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We need to know two of the following three to calculate the third: redshifted color, baseline color, and velocity. The velocity is related to the difference between the redshifted color and the objects baseline color. How do we know the baseline color of distant objects to know the amount of redshifting?

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How do we know the baseline color of distant objects to know the amount of redshifting?

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From Wiki:

To determine the redshift, one searches for features in the spectrum such as absorption lines, emission lines, or other variations in light intensity. If found, these features can be compared with known features in the spectrum of various chemical compounds found in experiments where that compound is located on earth. A very common atomic element in space is hydrogen. The spectrum of originally featureless light shone through hydrogen will show a signature spectrum specific to hydrogen that has features at regular intervals. If restricted to absorption lines it would look similar to the illustration (top right). If the same pattern of intervals is seen in an observed spectrum from a distant source but occurring at shifted wavelengths, it can be identified as hydrogen too. If the same spectral line is identified in both spectra but at different wavelengths then the redshift can be calculated

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Most of space is made up of Hydrogen. Hydrogen has distinct emission and absorption lines that correspond to electron orbital transitions. The 2 -> 3 line at 656 nm (so orange / reddish) corresponds to an electron going from a state with principal quantum number two to three. This will be the same everywhere (except perhaps near black holes) so there you have your baseline.

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