The brighter the star is overall, the longer the period of luminosity. Since the cepheids ... Leavitt published her findings in 1912 -- in a chart of 25 cepheid periods and their apparent brightness.

Previous studies that used the Hertzsprung-Russell diagram — a tried-and-true chart that traces a star's age through its luminosity and temperature — placed the star at about 8 billion years old.