Henrietta Leavitt
How do you measure the distance to a star?
That time, that place
At the Harvard Observatory a group of women worked as computers, examining photographic plates one by one and recording the positions and brightnesses of stars. They were paid far less than male assistants and were not permitted to use the telescopes.
Henrietta Leavitt was one of them, and had lost most of her hearing while still young.
Why this question
Her assignment was to find and catalogue stars whose brightness varies periodically on plates of the Small Magellanic Cloud. It was tedious work, and she found 1,777 of them.
Then a pattern surfaced: the brighter the star, the longer its cycle from bright to dim and back.
What was found
What she recognized here was decisive.
The stars in the Small Magellanic Cloud all lie at nearly the same distance from us. So differences in apparent brightness are differences in true brightness.
Measure the period, then, and you know how bright the star actually is. Compare true brightness against apparent brightness and you have the distance, since farther means fainter.
A ruler had been laid against the universe. Until then, distances could be measured only to a handful of very nearby stars.
There was no way to measure the distance to a far star
The period-brightness relation revealed among stars all at one distance
Hubble would use her method to measure the distance to Andromeda. Leavitt died of cancer in 1921; four years later a Swedish scholar seeking to nominate her for the Nobel learned she was already gone