You’ve likely heard of the Big Bang theory and the name of Edwin Hubble associated with it. But a person you may not have heard of is Henrietta Leavitt. Leavitt played a critical role in enabling Hubble’s accomplishment. Seeing as today’s her birthday, let’s celebrate her, her achievement, and her impact on astronomy and cosmology.
Born on the 4th of July, 1868, in Lancaster, Massachusetts, to a church minister, Henrietta Swan Leavitt attended Oberlin College, transferred to Harvard University’s women’s college, later to become Radcliffe, studied a broad range of curriculum, and received her bachelor’s degree in 1892. In her final year, she signed up for a course on astronomy and so took her life in a new direction. She began working for the Harvard College Observatory, then under the direction of Edward Charles Pickering, and after years of this work, which were interspersed with some travel and teaching elsewhere, she became a permanent member of the Observatory in 1902.
Leavitt’s focus at the Harvard Observatory was on photographic plates, specifically the measuring and cataloging of the brightness of stars. It was in this role that she saw something no one else had yet seen. While that moment was significant in its own right, its subsequent impact on the field of astronomy was tremendous. To fully understand why, one must first understand the scientific context in which Leavitt’s discovery was made.
In Leavitt’s time, scientists didn’t yet suspect that the stars were all moving away from Earth. They seemed to be just sitting there in the sky. Yes, they rotated across the sky throughout the night, but that’s because we have known since the time of Copernicus that that’s because the Earth itself is spinning on its axis. And yes, some objects among the stars were moving, but others explained them to be “wanderers” or planets, some asteroids too. But the huge mass of stars? They were really just sitting there. Who knew they were moving away from us?
This is how things stood until the early 1900s when scientists first started pointing spectroscopes towards the stars to see what they were made of, these devices providing a fascinating means of capturing electron-orbital fingerprints and so identifying elemental composition. But the curious astronomers eventually noticed that the resulting spectral lines didn’t line up exactly with those measured on Earth and thus concluded that the stars must be moving relative to Earth, causing a “Doppler” shift in the lines, with a shift towards the blue end of the spectrum signifying motion “toward” and a shift towards the red signifying motion “away.” Vesto Slipher, a leader in this field, took the first set of accurate line-shift measurements and noted that the majority of the stars were red-shifted, thus moving away from Earth. But there was a large paradigm-gap between these observations and the thought that the universe itself was expanding and so moving everything away from everything else. The missing data needed to discover this expansion was distance, specifically the distance from Earth to the stars. Without it, attempts to better understand any structure behind the motion of the stars were impossible.
The primary method available for measuring the distance from Earth to the stars when Leavitt started her work was the parallax method, which relied on the observed change in position of a star against a non-moving background of more distant stars as the Earth rotated around the Sun. But this method could only be used on stars closer than 100 light years away from Earth and most stars and other galaxies are beyond this distance. Clearly another method was needed.
Consider a flashlight. Flip it on and have someone walk it away from you. Its brightness decreases. Using basic physics, you could calculate its distance from you based on its observed brightness. Ah, but you would have to know one more variable. How inherently bright is the flashlight when it’s right next to you? You need to know the relation between observed brightness and inherent brightness to calculate distance. In astronomy, the challenge in knowing inherent star brightness lies in the fact that there are many different types of stars, each with its own inherent brightness, which itself could vary depending on other variables related to that star. What was needed was a way to identify a known type of star with known variables and thus known inherent brightness.
We now return to Henrietta Leavitt. The Harvard College Observatory had a treasure trove of photographic plate images of stars taken by astronomers. Pickering assigned Leavitt the task of studying the plates to identify “variable stars” in the Small and Large Magellanic Clouds. These are stars whose brightness swings back and forth from bright to dim to bright on a regular basis of hours, days, or weeks. (This swing was later determined to be caused by the ionization and de-ionization of helium in the pulsating star atmosphere.) Leavitt’s strong work ethic and exceptional skill set led her to discover 1,777 variable stars in these clouds and by the end of her career more than 2,400 such stars in the universe, about half the total known at that time.
Within this effort, Leavitt narrowed her focus to a specific type of variable star called a Cepheid and started pursuing her interest in the relation between the observed maximum brightness and the rate of variation in brightness. Recall that observed brightness depends on both inherent brightness and distance. For her research, Leavitt wanted to remove distance as a variable and so needed to identify a set of these Cepheid variables all at the same distance from Earth. And this she did. She discovered 25 Cepheid variables in the Small Magellanic Cloud, assumed that they were all the same distance from the Earth, and for each compared maximum brightness against the period of variation.
Have you ever experienced that eureka moment of excitement after unveiling some aspect of nature for the first time? Stuart Firestein describes this well in his book Ignorance – How It Drives Science (p. 160). “I am afraid that it is impossible to convey completely the excitement of discovery, of seeing the result of an experiment and knowing that you know something new, something fundamental, and that for this moment at least, only you, in the entire world, knows it.” I can imagine that this is how Leavitt must have felt when she plotted her data, because the correlation was right there in front of her. What a wonderful moment in science this was. As the rate of variation slows down (longer period of time), maximum brightness increases.
Leavitt’s discovery was not the final step in the process of measuring inter-galactic distances. Instead, it was the penultimate step. The reason? Her data weren’t calibrated to the distance from the Small Magellanic Cloud (SMC) to Earth. This situation was rectified when Ejnar Hertzsprung and later Harlow Shapley measured the distance from Earth to a single Cepheid outside the SMC and close enough for the parallax method to be used. With this distance in hand, and with the distance between the Cepheid and the SMC calculated based on maximum brightness difference (for the same variation rate), astronomers could calculate the distance from Earth to the SMC and so convert Leavitt’s brightness scale from “observed” to “inherent.” This then enabled the following process: identify a distant cloud, find a Cepheid variable in that cloud, measure its rate of brightness variation, determine its inherent brightness from Leavitt, Hertzsprung, and Shapley’s work, compare this with the observed brightness, and thus calculate distance from Earth to that Cepheid.
The creation of this new distance-measuring methodology from Leavitt’s work was a true breakthrough moment in the fields of not only astronomy but also cosmology, for in 1929-1931 Edwin Hubble combined Slipher’s work with Leavitt’s to reach deep into the universe and gather the data used to support the hypothesis of an expanding universe, which then led to the Big Bang theory. The figure below from my book Block by Block illustrates this process.
Henrietta Leavitt unfortunately didn’t live to see Hubble’s amazing results. She died of stomach cancer on December 12, 1921, at the age of 53. In her obituary, fellow astronomer Solon I. Bailey shared the following:
“She took life seriously. Her sense of duty, justice and loyalty was strong. For light amusements she appeared to care little. She was a devoted member of her intimate family circle, unselfishly considerate in her friendships, steadfastly loyal to her principles, and deeply conscientious and sincere in her attachment to her religion and church. She had the happy faculty of appreciating all that was worthy and lovable in others, and was possessed of a nature so full of sunshine that to her all of life became beautiful and full of meaning… Miss Leavitt was of an especially quiet and retiring nature, and absorbed in her work to an unusual degree. She had the highest esteem of all her associates at the Harvard Observatory, where her loss is keenly felt.” (Popular Astronomy, 30, no. 4, April 1922, pp. 197-199)
And so on this July 4, her birthday, let us recognize Henrietta Leavitt for her curiosity, intuition, intelligence, and roll-up-your sleeves perseverance in collecting and interpreting plate after countless plate of data, thereby contributing to the most wondrous of human achievements, the discovery of the Big Bang origin of our universe. Here’s to Henrietta Leavitt, a true scientist and role model.