If stars moving away/towards Earth appear blue/red shifted, how do we know their colours are red/blue because they are moving, and not just that they are a red/blue coloured star that’s relatively stationary?

Question

I copied the wording from a Quora post because all the answers were ELIhave a PHD in astrophysics.

In: 6

Answer 1

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.