Let's say that a particular emission line of helium is originally emitted with an original wavelength of 505.5 nm from a star traveling quickly across our galaxy. Using a research telescope with an attached spectrometer, we observe and measure the wavelength of this emission line coming from the star to be 506.1 nm. Based on these numbers, how fast is this star moving toward or away from us here on Earth? (see example of this type of question in lecture slides or in textbook)
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