Passage:
In 1912, astronomer Henrietta Swan Leavitt published an analysis of 25 variable stars located within the Small Magellanic Cloud, establishing a mathematical correlation between the stars' pulsation periods and their brightness. Prior to Leavitt’s findings, astronomical distance measurement relied primarily on trigonometric parallax, a geometric method restricted to stellar bodies within a few hundred light-years of Earth. Leavitt recognized that because all stars in the Small Magellanic Cloud were situated at roughly the same distance from Earth, differences in their apparent magnitude directly reflected differences in their intrinsic luminosity. By plotting the logarithmic periods of these Cepheid variables against their observed brightness, she identified a linear relationship: stars with longer pulsation cycles were consistently brighter than those with shorter cycles. While Leavitt did not determine the absolute physical distances to these stars—as the baseline distance to the cloud remained uncalibrated until Ejnar Hertzsprung measured nearby Cepheids in 1913—her period-luminosity relation provided the foundational standard candle later utilized to measure extra-galactic distances.
Statement: According to the passage, Leavitt determined that differences in the apparent brightness of Cepheid variables in the Small Magellanic Cloud corresponded directly to differences in intrinsic luminosity because all stars in the cloud were located at roughly equivalent distances from Earth.
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