If you had been alive from about the mid 17th century to the middle part of the 20th century, and someone talked about a computer, you wouldn’t think of a mechanical device that did calculations. You would have thought of an actual person. Henrietta Swan Leavitt was one of these people. She was hired to be a human-computer, so let’s start with a little background.
First, Henrietta Leavitt was born on July 4th, 1868. She graduated from Radcliffe College in 1892, although it was called the Society for the Collegiate Instruction of Women at that time. Being a smart, educated woman at this time was of little use because they were not accepted into the workforce. Most women were expected to stay home and raise the children.
When she was around twenty-five years of age, Leavitt volunteered for a position at Harvard Observatory in 1895. As a human-computer, her job was to use her brain to make very complex observations and mathematical calculations and report the data back to her supervisors.
In 1902, she was offered a real job at the Harvard Observatory, although the pay was low. Working under the famed astronomer Edward Pickering, her a very tedious job. Her assignment was to go through many thousands of glass plates (photos) of the sky and compare them. What Leavitt was looking for was pulsating stars, also known as variable stars. A star that would get brighter and dimmer, brighter and dimmer. In particular, she was assigned to look in the Magellanic Clouds, which are two neighborhood galaxies of the Milky Way.
She would take one image of the sky and compare it with a picture that was taken at another time on a different night in the same area of the sky. One was a positive image and the other was a negative image of the sky. On one slide the stars would appear as bright spots, while they would appear as dark spots on the other. She would carefully lay one slide on top of the other and, once she had the two plates lined up, she would then use a hand lens to magnify the image to see each individual star.
If nothing was changing on the star between the two nights, then the star would just become blackened out. She wouldn’t see anything. On the other hand, if the star was pulsating or getting brighter and dimmer, she would see the dark spot of the negative slide with a halo around it. This would indicate that the star was blinking, getting brighter and dimmer over a period.
She would then look at additional slides taken on different nights over a period of a year or so. Leavitt would attempt to get an idea of where the star was, what its period was, and collect as much information as possible.
Yet, she worked on this project for more than ten years. Day after day, night after night, spending endless hours looking at slide after slide after slide, picture after picture of plate after plate. Imagine how tedious the work must have been.
After cataloging all these variable stars, in 1912 Edward Pickering published a paper at Harvard giving full credit to Levitt for detailing her observation of 1777 of these variable stars in the Magellanic Clouds. Every one of these stars waxed and waned like slow motion beacons in the Sky. Some took days, some took weeks, some took months to go through their cycle.
Overall, there is nothing really shocking about this until you got to the very end of the paper, at which point he discusses the nature of twenty-five of these particular stars. These were a certain type known as Cepheid variable stars and what she noticed was that there is a relationship between the period of their brightness and how bright they were. What this means is the brighter the Cepheid variable star was, the longer its period was. In other words, brighter stars were observed to blink more slowly.
This was a revolutionary observation because at this time no one had a way of measuring the distance to very distant stars. They could measure some of the closer stars, but even that wasn’t a very accurate science. Yet, no one really knew how far away the more distant stars were. Neither the brightness nor the size would be of any help here. For example, you can have a large, bright star that’s so far away that it will appear to be very small and dim.
And here’s where her true genius came in: Leavitt realized that all the stars in the Magellanic Clouds must be approximately the same distance from the Earth. So, if you can determine the distance to one of these seven variable stars in the Magellanic Clouds, then you can use that as a key to figuring out the distance to every other variable separate star anywhere in the universe.
All one would need to do is locate a Cepheid variable somewhere out there in our great expanse of the universe and measure its pulse. Observe how long it takes to dim and brighten. Once you have that information, you now know how bright the star really is.
Brightness drops off exponentially with distance. So, if you know how bright the star should be and it appears four times dimmer, then you know it’s twice as far away from you. It appears 9 times dimmer then the star is three times farther from you, and so on.
The real trick, however, was determining how far away are any of these Cepheid variables were. That would be the job of other scientists to figure out. Once Pickering’s paper was published, it didn’t take long for other astronomers to hop on the bandwagon. They wanted to quickly find out how far out is it to one of the Cepheid variables.
Once you determine the distance to one, then you’ve unlocked the key to the entire universe. And it didn’t take long. Just shortly after her discovery, Ejnar Hertzsprung of the famed Hertzsprung Russell diagram and another astronomer named Harlow Shapley, both went to work on this problem. It was soon determined that the Magellanic Clouds were about 100,000 light-years from us.
To give you an idea of how important this was, prior to Henrietta Swan’s discovery, astronomers could estimate the distance to a star approximately 100 light-years out into space. With Leavitt’s new discovery, could now estimate distances up to ten million light-years out into space.
Sadly, Levitt became ill shortly after this discovery and died of cancer on December 12, 1921. It has been reported in the literature that she was nominated for a Nobel Prize in 1925, but that’s not true. It was suggested that she should be awarded the Nobel Prize, but she was deceased and therefore not eligible.
Useless? Useful? I’ll leave that for you to decide.