The only thing we know of so far that can shift light to longer wavelengths is the "Doppler" effect. If the source and the observer are moving apart, then the observer sees wavelengths that are longer than they should be. If the source and the observer are moving toward each other, then the observer sees wavelengths that are shorter than they should be. It works for ANY wave ... sound, light, water etc. The trick is to know what the wavelength SHOULD be. If you know that, then you can tell whether you and the source are moving together or apart, and you can even tell how fast. If the lines in a star"s spectrum are at wavelengths that are too long, then from everything we know right now, the star and Earth are moving apart.
