Cepheid variable stars are a special type of star that pulsate in brightness over a regular period. These stars expand and contract, causing their light to vary in intensity. The cycle can last from a few days to several weeks, making them easy to observe. Astronomers use Cepheid variables as "standard candles" to measure distances in the universe. By knowing the relationship between a Cepheid's pulsation period and its brightness, scientists can determine how far away it is. This helps us understand the scale of the universe and the location of galaxies, including our own Milky Way.