The knowing man!

Three If you let the light of any star through the prism, you will get a rainbow. If you make a rainbow large and high-quality (good optics) and look at it through a microscope, it will become apparent that it is not a single spot, smoothly flowing flowing flowers of red-orange-yellow-...", but consists of many strips with spaces between them. These bands, their width and size of spaces depend on what a particular star is made of, what is shining there and in what proportions - hydrogen, helium and what else. Each matter, if heated so that it begins to shine, has its "barcode" (spectra) of these strips, they are always the same in all parts of the universe. When mixing substances, the stripes are simply applied to each other. If you look at which stripes are obtained from the light of a distant star through the prism, then you can understand what it consists of (this is called spectroscopy). Well, if it flies away from us, then, for example, the barcode of hydrogen (and it is in any star) will be visible the same, but it will be slightly red. When you measure how much, you can calculate the speed.