Women in Astronomy: Henrietta Swan Leavitt

B/W photo of Henrietta Swan Leavitt at workHow big is everything? How much everything even is there? We have answers to these questions because of Henrietta Swan Leavitt, one of my favorite astronomers. Leavitt passed away on this day, December 12, in 1921, a too-young 53, taken by stomach cancer. 

Leavitt was one of the computers at the Harvard College Observatory. Before adopting the name to describe machines that crunch numbers, computers were low-level staff, often women, who did the complicated mathematics needed for scientific research, engineering projects, financial applications, and any other endeavor that relied on accurate and complicated mathematics. Women were often preferred in this position because a) they cost far less to employ than men, and b) they were considered well-suited to the repetitious work.  

Leavitt’s task at the observatory was to catalog variable stars from the photographic plates taken by Harvard’s satellite observatory in Peru. Each glass plate recorded dozens or even hundreds of stars, which computers would pore over under high magnification, identifying each star, planet, and other phenomenon for use by researchers. Among these stars were variables, stars whose brightness changes over time. 

Stars can be variable for a number of reasons. One kind of variable star is actually two stars, a binary system. From Earth, the star appears to fluctuate in brightness as each star passes in front of the other. Another kind of variable, the kind that becomes important here, are Cepheid variables. Cepheids are single stars that grow as they get hotter as they grow they cool, which causes them to sink back. Kind of like oatmeal on the stove, getting larger as steam builds up and then falling back as the steam pops through the surface. Cepheids go through this hotter/colder cycle on an extremely regular schedule (called its “period”), growing brighter as the star expands and then darker when it cools and recedes. 

What Leavitt noticed as she catalogued thousands of variables is that not only was the period of Cepheid stars remarkably stable, but that the faster the period, the brighter the star. Comparing data from many stars, she found that all Cepheids with the same period were the same brightness. This is important because it means that Cepheid variables could be used as a “standard candle”, something in space whose brightness can be absolutely determined. Since light falls away as it travels through space at a constant rate (proportional to the square of its distance), if you know the period of a Cepheid, you can easily determine its distance. 

At the time, the only way to measure distance in space was to very precisely measure the angle it made when viewed from one side of the Earth’s orbit and the other, 6 months later. If you knew the diameter of the Earth’s orbit and the angle made on each side of the orbit, you could triangulate the distance to the star. This has drawbacks: first, it demands precision that even today is difficult to achieve, and after a few hundred light years, the angles are far too small to be measured at all; second, the width of the Earth’s orbit was barely known, having only been accurately established at the end of the 19th century.  

Leavitt’s work meant that you could measure the distance to anything, if you could find a Cepheid variable (which aren’t common but not incredibly rare either). In 1924, a couple years after Leavitt’s untimely death, Edwin Leavitt applied her work (which has become known as “Leavitt’s Law) to determining the distance to Cepheid’s found in what was then known as the Andromeda Nebula. Hubble showed that the nebula (which we now know is some 2.5 million light-years away) was far too distant to be part of our own galaxy, that indeed, it was a whole other galaxy in its own right.  

Up until this point, astronomers had differed sharply on the extent of the universe, with many believing that the Milky Way galaxy, or home, was the entirety of the universe, and that what we today know are distant galaxies were in fact “spiral nebulae” within the Milky Way. Hubble’s application of Leavitt’s Law shattered that view, placing the Milky Way as just one of millions of galaxies.  

Leavitt continued at the Harvard College Observatory off and on until her death. She had independent means and often took long leaves, increasing as she battled her illness in the last part of her life. But she continued to make advances in astronomy after her contribution on variable stars; among other things, she developed the brightness scale for categorizing stars which was adopted as the standard in 1913. She never taught and was little recognized in her own life, but has had an asteroid, a moon crater, and a telescope at Texas’ McDonald Observatory named after her. Hubble insisted she should have won a Nobel Prize for her work on Cepheid variables, and indeed another astronomer tried to nominate her in 1926, only to find that she had passed away and thus was not eligible (Nobel Prizes are not awarded posthumously). 

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