Measuring The Universe
Around two years ago, my dad recommended a book to me called “Miss Leavitt’s Stars” by George Johnson. The story details the life of Henrietta Swan Leavitt, a 20th-century American astronomer working with notable astronomer Edward C. Pickering at the Harvard Observatory.
The story stayed with me for a while not only because it’s about the work of a woman in science, but because it was about a discovery we all take for granted. When NASA released photos of the galaxy M87 black hole, media sources used statistics like “55 million light-years away”, as if the magnitude of the distance was analogous to how important the milestone was. But where do those numbers come from? I’ve always wondered what a light-year actually is. I asked someone when I was younger and was told to imagine a field with ~many~ blades of grass. If each of those blades was a mile, the whole field was a light year.
But is that big? I still don’t think I know. It’s hard to convert an understanding of volume to an understanding of distance. My lack of measurement-related imagination aside, where do we get these numbers from? How can astronomers definitively say that an object is 55 million years away? How do we measure the universe?
In the early 1800s, the answer involved a concept called parallax. To understand parallax, you can hold your finger a few inches away from your face and close one eye. Now open it, and close the other. Your finger seems to change position, allowing us to measure the angles between your eyes and your finger. We can easily measure the distance between your eyes, so simply by using trigonometry, we can also find how far away your finger is. This system can easily be scaled up, replacing the distance between your eyes with the Earth’s orbit. At different times in the year, an astronomer can chart the location of a close-by celestial object, and easily calculate the distance.
But what if it’s not just your finger you’re looking at? A flagpole many yards away might not seem to change position at all when you blink your eyes, and there is no longer a reliable way to predict how far away it is. Astronomers of the 19th century soon realized that when measuring the distances of various stars, other factors could be taken into account, namely, brightness.
This is best explained in an example involving streetlights. The brighter a light is, the closer it likely is to you. The dimmer, the farther away. It seems foolproof, but once again, there’s a catch. What if something far away from you is very bright (ex. the sun), and something very close to you is very dim (ex. a candle)? This method alone no longer proves useful.
Astronomers in the 19th century refused to forget about using brightness to measure the universe–it just seemed too good to give up on. Nonetheless, the difficulty with comparing very bright and very dim objects was proving nearly inescapable.
In 1908, Henrietta Swan Leavitt, our Harvard astronomer, discovered something that would change everything. She worked with Cepheid variables, types of stars that vary their brightness over predictable amounts of time. By keeping track of many stars along with their apparent luminosities, she was able to determine that the brighter a Cepheid appeared to be, the longer the period of brightness variation was. The relationship was remarkably simple and could be plugged into modular equations to serve as the yardstick that could measure relative celestial distance.
“A straight line can readily be drawn among each of the two series of points corresponding to the maxima and minima, thus showing that there is a simple relation between the brightness of the variables and their periods.”
Henrietta Swan Leavitt
But the yardstick still had to be calibrated. We didn’t just want to know that one object is farther away than another — we needed numeric, quantitative, distance. Leavitt tracked hundreds of Cepheid variables in the Small Magellanic Cloud, a dwarf galaxy near the milky way, and determined their periods and apparent brightnesses.
Because the Magellanic Cloud is so small and so close to us, nearly all the Cepheid variables there could be assumed to be the same distance away from us, and astronomy techniques of the day were sufficient to measure this distance. Astronomers Edwin Hubble and Harlow Shapley completed the “calibration” by precisely measuring the distance between Earth and the SMC, opening millions of square light-years of the universe to measurement. This technique has been used ever since, and we owe numbers like “55 million light-years” to Henrietta Swan Leavitt.
“She deserved the Nobel Prize for her work.”
Edwin Hubble
Edwin Hubble, among others, remarked that Leavitt deserved a Nobel Prize for her calculations, and yet she never received one. For years, it was much more common for Hubble’s name to be associated with the Cepheid work. At the time, women in astronomy were not often leaders of observatory research projects. Their work often involved tracking the paths of asteroids and stars, a task often looked down upon at the time.
Leavitt broke the stigma. By using such a “trivial” task to accomplish greatness, she broke expectations around what astronomical work was considered valuable. She also proved, as many women have throughout history, that skill and ambition can outshine prescribed limitations. If she believed in her ideas, it didn’t matter if anyone else did. The work would start regardless, and the others would come on board eventually.
Leavitt’s story is important to me because it’s about a woman’s drive to do work she found interesting. It bothers me that the work was so readily miscredited, even by Leavitt’s coworkers at the observatory, but I see evidence of positive change all the time now. For my eighteenth birthday, I attended a play about Leavitt’s work. I’ve read a book about her, and a few years ago I saw a mug at a gift shop with her portrait on it.
People remember Henrietta and her contributions to astronomy. We celebrate her dedication when her peers didn’t always do so. I feel strangely connected to Ms Leavitt sometimes, especially when I’m working through difficult math or physics problems. I solve them for her. She paved a way for women like me, so I must do my best in her honor.
Sources
- Miss Leavitt’s Stars by George Johnson
- https://www.britannica.com/biography/Henrietta-Swan-Leavitt
