Audio Guide：

Galaxies are extremely far away. This makes it exceedingly difficult to measure their distance. Astronomers usually apply different measuring techniques to verify the results and achieve the most accurate measurements. There are two most commonly used methods. For nearer galaxies, length can be measured based on the relationship between the light variation period of Cepheid variables and the absolute magnitude. For galaxies that are farther away, their distances can be gauged by the observed brightness of type la supernovas.