American astronomer Henrietta Swan Leavitt (1868–1921) worked at the Harvard College Observatory in Cambridge, Massachusetts. In 1904, Leavitt noticed that a particular star in the constellation Cepheus would regularly change its brightness. Careful study showed that the star varied its brightness in a predictable, “saw-tooth” pattern. Eventually, other variable stars with this same saw-tooth pattern were found and were named Cepheid variables after the first star of this type that was discovered.

In 1913 Leavitt and the Danish astronomer Ejnar Hertzsprung (1873–1967) worked together to deduce that Cepheid variables varied in a very specific way: The time it takes a Cepheid variable to go through one cycle of brightness variation—its period—is mathematically related to the peak luminosity of the star. This kind of “period-luminosity relation” meant that it was possible to use Cepheid variables as standard candles: To know the luminosity of a Cepheid, just measure its period of variability, and from that, the distance to the star or the object it is in.