eli5: Colder stars are usually red, hotter ones are more blue. But how do scientists how much “redness” is from the star itself and how much is due to the red shift of the expanding universe?
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A little history as background. Once people figured out that there are distinct chemical elements (early 19th century) and how to reliably generate electric current (later 19th century), the first thing that scientists did was to isolate elements and zap the shit out of them with high voltage. Turns out that many elements give off visible light when you get them into gasseous form and put 40,000 volts through them. (This led to neon lights (early 20th century) and other fluorescent lights in a zillion different colors.) The NEXT thing scientists did–like, that same afternoon after a couple of beers–was to put the light through a prism. Instead of a rainbow effect, when there’s just one element being zapped, the resulting “spectrum” has just a few very VERY distinct lines. Neon, for example, gives a bright red line and a few faint lines in other colors. Every element, it was discovered, has its very own completely unique set of lines!
A couple beers later, and it was discovered that the spectrum of a given element DOES NOT CHANGE, NO MATTER WHAT. Freaking amazing! You can squeeze it under pressure, you can heat it, you can freeze it, yet the spectrum is identical under all conditions.
Spectral analysis is now one of the basic tools of chemistry. You want to know what something’s made of, you zap it real good and look at it through a prism.
The next natural step (late 19th century) was to have another look at the sun, only with a way better prism than Isaac Newton used in the 1600s. Lo! And behold, instead of a full spectrum rainbow, a whole lot of distinct lines of color appear. Scientists were immediately able to measure the exact chemical makeup of the sun by: 1) lining up the many lines of the sun’s spectrum against the few lines given off by various elements; and 2) comparing how bright the hydrogen lines are vs. the helium lines vs. the iron, and so on.
So then we knew: Stars get their colors due to their exact chemical composition. This was all figured out by 1915 or so.
No more than six beers later, someone said, “Let’s fix up a really good telescope with one of these new prisms and see what OTHER stars are made of!” It was noticed pretty soon (by 1930) that the spectra of stars and galaxies lined up just fine, BUT were often shifted to the red. This red shift needed an explanation because, as noted earlier, nobody could make it happen in any way on Earth. The individual elements give you certain colors, and that’s final. Indeed, once quantum mechanics got developed, it became possible to predict which colors a given element would give off, again with startling accuracy and precision.
Relativity came to the rescue! Someone got a nice prize for realizing that light will come to an observer in a different color if the source and observer are moving towards or away from one another. Some astronomers naturally scoffed at this explanation of the red shift because it requires that most of the universe is moving away from Earth at a measurable fraction of the speed of light. But further experience bore this out: the farther the star, the more its spectrum is shifted to the red; the nearer the object, the less shifted its spectrum will be (with notable exceptions where some near objects happen to be coming our way; on a “whole universes” scale, the effect is pretty unifrom).
So, your question is answered thusly: the elements in a given star CAN ONLY do certain very distinct colors. Anything outside that is due to movement and not chemistry.
(Edited dates: some of this stuff was figured out even earlier than I first remembered)
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