Cefeids are a type of variable star that pulsate in brightness over a regular cycle. These stars expand and contract, causing their luminosity to change, which can be observed from Earth. The period of their pulsation is directly related to their intrinsic brightness, making them useful as standard candles in astronomy. Astronomers use Cefeids to measure distances in the universe. By comparing the observed brightness of a Cefeid to its known intrinsic brightness, they can determine how far away it is. This method has been crucial in establishing the scale of the universe and understanding the expansion of the universe, including the work of Edwin Hubble.