Unlike exoplanets and binary systems, Cepheid variable stars actually change in brightness over time, rather than appearing to because of objects blocking light coming from the star(s).

Cepheids pulse in size (and brightness) in a very regular pattern, which takes longer to complete the bigger they are. By watching how long it takes to go from bright to dim to bright again you can determine how bright a Cepheid star really is. By comparing that to how bright a Cepheid star looks in the sky you can figure out how far away it actually is, in the same way you can figure out how far away a streetlight is on the highway…the farther it is the dimmer it looks. Astronomers used Cepheid variable stars to determine distances to galaxies for the first time, when no one really understood just how far away they were. NASA’s Hubble Space Telescope still performs distance measurements using Cepheid stars today, in order to more precisely measure the rate at which our universe is expanding.