Same thing with blueshift, if something is moving towards us, then the spectral lines will move towards the blue end of the spectrum. All it means is that if something is moving away from us, then its spectral lines will move towards the red end of the spectrum. What do those things mean though? What does it mean for something to be redshifted or blueshifted? Conversely, any source moving towards us will have its spectral lines blueshifted. We also know that any source moving away from us will have its spectral lines redshifted. In other words, its stationary relative to Earth. This is because these are the spectral lines that we would expect from a source that’s not moving at all relative to Earth. Therefore, the spectral lines that we find from the laboratory source are in a way our reference spectral lights. Now, since the laboratory source is on Earth, that means that it cannot be moving relative to Earth because it’s on Earth. So that obviously has to be back here on Earth. As well as this, we’ve got the same spectral lines from a laboratory source. So we can see that we’ve got spectral lines from two galaxies in the diagram: the labelled Galaxy A and Galaxy B. How is Galaxy A moving relative to Earth? How is Galaxy B moving relative to Earth? Which galaxy is moving faster relative to Earth? The diagram shows the spectral lines in the light from two galaxies as well as the same spectral lines in the light from a laboratory source.
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