Elliptical galaxies are universally old and yellow; about three-quarters of all ellipticals have no significant star-forming gas or dust left, and even the quarter or so of ellipticals that are still forming stars do so at much lower rates (and with gas and dust reservoirs that are far closer to depletion) than is the case in irregular and spiral galaxies.

In contrast, spirals and irregulars have large amounts of gas and dust, breed stars like rabbits, and, as a result, contain huge quantities of young, bright, blue stars.

Why are elliptical galaxies so universally old, yellow, and star-forming-material-starved, while other types of galaxies are young, blue, and rich in gas and dust? Shouldn't there be a bunch of young, blue, gas/dust-rich ellipticals complementing their yellowed seniors, and many yellow, geriatric, no-longer-star-forming spirals and irregulars to go along with the young blue star factories?

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    $\begingroup$ There are hypotheses that elliptical are result of merger of two spirals $\endgroup$ – planetmaker Feb 27 '20 at 5:11
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    $\begingroup$ As planetmaker has said above, elliptical galaxies are likely mergers of spirals. As the spirals merge, they result in a starburst galaxy, and after all or most of the gas has been used, then it effectively becomes the stereotypical elliptical. $\endgroup$ – Max0815 Mar 5 '20 at 14:30
  • $\begingroup$ Pink Hearts, Orange Stars, Yellow Moons, Green Clovers, and Blue Diamonds! $\endgroup$ – uhoh Sep 28 '20 at 1:31

The main reason the late-type galaxies (spirals and irregulars) are blue is that the brightness contribution of the hotter stars (Main sequence O, B, and such) surpasses the contribution of colder, less massive stars (even though, there are more low mass stars than high mass stars). Read more about it here and here.

In early-type galaxies (elliptical and lenticular), we have an older population of stars, no dust, no star-formation. The luminosity contribution of massive young stars does not exist here. If you look at the spectrum of an elliptical galaxy, you'll notice that is similar to the spectrum of a KIII star (that's a red gigant, a very much evolved sun-type star), this tells us that the main contribution of light comes from this kind of stars.

We don't have blue elliptical galaxies because there are no young stars, star formation, or dust to make stars, that can give us a significant blue light contribution.

There aren't yellow spiral ones, because the massive stars will shine brighter than the less-massive stars.

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    $\begingroup$ Thus is a restatement of information already in the question. The question was, why? $\endgroup$ – ProfRob Sep 28 '20 at 6:39

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