The Star-women (pun intended)

Updated: Nov 15, 2020

It has been common knowledge for close to a century that Stars are primarily made of Hydrogen. This crucial insight comes from the work of Cecilia Payne, who in her thesis “Stellar Atmospheres; A Contribution to the Observational Study of High Temperature in the Reversing Layers of Stars” outlined her discovery.

The late 1800s and early 1900s saw a rise in women being employed by universities, particularly in the science department. They were hired at low wages to do menial tasks, such as performing simple calculations on huge sets of data over and over and over, or perhaps classify huge sets of objects into various categories based on various properties. Today, we can do these jobs with relative ease on our electronic computers. And the people who did such a job were called Human computers. In fact, if you go back to the early days of computing, the videos clearly mention “Electronic Computers”, because the general use of the word “Computer” was to refer to these women who crunched huge sets of data.

Annie Jump Cannon

One such Human-Computer who worked at Harvard, Annie Jump Cannon, along with Professor Edward C Pickering developed a classification system, now known as the “Harvard Classification System.” It was perhaps the first serious attempt at classifying various stars we observe, based on various factors such as temperature, spectra, and size. 

One method to identify the composition of stars and nebulae far far away comes from an insight from Gustav Kichhroff, who propounded his three laws of spectroscopy, based on observations of how Gases and solids interacted with radiation.

  1. A solid, liquid, or dense gas excited to emit light will radiate at all wavelengths and thus produce a continuous spectrum.

  2. A low-density gas excited to emit light will do so at specific wavelengths and this produces an emission spectrum.

  3. If light composing a continuous spectrum passes through a cool, low-density gas, the result will be an absorption spectrum.

With knowledge of these laws, it quickly became apparent that every material, atom, or molecule had its unique way of interacting with radiation and producing a unique spectrum. The emission spectrum of various gases is what makes the neon lights glow in the bright colours we see. The tubes are filled with specific gases, that give a unique colour. When put together, it can be used to light up beautiful artwork.

Fraunhofer lines

This also means, when looking at the spectrum of a distant object, it would be easy to reconstruct what elements it had based on the light it emits/blocks. Joseph Von Fraunhofer had studied these lines for our sun, and spent years cataloguing them, but was unable to make sense of them. But with the laws of spectroscopy, Kichhroff was able to reconstruct the elemental composition of the sun out of the Fraunhofer lines. The first element to be discovered in the sun was Sodium, but many other elements found on earth were discovered. Helium was identified as a part of the sun, much much before it was ever discovered here on earth. Because the elements observed were similar to what was seen on earth, it was assumed that the stars were also composed similarly.

It was unknown at the time, but these laws of spectroscopy were some of the most crucial details in determining the structure of an atom.

In the early 1900s, Quantum Physics was a rising new field, and it’s similarities to Statistical mechanics inspired Meghnand Saha to study the potential of an atom being ionized and it’s relation with temperature and pressure. His work concluded with the Saha equation, which described the ionization trend taking into account features of Statistical Mechanics. 

Meghnad Saha

Among the many other things this result predicted, it brought up a correlation between the spectrum of a star and the Classification that was pioneered by Annie Jump Cannon. Today, this equation stands as the backbone of astronomy.

Cecilia Payne

Cecilia Payne was a British astronomer who moved to the USA to further her studies. She had graduated from Cambridge University but was not awarded a degree because of university policy. When Harvard opened it’s Graduate programme in Astronomy, she was the second person to join under the newly instated fellowship to encourage women to study at Harvard College Observatory.

She was the first person to finish her PhD at Radcliffe college. Her thesis, not just proved the correlation between temperature and spectral lines (From the Saha equation), she also noticed that the different lines in the spectrum were because of different concentrations of elements, not because of different compositions. 

As a corollary, she also arrived at the result that the reason Hydrogen and Helium lines were more common, was because they were more abundant in the stellar atmosphere, by a SIGNIFICANT margin. But this result was rejected by Henry Russel, because of it’s contradiction to common knowledge about the stellar compositions. The idea that the stars were composed differently than the earth was revolting at the time.  She dejectedly changed the conclusion of her thesis to match the advice of Henry Russel, and described her results as “Spurious.” 

A few years later, Henry Russel himself arrived at the same conclusion, through a different method, which not just confirmed, but even strengthened her discovery. When he realised his mistake, he made sure to credit Payne for her discovery. But even today, it’s often remembered as Russel’s work, and Payne’s contribution is less known. However, a few years later, her PhD was recognized by Otto Struve as “the most brilliant PhD thesis ever written in astronomy.”

Illustration of our Solar system

This discovery also formed the basis of the modern idea that the elements around us were formed in the heart of stars as a result of Nuclear Fusion and forms the basis of the current model of how the solar system was formed.

Every single discovery in the world is built upon something behind it. When we look at the beautiful theories that explain our world and take them for granted as common facts, we often forget the decades, if not centuries of effort that went behind that discovery.

Every beautiful theorem that describes our world was built on top of something just as beautiful underneath it. And it’s always worth exploring it.

Image Sources: 

Solar System:

Cecilia Payne:

Meghnad Saha:

Fraunhofer Lines:

Annie Jump Cannon:


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