Influence
The period-luminosity relationship for Cepheids made them the first "standard candle" in astronomy, allowing scientists to compute the distances to galaxies too remote for stellar parallax observations to be useful. One year after Leavitt reported her results, Ejnar Hertzsprung determined the distance of several Cepheids in the Milky Way, and with this calibration the distance to any Cepheid could be accurately determined.
Cepheids were soon detected in other galaxies, such as Andromeda (notably by Edwin Hubble in 1923–24), and they became an important part of the evidence that "spiral nebulae" are actually independent galaxies located far outside of our own Milky Way. Thus, Leavitt's discovery helped to settled the "Great Debate" on whether the Universe was larger than the Milky Way, and thus changed our picture of the Universe forever.
The accomplishments of the American astronomer Edwin Hubble, who established that the Universe is expanding, were also made possible by Leavitt's groundbreaking research. "If Henrietta Leavitt had provided the key to determine the size of the cosmos, then it was Edwin Powell Hubble who inserted it in the lock and provided the observations that allowed it to be turned," wrote David H. and Matthew D.H. Clark in their book Measuring the Cosmos. To his credit, Hubble himself often said that Leavitt deserved the Nobel Prize for her work. Gösta Mittag-Leffler of the Swedish Academy of Sciences tried to nominate her for that prize in 1924, only to learn that she had died of cancer three years earlier (the Nobel Prize is not awarded posthumously).
Read more about this topic: Henrietta Swan Leavitt
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