This season is full of stories about stars guiding travelers to far-off destinations, but imagery from NASA's archive shows off a stellar guide of a different sort. The central point of this holiday "wreath" is a bright star known as RS Puppis, or RS Pup for short. RS Pup is what's called a Cepheid variable: a star that periodically dims and gets brighter. The change in luminosity is due to instability—these are large, short-lived stars that are already nearing the end of their chemical fuel reserves, and don't maintain steady transfers of temperature from layer to layer. Astrophysicists can use the variability of these stars' light to calculate how far away they are (6,500 light-years, in this case), allowing them to serve as a sort of cosmic yardstick.