 Like Polaris, Delta Siphi is a binary star system and a Siphiid variable star. Siphiid stars undergo periodic changes in luminosity. Delta Siphi is among the closest stars of this type of variable with only Polaris being closer. Most stars have some variability in their luminosity. Even our Sun varies on an 11-year cycle of sunspots, but Delta Siphi's variability is caused by regular pulsation in the outer layer of the star. Here's its light curve showing luminosity changes over time. The pattern is quite regular. Early in the 1900s Henrietta Levitt thought to plot Siphiid luminosity cycle periods against luminosity. She found that the period of these stars varied in proportion to their absolute brightness. This was very interesting because as we have discussed once we know the intrinsic luminosity of a star we can easily calculate its distance. Levitt's discovery made Siphiid stars true standard candles and changed the history of astronomy. Rr Lyra is a variable star like Delta Siphi. As the brightest star in its class it became the namesake for the Rr Lyra variable class of stars. The relationship between pulsation period and absolute magnitude of the Rr Lyras make them good standard candles. They are not as bright as Siphiid variables, but there are a lot more of them. They are extensively used in globular cluster studies, including the studies that helped us understand the form and size of our Milky Way galaxy.