A particularly bright kind of star acts as a beacon for measuring the distance to the near galaxies. These stars are called ‘Cepheid variables’, which change in brightness in the regular way.
Astronomers cannot measure the time it takes a Cepheid to flash from maximum brightness, down to minimum, and back to maximum. This time is called its period. Brightness Cepheid flash more slowly than fainter ones, so once Cepheid period has been established, its brightness can be deduced. If a Cepheid is found with a period of two weeks, for example astronomers can say that it is 4000 times brighter than the sun. By investigating the apparent dimness of Cepheid in distant galaxies they can tell how far away the galaxies life.
Picture Credit : Google