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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
    Commented Feb 27, 2020 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
    Commented Mar 5, 2020 at 14:30
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    $\begingroup$ Pink Hearts, Orange Stars, Yellow Moons, Green Clovers, and Blue Diamonds! $\endgroup$
    – uhoh
    Commented Sep 28, 2020 at 1:31
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    $\begingroup$ @uhoh - That did put a smile on my face! Thanks for that. $\endgroup$
    – Astrid_Redfern
    Commented Apr 5, 2021 at 19:18

2 Answers 2

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Galaxies consist of dark matter, stars, and gas. While gas is "collisional", i.e. it may interact hydrodynamically and cool, dark matter and, effectively, stars are collisionless. Hence, it is relative easy for an originally more or less spherical, gas-rich galaxy to collapse along the axis of rotation, while centrifugal forces prevents to collapse in the plane. In contrast, a spherical gas-poor galaxy will tend to keep its shape.

Spiral galaxies are blue

That is, a gas-rich galaxy will tend to settle down into a flat disk. Being gas-rich, such a galaxy will keep forming stars, and a spiral pattern will form.

Stars come in all sizes, but the most massive ones dominate the total luminosity (since $L\propto M^4$), and since massive stars shine with very energetic light, they are bluish/white.

Elliptical galaxies are yellow

Elliptical galaxies are the result of a merger between two or more galaxies. If the merging galaxies are very different in size, the smaller will just be "eaten" by the larger. But if the galaxies are similar-sized (and don't have too similar angular momenta), the event can be quite dramatic:

Tidal forces mess up the shape, gas is stripped from the galaxies, and collisions of gaseous clouds ignite massive starbursts. Such major mergers are quite efficient at suppressing angular momentum, allowing large amounts of gas to reach the galactic centers, feeding the central supermassive black hole leading to vigorous quasar activity (e.g. Hopkins et al. 2008).

Both the gas stripping, the radiation pressure from newborn, massive stars, the feedback from their fast and explosive deaths, and the active galactic nuclei, cause the merged galaxy to lose its gas. As the massive, blue stars die out, and with no gas to form new stars, only the smaller, yellowish/orange/reddish stars are left. The galaxy is said to be "quenched", ending up "red and dead".

Why not the opposite?

So the answer to your question is: The reason spiral galaxies are not red is that they keep forming new stars, the most luminous of which are blue.

And the reason elliptical galaxies are not blue is that they're unable to form new stars, so only the red ones are left.

Addendum thanks to Peter Erwin: Spiral galaxies may eventually deplete most of their interstellar gas (partially through outflows) and stop forming stars. This cause them to gradually change color toward yellow. However, without the feedback from star formation, such a galaxy will also tend to lose their spiral structure, ending up instead as a lenticular (or "anaemic") galaxy (see e.g. Elmegreen et al. 2002).

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    $\begingroup$ "Ram pressure stripping" refers to the removal of gas from a galaxy when it passes at high velocity through a large-scale (group or cluster) gaseous medium. It's not something that happens in a merger. $\endgroup$
    – Peter Erwin
    Commented Apr 6, 2021 at 16:30
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    $\begingroup$ @PeterErwin Yes, thanks, although I’d probably also use the word in the case of minor mergers, where small satellites plunge through the interstellar medium of a large galaxy. I removed the words, thanks. $\endgroup$
    – pela
    Commented Apr 6, 2021 at 18:40
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    $\begingroup$ I also realized I wrote "the smaller reddish galaxies" instead of stars, but it's fixed now. $\endgroup$
    – pela
    Commented Apr 6, 2021 at 19:15
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    $\begingroup$ @Vikki-formerlySean There are, but they're a bit rare, partly because once they stop forming stars, the spiral arms fade away and you're left with an S0/lenticular galaxy, which still has a dominant disk, but without spiral arms (and thus not a "spiral" galaxy). $\endgroup$
    – Peter Erwin
    Commented Apr 6, 2021 at 21:32
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    $\begingroup$ @PeterErwin: Ah, makes sense. $\endgroup$
    – Vikki
    Commented Apr 6, 2021 at 21:42
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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
    Commented Sep 28, 2020 at 6:39

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