I copied the wording from a Quora post because all the answers were ELIhave a PHD in astrophysics.
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Gaseous atoms produce light of *very* specific frequencies. E.g. hydrogen produces the [Balmer series](https://en.wikipedia.org/wiki/Balmer_series). If you look for that pattern, and see it — but shifted up or down in frequency — then you can say you’re looking at a cloud of hydrogen moving toward or away from you at a particular speed.
Each gas when it gets hot enough, emits light. The color of this light is very specific to that gas and will always be the same. Like sodium street lamps are always that yellow colour. If we know what colour it should be, because we know what gases are there, we can measure how much the colour has shifted.
In reality stars shine in different specific colours, that creates a pattern, that tells us of what elements the star is made of. And we can see how much the pattern has shifted.
Okay, stars are made of different elements depending on their size and age. Each element of the periodic table emits, and absorbs light differently. (It has to do with the electron orbits, but that’s a bit complicated and not necessary to know for this question. )
Anyway, because each element absorbs light differently, they leave unique patterns. It’s like a barcode in the light spectrum. Each barcode pattern is located at precise points on the electromagnetic spectrum. I’m talking with nanometer precision.
Okay, so we know the exact pattern for every element, and we know the exact location in the spectrum for every element. Remember this.
Now, the shifting occurs when the universe itself is expanding. It’s pretty much all redshifted. An object would have to be traveling immensely fast towards us to blue shift.
As the universe expands it stretches the Lightwave out with it. So, the part that was orange is now red, yellow has become orange, etc.
BTW, this is oversimplified, the shifting is much more subtle, but the idea it conveys is correct.
Anyway, say we find the pattern for oxygen, which is green, in the yellow. How far into the yellow can tell us how far the star is. We know the light has shifted because the oxygen barcode is in the wrong place.
These barcodes are also how we know what elements are in stars and nebulae and such.
I think I got all the relevant information in here, ask about anything confusing, I’ll do me best.
You know how a prism splits up white light into a rainbow of individual colors? [This](https://www.shutterstock.com/image-vector/emission-spectrum-sodium-element-2025986870) is what it looks like when you take a pure element, sodium in this case (salt if you want to try it at home), and split up the light it gives off when really hot – it forms bands that make up a distinct pattern. Each element forms a distinct pattern. This is what the pattern for [hydrogen](https://en.wikipedia.org/wiki/Hydrogen_spectral_series#/media/File:Hydrogen_spectrum.svg) looks like.
Stars have a lot of hydrogen, so when you split up the light using in a spectrograph, you can look for that distinctive pattern (distinctive to astronomers, at any rate) in the spectrum of light from the star. If the pattern is shifted to the red then you know the star is going away. Shifted towards the blue and it’s coming at you. How much shift tells you how fast.
This technique can tell you more! When a star is spinning, one side may be going away and the other side coming towards you, and if your equipment is super sensitive, you can tell how fast the star is spinning!
Spectral Lines
Stars may appear “White” to our eyes but in reality they give off light in all sorts of different “colors” that mixed together only appear white to our eyes.
When we say that the light was red-shifted it doesn’t mean that the star itself appears red, just that all of its light was shifted towards the red end of the spectrum.
“Red” in this case means lower frequency and “blue” higher frequency. Red-shifted means all the light the star gives of was shifted a bit towards the lower frequency end of the spectrum. In practice that means that some light that was to high frequency to see otherwise will move into the part of the spectrum people can see and some part at the lower end move out of it.
Red is not really a description of the end result as much as a direction describing the change.
If a star gives of light in all the colors and the light gets red shifted or blue shifted it will not look different. However it isn’t quite “all the colors” in the real world.
Atoms and molecules give of and absorb light in very specific frequencies.
Think of it as a rainbow with some dark bands interrupting the usual continuous spectrum of colors.
Those lines are linke fingerprints for elements. We can tell what elements there are in an object by the lines in its spectrum.
If we see light from a distant star that looks exactly like we expect from such a star only everything is moved a bit towards the red or blue side, we can guess that the star was red-shifted or blue shifted.
We see a distinctive and recognizable pattern of lines in the spectrum shifted a bit towards the red or the blue side of the spectrum.
When we examine starlight we find it has a fingerprint that tells us the size, temperature and chemical makeup of the star. From that we can determine what color it should be. It is called astronomical spectroscopy.
Fingerprints of elements in the starlight, basically.
Stars can contain lots of different elements. Each element leaves its own distinctive pattern in the light that arrives here (its “[absorbtion spectrum](https://en.wikipedia.org/wiki/Absorption_spectroscopy)”). If you smear the light from the Sun out into a long strip using a prism, for example, it isn’t, as you might guess, one wide blur of light; there are [thousands of dark lines](http://www.pas.rochester.edu/~blackman/ast104/spectrumsun.html) caused by the elements within the Sun. And because each element leaves its own distinctive pattern, you can look for and identify those patterns in the light from a distant star, just as you can look at them in the light from the Sun (or even in a school lab). So you can tell things about the star’s makeup just from its light.
So – why “red shift” (or “blue shift”)? When a star is moving towards or away from us, the [Doppler effect](https://en.wikipedia.org/wiki/Doppler_effect) means that those patterns won’t show up at the same frequencies as we’d see if it were stationary. Further toward the red end of the spectrum? The light is “red shifted”. And where they DO show up is precisely related to how the distance between us is changing. The bigger the shift, the bigger the relative velocity. “Hey, that pattern’s carbon ((or whatever)) – but it’s waaay over towards the red end, almost into the infrared. So the star is moving away from us really fast.” Simple as that.
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