03/08/2025
In light of it being international womens day I would like to highlight the work of Henrietta Swan Leavitt. Astronomer at Harvard and Oberlin college
Edwin hubble is someone most people into astronomy can tell you who he is and what he did. For the uninitiated his discovery and measurement of a cepheid variable star in the Andromeda galaxy, proved that it existed outside of the milky way and was its own "island univerise".
He had for years argued that the spiral nebulae we were observing, could be resolved into individual stars, and suggested that they existed outside of our galaxy. It was at that time, "understood", that the entirety of the observable universe, was entirely within the milky way. We had developed the ability to calculate distances but to a limited degree, but only enough to roughly estimate the size of the galaxy. The leading estimate at that point was some 300,000 light years. Well above current estimates, but broadly speaking, in the ball park.
So why was that cepheid variable star so important?
In the 1780's, it was discovered that there was a certain type of star that pulsated at regular intervals. What that interval was varied, but what ever it was, it followed that interval tightly. Both in brightness and size if its 10 days or 2 days it follows the same timing.
Henrietta Leavitt was from Lancaster mass and studied at Oberlin college and Harvard, earning her bachelors at Harvard. She worked towards a graduate degree from Harvard College Observatory, but illness kept her from completing it. After college she joined the staff at the observatory and was paid adjusted for todays inflation, $10 an hour for her work. She was assigned the task of identifying these variable stars that existed in the Small and Large Magellanic cloud catalog them and, to research them. While she never had access to make observations of her own, because of Harvards policy against allowing women to touch the telescopes, she did have access to their library of photographic plates. She recorded nearly 1800 of these variables.
in 1912 she released a paper that clearly demonstrated that the total brightness of these variables was extremely similar if their timing interval was the same. And because these stars were all in the Magellanic clouds that meant they were all roughly of equal distance to us. Meaning all we would need is to measure it at its brightest. We would have a way to measure how far away it was because if it was dimmer that would have to be because it was further away.
Suddenly, the ability to measure space distances came down to recording the interval of one of these cepheid variables. Find one within distance for us to parallax measure from earth to calibrate the system and boom, the yard stick we had to measure truly great distances became much much larger.
By finding one of these variables in andromeda hubble used this system of measurement to prove that the cepheid variable star he had been studying had to of been at least 1 million light years away. Far far beyond even the largest estimate of the size of the milky way. Showing that these thousands of spiral nebulae they were seeing, all were well outside of the milky way. Hubble, in effect, had proven the existence of a truly massive universe.
And it was because Leavitt all on her own, that made that discovery and did that for science. For 10 dollars an hour, with observations that weren't hers because she wasn't even allowed to make them.
And in a little change of pace I would like to point out that Hubble has many things named after him. Even the most important optical instrument of all time. But the unfortunate reality is that most likely very few of you have heard the name Leavitt. For the person that provided the yardstick for Hubble to measure the cosmos, it seems she deserves more notoriety. We should begin to honor her memory and contribution to science by saying her name more, and every conversation about Hubble be in the context of the discovery that she made.
