If z is calculated using the ratio between observable wavelength and emitted wavelength, how do we know the emitted wavelength of a star that is moving away from us? Wouldn’t we have to be at a constant distance from the star to know that value?


1 Answer 1


The light emitted by stars passes through various gases, these gases absorb very specific wavelengths of light. For example Hydrogen will absorb light at exactly 6563 Å. So we know that the spectrum of light would have a dark line at 6563 Å if the galaxy wasn't moving away from us.

If the light is redshifted, this line also gets redshifted. So if this line appears at 7830 Å, we can calculate the redshift.

Because gases absorb (and emit) light at particular known wavelengths, we do know emitted wavelength of the light.

  • $\begingroup$ Thank you - makes sense! (Just googled to work out that Å is Angstroms en.wikipedia.org/wiki/Angstrom - a way of representing very tiny lengths) $\endgroup$
    – tomh
    Jul 21, 2022 at 16:05
  • $\begingroup$ Hi James. If you don't mind, I'd love to ask couple of questions. Question 1 - at which step would light go through hydrogen ? you mean hydrogen gas that exists in a space(clouds of it -before forming the star from it) or you mean when fusion happens inside the core, energy (photon) is released and before it gets out of the surface, it gets scattered through other hydrogen atoms inside core. $\endgroup$ Apr 23, 2023 at 0:42
  • $\begingroup$ Question 2: if the answer from question 1 is both, then in terms of fusion in the core, released photon could scatter between multiple times in different hydrogen atoms, wouldn't this redshift the wavelength even more or once light is gone through hydrogen atom one time, it doesn't matter how many times it goes through with it later, its wavelength spectrum will stay the same ? $\endgroup$ Apr 23, 2023 at 0:42
  • $\begingroup$ Question 3: in the core, light would definately not only scatter hydrogen, but some other stuff as well before it would reach the outer shell and start moving to us. What if it scattered first through hydrogen and then helium after that it came to us ? $\endgroup$ Apr 23, 2023 at 0:44
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    $\begingroup$ @Giorgi Lagidze: No, that is not how Stack Exchange works (unless the intent is to prompt improvement of the answer). You need to consider the scary proposition of asking a new question. (It should also be one question at a time, unless they are very closely related (essentially one question).) $\endgroup$ Apr 23, 2023 at 13:58

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