He applied the period-luminosity relation to Cepheid variables.

American astronomer, Henrietta Swan Leavitt is renowned for her discovery of the period-luminosity relation of Cepheid variables.

Cepheids have a certain relationship between their period, and their absolute luminosity. Thus, their absolute luminosity can be determined. Comparing this with their apparent luminosity allows us to calculate their distance.

Cepheids have a certain relationship between their period, and their absolute luminosity. Thus, their absolute luminosity can be determined. Comparing this with their apparent luminosity allows us to calculate their distance.

Cepheids have a certain relationship between their period, and their absolute luminosity. Thus, their absolute luminosity can be determined. Comparing this with their apparent luminosity allows us to calculate their distance.

Cepheids have a certain relationship between their period, and their absolute luminosity. Thus, their absolute luminosity can be determined. Comparing this with their apparent luminosity allows us to calculate their distance.

Main sequence stars

The mass/luminosity relation is important because it can be used to find the distance to binary systems which are too far for normal parallax measurements.