Dmitry Shteynbuk — How Henrietta Leavitt Measured the Universe
In 1912, a woman working as a "computer" at Harvard found the pattern that made the extragalactic distance scale possible.
In the early 1900s, the Harvard College Observatory employed a small army of women — the so-called "Harvard Computers" — to catalog and measure the brightness of stars from photographic plates. They were paid about 25 cents an hour. One of them, Henrietta Swan Leavitt, was assigned the tedious job of examining photographs of the Small Magellanic Cloud (SMC) and marking every variable star she could find. She found roughly 1,700.
Most variable stars are erratic. But a subset of them — Cepheid variables — pulse in a very regular way, brightening and dimming over periods ranging from a day to about 100 days. Because all the Cepheids in the SMC are at approximately the same distance from Earth (the SMC is much smaller than its distance to us), Leavitt could compare their apparent brightnesses directly, without worrying that a bright-looking one was just closer.
In 1912 she published a two-page paper noting a startling regularity: the longer a Cepheid's period, the brighter it intrinsically was. This is the period–luminosity relation. Measure a Cepheid's period, and you know its true luminosity. Compare true luminosity to apparent brightness, and you get distance. It is the closest thing astronomy has to a universal ruler.
Within a decade, the relation was being applied to Cepheids in other galaxies. Edwin Hubble, using the 100-inch telescope at Mount Wilson, identified Cepheids in what was then called the "Andromeda Nebula" and calculated that it lay hundreds of thousands of parsecs away — far outside the Milky Way. Andromeda was not a nebula. It was another galaxy. In one measurement, the size of the known universe grew by orders of magnitude.
Leavitt never received the credit her work deserved during her lifetime. She died of cancer in 1921 at 53, and a member of the Swedish Academy tried to nominate her for the Nobel Prize three years later, only to learn she was ineligible because she had died. Her period–luminosity relation is still the anchor of the extragalactic distance ladder. Every value of the Hubble constant published since — every measurement of the expansion rate of the universe — traces back to those 1,700 dots on a photographic plate.