallyelliptical galaxies. The blue cloud includes most blue galaxies, which are generallyspirals. In between the two distributions is an underpopulated space known as the green valley which includes a number of red spirals.
Like the comparableHertzsprung–Russell diagram forstars, galaxy properties are not necessarily completely determined by their location on the color–magnitude diagram. The diagram also shows considerable evolution through time. The red sequence earlier in theevolution of theuniverse was more constant in color across magnitudes and the blue cloud was not as uniformly distributed but showed sequence progression.
One of five patches of sky covered by the COMBO-17 survey.[4]
New research suggests the green valley is actually composed of two different populations of galaxies: one of late-type galaxies, where star formation has beenquenched due to their gas supplies being shut off followed by exhaustion of their gas reservoirs for several billion years, and another of early-type galaxies where both the gas supplies and gas reservoirs have been destroyed very quickly, likely because ofmergers with other galaxies and/or the presence of anactive galactic nucleus.[5][6]
TheMilky Way and theAndromeda Galaxy are assumed to lie in the green valley due to their star formation slowing down as a result of both running out of gas.[7]
^Bell, Eric F. et al.Nearly 5000 Distant Early‐Type Galaxies in COMBO‐17: A Red Sequence and Its Evolution since z=1,The Astrophysical Journal, 608:752–767, 2004 June 20.[1]
^Strateva, I., et al.Color Separation of Galaxy Types in the Sloan Digital Sky Survey Imaging Data, 2001, TheAstronomical Journal, 122, 1861[2]
