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 generally
elliptical galaxies. The blue cloud includes most blue galaxies, which are generally
spirals. In between the two distributions is an underpopulated space known as the green valley which includes a number of red spirals.
Like the comparable
Hertzsprung–Russell diagram for
stars, 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 the
evolution of the
universe was more constant in color across magnitudes and the blue cloud was not as uniformly distributed but showed sequence progression.
New research suggests the green valley is actually composed of two different populations of galaxies: one of late-type galaxies, where star formation has been
quenched 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 of
mergers with other galaxies and/or the presence of an
active galactic nucleus.[5][6]
The
Milky Way and the
Andromeda 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]
See also
Galaxy Zoo – Crowdsourced astronomy project, citizen science projects from 2007 to classify galaxy images
Hertzsprung–Russell diagram – Scatter plot of stars showing the relationship of luminosity to stellar classification, used for stars in clusters
References
^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, The
Astronomical Journal, 122, 1861
[2]
^Schawinski, Kevin; Urry, C. Megan; Simmons, Brooke D.;
Fortson, Lucy; Kaviraj, Sugata; Keel, William C.; Lintott, Chris J.; Masters, Karen L.; Nichol, Robert C.; Sarzi, Marc; Ramin; Skibba; Treister, Ezequiel; Willett, Kyle W.; Wong, O. Ivy; Yi, Sukyoung K. (2014). "The Green Valley is a Red Herring: Galaxy Zoo reveals two evolutionary pathways towards quenching of star formation in early- and late-type galaxies". Monthly Notices of the Royal Astronomical Society. 440 (1): 889.
arXiv:1402.4814.
Bibcode:
2014MNRAS.440..889S.
doi:
10.1093/mnras/stu327.