what is red shift and how do we know about it?
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Light from distant stars is shifted in the red direction (lower frequency = longer wavelength, as opposed to blue shift to a higher frequency). We know this by observing fine grain structures in the light they emit. For example, sodium radiates at two specific frequencies which are close to each other (588.9950 and 589.5924 nanometers). When you observe a pair of identical magnitude at 600nm and 600.5974nm you know you’re seeing sodium, but red shifted about 11nm.
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