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EXPLAIN HOW CEPHEID VARIABLE STARS ARE USED TO DETERMINE GALACTIC DISTANCES.
What is THE OUTER LAYERS OF THE STAR UNDERGO PERIODIC EXPANSION AND CONTRACTION WHICH PRODUCES A PERIODIC VARIATION IN LUMINOSITY. BY MEASURING THE PERIOD, THE LUMINOSITY CAN BE FOUND; THEN BY MEASURE ITS APPARENT BRIGHTNESS, ITS DISTANCE FROM EARTH CAN BE FOUND.