What profound relationship did Henrietta Swan Leavitt discover while studying the Magellanic Clouds?

Answer

The Period-Luminosity Relationship

Leavitt discovered the Period-Luminosity Relationship, linking the time it took for a Cepheid variable to cycle (period) to its true average brightness (absolute magnitude).

What profound relationship did Henrietta Swan Leavitt discover while studying the Magellanic Clouds?
measurementgalaxyAstronomerDistancecalculation