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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 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 at 14:30

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