In the first story under this series, we learned how Antonia Maury helped us understand the stars better and fought for recognition of her work. In this article, let’s understand how the more brilliant human computers of Harvard helped us better understand our universe.
From Fleming to Cannon’s way of classifying stars
Along with Antonia Maury, many brilliant women joined the efforts to establish observational documentation of stars to produce the HD star catalogue back in the 19th century. One such lady was Ms Annie Cannon, who eventually took charge of the Henry Draper Catalogue group. Annie started astronomy under the guidance of her mother, Jump Cannon, who was a science enthusiast. They had a small roof door in their home from where they could see the beauty of the stars. Her mother encouraged Annie to take up science studies.
Annie took up physics and astronomy at Wellesley College and eventually landed in the Pickering group as an underpaid ‘Human Computer’. She joined the efforts of A. Maury, W. Fleming and later Henrietta Leavitt. This was the time when Maury had a disagreement with Prof. Pickering over giving credits to female researchers. One more disagreement was heating up, and it was between the ladies.
Ms W Fleming developed a simple classification system wherein stars were classified based on hydrogen abundance in their spectra. Spectra with the strongest lines were classified as “A” stars, the next strongest “B,” and so on down to the alphabet O, for which the hydrogen lines were feeble. Eventually, two decades later, astronomers realised that hydrogen lines are not a good indicator for classifying stars since these lines disappear from the visible light spectrum when the stars get too hot or too cold. Meanwhile, Ms Maury came up with a complex system based on the surface temperature of stars, classifying them as Type-a, Type-ac and so on. The differences became so vast that Ms Fleming opted out and gave up the In-charge position, which then went to Ms Cannon. Ms Fleming is now well known for two significant contributions. One is the discovery of the famous horsehead nebula in the Orion constellation, and the other is discovering a white dwarf star.
The new in-charge, Ms Cannon, looked at the data carefully and understood the viewpoints of both her predecessors. She simplified the spectral classification and based it on the surface temperature of the stars. Thus the stars with high temperatures were called Class ‘O’, then B, then A and so on till dim stars of class M. This scheme is now known as Harvard classification and even today is followed as the standard for stellar classifications. The spectral classes are thus O-B-A-F-G-K-M, popularly memorised as Oh, Be A Fine Girl Kiss Me! (Guy instead of Girl, for those who wish to substitute!). In recent, the mnemonic is now getting changed to: “Oh Boy! An F Grade Kills Me!”
Annie Cannon was the fastest person at classifying stars, and she could easily classify 5,000 stars per month. In her career, she identified and catalogued a whopping 3,50,000 stars. In 1921, Cannon became the first woman to receive a Doctor of Astronomy Degree from Groningen University. Later, she became the first woman to receive an honorary degree from Oxford University. In 1922, the International Astronomical Union adopted Cannon’s method as the official spectral classification system. In 1923, Cannon was voted one of the 12 greatest living women in America by the National League of Women Voters. After decades of hard work, Cannon was finally appointed as a permanent faculty at the Harvard College Observatory in 1938.
Leavitt’s contribution to our understanding of the universe
During the same time, another exceptional astronomer was working at Harvard Computer—Henrietta Swan Leavitt, and she focussed on variable stars. She started to look into stars from the Cepheus constellation and found a type of variable star called the Cepheus Type. A variable star changes its brightness in a fixed time period. For example, the famous star Algol changes its apparent brightness in about three days by nearly five times. So, today it is a bright star, while tomorrow night it will be five times dimmer, and on the third day, it will be bright again.
Ms Leavitt found an astonishing fact for Cepheid type variables (named after d-Cephei star from Cepheus constellation)—the period of such stars is proportional to the luminosity. Higher the luminosity, longer is the pulsation period. Now, if you know the period of variation, you can find the luminosity, and from that, we can easily find the star’s distance. So for stars at very large distances, astronomers now had a way to measure distances if you find a Cepheid type variable. This relationship is called Period-Luminosity Law, and it opened up doors to measure distances of faraway stars.
Henrietta further went to study over 2000 such stars in the nearby ‘clouds’ called Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC). She found that the distances between these two objects are very large as compared to most of the stars or ‘nebulae’ in the nearby region. Thus came the concept of the galaxy, and eventually, astronomers confirmed that LMC, as well as SMC, are outside our Milky Way galaxy. Today, we know that these are satellite companions of our galaxy.
The beautiful Andromeda galaxy was found at an astonishing distance of 2.2 million light-years, and hence we understood that the Milky Way is one of many such galaxies in the universe. The concept of how our universe looks changed dramatically after Leavitt’s results. Prof. Harlow Shapley (who invited Anni Cannon) came out with a working model of the spiral arm Milky Way galaxy. The interest in the field of cosmology went up exponentially. Based on the methods of Leavitt, Cannon, Maury and others, astronomers measured distances of remote galaxies and also understood that all the galaxies have a great speed in the universe.
In 1916, Prof Einstein came with the theory of General Relativity. Physicists and Mathematicians used this path-breaking theory and started to develop models of our universe. Einstein himself came up with solutions for his theory, and to his surprise, he found that the universe is expanding and not static! Such was the firmness of the concept of a static universe that he introduced a constant in his theory and made the universes a ‘static universe’. His contemporary scientists were not bound by such stationary ideas and mathematician Alexander Friedmann came with a model of the universe which was expanding and could be contracting in future. So the debate of how the universe was heating up.
Edwin Hubble applied Leavitt’s methods to measure galactic distances and, at the same time, started measuring the speed at which galaxies move. To his surprise, he found that all the galaxies were moving away from each other and farther the galaxy, faster was its speed of running away. He thus concluded that the universe is expanding! Unfortunately, Ms Leavitt was not alive to see the biggest triumph of her work. She passed away in 1921 at the young age of 53 years.
The work done by these ladies in the late nineteenth and early twentieth century was groundbreaking. It was done in a very depressing situation with inadequate salary, no assurance of job continuity, minimal credit for the work and no scientific benefit apart from the pleasure of doing things. It required a great zeal towards their work and perhaps hope that the world will acknowledge their work someday. Today, when astronomers use these concepts, we hardly think who invented these ideas. Even today, many scientists forget to credit the work to these brilliant female computers.
Nevertheless, the brilliance of their ideas and the simplicity of their methods are impossible to forget. These Harvard computers continue to shine as few of the brightest stars of astronomy!
This article is a part of a two-part series of articles on World Space Week 2021. The first article in the series can be accessed here:
Dr Abhay Deshpande is a Senior Scientist (Physicist) working for SAMEER, R&D Lab of MeitY, Government of India. He is also the Honorary Secretary of Khagol Mandal.
This article is a guest column reflecting the author’s opinions and does not necessarily represent the official views of The Weather Channel.
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