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My understanding is that we can measure the distance to far away stars by using their color to infer the absolute magnitude of the light they emit and compare that to the observed magnitude of the light we actually receive. The difference between those values tells us how far away the star is. From my amateur perspective, this makes sense, even if I don't know the exact equation that describes the relationship.

For speed away from us, I've been told that we measure the redshift of the light. However this presents two issues to me.

1) How is the redshift actually measured? I haven't found any website that attempts to explain this. I understand the math and the Doppler effect, so I get the relationship, but I don't see how we get the z value in the first place.

2) Doesn't this mess up the distance calculation? If the light is redshifted, then the color would be different, so in order to get the distance from Earth, we'd first have to "de-shift" the light once we know what it is, right?

I'm asking about both because they are related topics and I want to make sure a misconception on distance isn't confusing me about speed. If someone could clear up both for me, I would appreciate it.

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Redshift is gotten from spectroscopic analysis. The idea is that every element absorbs light at a specific frequency - see for example the Wikipedia article on the "Hydrogen line", which is an absorption at 21 cm. However this 21 cm applies only in the laboratory frame. If the star is moving relative to us, then we won't actually measure 21 cm. Instead we'll get some other wavelength value which we can use to calculate the redshift/blueshift from the definition.

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  • $\begingroup$ So that requires us to know the star's elemental makeup, right? How do we determine that? $\endgroup$ – TechnoSam Jun 24 at 13:21
  • $\begingroup$ 75% of the universe is hydrogen, and it's the major component of stars as well. After identifying the hydrogen line (which is going to be the biggest absorption line) everything else should fall into place. $\endgroup$ – Allure Jun 24 at 21:21

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