Let's ask a question first: when scientists receive the spectrum emitted by stars hundreds of millions of light years away, how do they come to the conclusion that the spectrum of the stars is redshifted by tens of nanometers, rather than that the original spectrum of the stars is like this? Because there are many articles about redshift, few people explain this problem in detail or make mistakes, including many big coffees.
Friends who have read similar articles will say, "I know, judge by atomic absorption spectrometry!" " "
It can be said that this answer is basically correct, but it is not accurate. Below I use the metaphor of "enlarging photos" to explain how scientists judge and measure the redshift of stars in an easy-to-understand way.
If we actually receive the spectrogram shown in the second spectrogram above and find that the four black spectral lines are at the positions of A+ 100, B+ 100, C+ 100 and D+ 100, we can still conclude that this is the absorption spectral line of methane molecules, but only the positions of the four spectral lines. People can still see you at a glance. As for how many times it is enlarged, just measure the eye size of the enlarged photo compared with the original one.
Similarly, as long as we measure how many nanometers the characteristic absorption line of methane molecule has shifted to the red end, we can measure how much the galaxy containing the molecule has shifted red.
The above is what I learned and understood in a popular science book about radio astronomy I read 30 years ago. If I spend too long, my memory may be wrong.
You got it?