eli5: When analysing light spectrum of distant galaxies, how do we know if its red shift, AND NOT just the actual color of galaxy?

Question

since different gases give different color when burning, how can we know that the red colour we obtained is not just the actual color of gases burning, but rather a red shift?

In: 3

Answer 1

Gasses don’t burn in stars and since 75 % of mass in any galaxy is hydrogen, we know what hydrogen fusion spectrum needs to be.