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I know that elliptical galaxies are old galaxies, with almost no star formation and an old stellar population. This can be seen by their dominantly red spectra in the optical. I am trying to get a more detailed statistical overview on what are the spectral classes of stars that constitute the stellar populations of ellipticals. In this graphic, the size of the rectangles in the HR diagram is correlated with the relative contribution to light coming from the galaxy from this stellar spectral type. I am looking for new research results/papers on this.

enter image description here

source: https://www.usm.uni-muenchen.de/people/saglia/dm/galaxien/alldt/alldt.html

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    $\begingroup$ That's a terrible temperature color coding 😳 $\endgroup$
    – pela
    Jan 18, 2023 at 12:35
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    $\begingroup$ I dont know how many years its old, but certainly from the last century. Do you know of similar graphics like this, since I find the information content very appealing $\endgroup$
    – trynerror
    Jan 18, 2023 at 13:05
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    $\begingroup$ Sorry, it wasn't a very helpful comment. And I realize that if you image google "HR diagram" you'll find many versions colored like this, or wrong in other ways. The color should go from red (at low T), to orange, white, and finally light blue (at high T), as seen e.g. here. As for your question, it looks about right for an elliptical, though you don't normally depict it like that. Usually you will look at the total spectrum or, if you don't have a spectrum but have magnitudes in… $\endgroup$
    – pela
    Jan 18, 2023 at 14:33
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    $\begingroup$ …multiple filters, you'll fit a synthetic spectrum to these magnitudes. The synthetic spectrum is then created by varying the various parameters that affect the spectrum, e.g. mass, age, metallicity, until it fits the magnitudes (so-called "SED-fitting). $\endgroup$
    – pela
    Jan 18, 2023 at 14:35
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    $\begingroup$ That figure does not show "the number frequency of stars inside the galaxy"; it's probably trying to show the relative contribution to light coming from the galaxy due to different spectral types. The overwhelming majority of stars in an elliptical galaxy are red dwarfs, but they contribute relativey little light. $\endgroup$ Jan 21, 2023 at 11:06

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