That's really not the answer to the question. That's just "what is red shift"... I would guess that the answer is something to do with the fact that because our galaxy is spinning, some stars are moving towards us, and some away from us. Or at least, some stars are moving away from us faster than others. Therefore some stars will be more red-shifted than others. Some might be blue-shifted if we are spinning fast enough, but I don't know if that ever happens. Or on a long enough timespan, you could observe one particular star, and see minute changes the amount of red-shift that star exhibits. But I think you'd have to watch that one star for millions of years for that method. Anyway, it's a possibility (Y)