Because of its precision, the Cepheid-variable method of correlating the luminosity of Cepheid stars with those of known types of ordinary stars is one of the most important tools that astronomers have for ascertaining distances to Line proximate galaxies. Fortunately, Cepheids are luminous enough to be observed (5) in other galaxies, not just in our own, such that scientists have established a benchmark for their true brightness. One first observes a Cepheid, notes the time span for its luminance to vary, and plots that information on an already established graph to derive its intrinsic luminosity. Comparing intrinsic luminosity with apparent brightness allows one to calculate the star's distance (10) as long as the period-luminosity relation for the star in question is first corrected for gravitational interference, parallax, and experimental error. Data gathered by this process would suggest that the universe is expanding, but the method is less reliable in the nether reaches of the universe, where the crucial evidence for cosmic expansion would likely be spotted. The passage is primarily concerned with