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

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.