(Great Minds intro music)

How far away is the Andromeda galaxy? What's, what's the diameter of the Milky Way? How do we even know that the Universe is expanding?

These are questions that give me a headache. But, we can answer them, because we can measure the distance to stars. Astronomers do it all the time in a bunch of different ways, but we first learned how to do it only a hundred years ago from a great mind who is as unrecognized now as she was in her own time. Henrietta Swan Leavitt. Who was born in Massachusetts in 1868 and attended what would be known as Radcliffe College, where she studied and excelled at astronomy.

But after graduating in 1893, she couldn't turn her skill into a paying career, so she give it away from free. She volunteered at Harvard College Observatory working in an office full of women who were known as...computers - catalogers who sorted, analyzed, and classified hundreds of thousands of photographs of the sky taken all over the world.

It was tedious work, but, for many women astronomers, it was the only work available. Women couldn't direct their own research at the time, and they weren't even allowed to operate Harvard's telescopes. 

In fact, women were only allowed to serve as computers because the Observatory couldn't find male scientists willing to do the work.

So, Harvard's "computer office" became a hothouse of some of America's greatest, if least known, astronomical minds. One of Leavitt's colleagues, for instance, was Annie Jump Cannon, who devised the spectral class system we still use to classify stars today.

Leavitt, however, focused on stars in a cluster called the Small Magellanic Cloud. She was tasked with cataloguing a class of stars known as Cepheid variables - stars that go through cycles of varying brightness over regular periods of time.

Through her computing, Leavitt identified 2,400 new Cepheids, doubling the number known to science. But more importantly, in 1912, she made a discovery that would revolutionize astronomy.

She found that the largest Cepheid stars in the cloud also have the longest period of peak luminosity: the bigger they were, the longer they were at their brightest. The correlation was so precise that Leavitt could measure a star size and immediately calculated its true luminosity.

Now this was a huge deal, because until then, there was no way of knowing whether a dim looking star was dim because it was just far away, or because it was actually dim. But since the stars she studied were all about the same distance from Earth, she had discovered a kind of yardstick.

You can pick a variable star anywhere in the sky, figure out its true luminosity using Leavitt's techniques and use that to calculate its distance.

This changed astronomy forever.

Before Leavitt, astronomers could only measure stars about a hundred light-years away. Thanks to her discovery, what is now known as Leavitt's Law was used to measure Cepheid stars millions of light years away.

It helped settle the debate about whether other galaxies existed beyond our own, and it was ultimately used by Edwin Hubble to determine that the Universe was expanding.

Of course Leavitt couldn't do any of this stuff herself, because she wasn't allowed to touch the menfolk's telescopes. But Hubble later said that Leavitt deserved a Nobel Prize for her work, and he wasn't alone. In 1923, Swedish physicists asked the Harvard Observatory for Leavitt's research to nominate her for the Nobel Prize in Physics. But, unfortunately, Leavitt had died of cancer three years earlier at the age of 53. And though women can get Nobel Prizes now, dead people cannot. That's just one of their rules.

So thank you to Henrietta Leavitt and all the other human computers who didn't get space telescopes named after them for helping us understand how big and how awesome the Universe is.

Thank you for watching this episode of scishow Great Minds. If you have other suggestions for great minds you'd like us to cover in the series, please leave them in the comments below. We're also on Facebook and Twitter, and if you wanna keep getting smarter with us here at scishow, you can go to http://youtube.com/scishow. And subscribe.