Caption: A cartoon galaxy color-magnitude diagram.

Galaxy color-magnitude diagrams are sort of the galaxy analogues to Hertzsprung-Russell (HR) diagrams for stars.

Features:

1. The horizontal axis is logarithmic luminosity for galaxies.

2. The vertical axis is color index for some representative imagined pair of passband filter magnitudes.

   A color index is the bluer minus the redder of the pair of passband filter magnitudes and bigger the difference, the redder the color index for the astronomical object. The color index is probably B-V.

   But note that in absolute true color galaxies look white???. "Blue" galaxies have more blue light relative to light red than "red" galaxies, but that is NOT the human psychophysical response in their absolute true color.

   See Extended Features below for more discussion of the color of galaxies.

3. Since circa 2000, there has been the suggestion that galaxies fall into two major populations on a galaxy color-magnitude diagram: a blue cloud (which are probably mostly spiral galaxies and have the spiral galaxy mix of colors with their blue color coming from OB stars mostly located in the spiral arms) and a red sequence (which are mostly elliptical galaxies and which look white hot in true-color). See Extended Features below for a discussion of white hot.

   Between the two major populations it has been suggested that there is a relatively unpopulated green valley which is just a whimsical name since the galaxies in the green valley do NOT look green in any true-color sense. They probably look a mixture of the other two populations in true-color.

   There are green bean galaxies and green pea galaxies, but they only look greenish in certain standard imaging passband filters, probably NOT in true-color.

4. Are the red sequence and blue cloud really distinct populations? Maybe NOT. Eales et al. (2018) argue that biased observations created the apparent distinction. There really is a CONTINUUM between the blue cloud and the red sequence, and there is NO relatively unpopulated green valley. But the issue is NOT fully settled circa the 2020s.

5. The essential difference between the two populations of galaxies (treating them as distinct by some conventionally defined line if NOT otherwise) is that the blue cloud galaxies have a lot of star formation and the red sequence galaxies do NOT have a lot: in the extreme limit very little to none.

   The red sequence galaxies are quenched galaxies: i.e., those for which star formation rate (SFR) is less than ∼ 10 % of SFR in galaxies recognized as star forming galaxies (mostly spiral galaxies and irregular galaxies) and maybe star formation is turned off completely (see, e.g., Cimatti 2020, p. 54). As a shorthand, we usually say quenched galaxies have NO star formation without qualification or star formation turned off without qualification.

1. The shown colors are meant to be representative. Unquenched galaxies have a relative excess blue light only near their star forming regions where hot young blue main-sequence stars (mainly OB stars) (hereafter just OB stars) live their whole stellar lifetimes which end when they explode as core-collapse supernovae (if their masses exceed ∼ 8 M_☉: all O stars and the larger B stars). However, in absolute true color the regions of excess blue light still just look white hot. Of course, the star forming regions themselves are dark dusty molecular clouds and do look dark or brown in absolute true color. Overall, the unquenched galaxies would look white hot (see Temperature of a "White Hot" Object, Carine Fang, 2001, bottom of page; blackbody_spectra.html).

   Similarly, quenched galaxies look white hot or somewhat yellow-white in absolute true color particularly as their stellar effective temperatures are n the white hot range ∼ 1500--11000 K see Temperature of a "White Hot" Object, Carine Fang, 2001, bottom of page; blackbody_spectra.html). Thousands of gigayears in the future (wildly extrapolating to the Λ-CDM model cosmic future), the quenched galaxies will become redder in the photometric sense (i.e., in B-V color index) since the only stars left doing nuclear burning will be red dwarfs which have lifetimes up to ∼ 10**4 gigayears (see Wikipedia: Red dwarf: Description and characteristics). However, the quenched galaxies will still probably look white hot in true color (as seen in space) because their stellar effective temperatures are ∼ > 2300 K which is still in the white hot range.

2. To confuse the discussion of galaxy color, astronomers measure color using the color index B-V which is frequently just called color. Lower B-V is bluer, higher B-V is redder. So blue cloud galaxy (which are star forming galaxies) are lower in B-V, and red sequence galaxies (which are quenched galaxies) are higher in B-V. And green valley galaxies are intermediate in B-V and do NOT look green in true color.