In the early 1900’s, five-year-old Cecilia Payne saw a meteor shooting across the sky for the first time. In that moment, she decided that she wanted to be an astronomer when she grew up. However, she was a little scared that, because she was so young, she would be too late to make her own discoveries about our universe.

“At a very early age, I made up my mind to do research and was seized with panic at the thought that everything might be found out before I was old enough to begin!”

Hi, my name is Rutuparna Das, and I am your host for NASA’s Universe of Learning’s Diaries of the Cosmos, where we explore the “how” of exciting discoveries about our universe, and the stories of the scientists behind those discoveries.  Today’s story is about Hydrogen Hunters!  First, we’re going to meet Dr. Cecilia Payne-Gaposchkin and find out how she discovered that stars are made up primarily of hydrogen and helium.  This story will be in four parts.  After today’s narrative, we’ll chat with Dr. Antara Basu-Zych, a scientist at NASA who uses Hydrogen to study the properties of stars today.  Then, stay tuned to hear about how you can join us in our explorations, how you can ask questions, and delve deeper into the science yourself!

 

So, at the time of Cecilia Payne Gaposchkin’s discovery, scientists actually thought that our Earth and Sun were made up of the same kinds of materials, in the same proportions. Of course, we now know that this isn’t the case. Cecilia’s discovery is what showed us this.  Astronomers have called her PhD thesis where she presented this work, “The most brilliant PhD thesis ever written in astronomy”. Her discovery that hydrogen and helium were the most abundant elements in stars would completely change our understanding of what the Universe is made up of. 

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Cecilia’s journey toward becoming an astronomer began with her insatiable curiosity about the world around her. As a young girl, she went to St Paul’s Girls School in West London, and there, she was taught and inspired by her science teacher, Miss Ivy Pendlebury.

 

Miss Pendlebury not only taught Cecilia physics but also nurtured her scientific identity. She allowed her to explore the subject by herself and create her own theories.       

 

Cecelia wrote about this period of her life in her autobiography:

“All motion, I had learned, was relative. Suddenly, as I was walking down a London street, I asked myself: ‘relative to what?’. The solid ground failed beneath my feet. With the familiar leaping of the heart, I had my first sense of the Cosmos. When I tried to tell Miss Pendlebury of the experience, she remarked calmly that I should find Relativity very interesting.”

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Afterwards, Cecilia went on to attend Newnham College, the women’s college of Cambridge University. 

 

As a student there, she attended a lecture by Professor Arthur Eddington on Einstein’s theory of Special Relativity--that same relativity Miss Pendlebury thought she might enjoy. This lecture completely blew her mind, and made her decide to study physics. It was:

“a complete transformation of my world picture...when I returned to my room I found that I could write down the lecture word for word. For three nights, I think, I did not sleep. My world had been so shaken that I experienced something like a nervous breakdown.” 

However, being a woman in physics at Cambridge in the 1920’s came with its own set of obstacles. 

“Women were segregated in the lecture room. Even in the laboratory they were paired off if possible, and (did I imagine it?) treated as second-class students.”

Even though Cecilia finished all her studies, she never actually got a degree. Cambridge wouldn’t let women earn degrees until 1948. The most she could do in England after all these years of study was be a teacher at a girl’s school. Given that she wanted to do research and learn about the universe, this was devastating.

 

Thankfully, one of her professors introduced her to astronomer Harlow Shapley at Harvard College Observatory. Cecilia spoke with him and asked if she could come to the U.S. and study under his guidance. That fall she sailed from Cambridge, England to Cambridge, MA to begin her PhD, with Harlow Shapley as her advisor. 

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At Harvard, she researched the atmospheres of stars – what the stars are made up of.  To do this, she used this powerful tool called spectroscopy. Spectroscopy basically means spreading light out into its different components. Now, what do we mean by “different components”? Well, the light we see around us can be broken apart into many different colors. And, you might have seen this when ordinary daylight gets split apart by water droplets and creates a rainbow. 

 

So, from the rainbow, we know that the Sun’s light is made up of all of these colors. Scientists can use an instrument called a spectrometer which basically acts as a prism – or like those water droplets we talked about – and spreads apart light into its component colors for a camera to then capture that light. The final image looks like a strip of light, where each position on the strip represents a different color. The red and orange colors of light would be represented on one end of the strip, and the blue and violet colors would be on the other end. So, let’s say that the camera detected more red light. In that case, the red end of the strip would be a lot brighter than the blue end.

 

If you looked closely at these rainbow images captured by a spectrometer, you would notice that there are lines where the color is missing. These areas appear dark and are called spectral lines. From these sharp, narrow, dark lines, astronomers can figure out what elements are present in the objects we’re looking at.

 

This is because, in the universe, each element has its own unique set of spectral lines.  Just like humans each have their own unique fingerprints, or how grocery store items each have their own unique barcode that identifies them, each element has its own unique spectrum. We can tell elements apart from each other by looking at the patterns of these lines on that strip of light. 

 

You might be wondering where these spectral lines, these spectral fingerprints, are coming from.  Well, it all has to do with how an atom is set up. All elements have a different atomic structure, and in particular, different numbers of electrons.  These electrons live in different levels of energy. The level they’re on depends, of course, on how much energy they have. Moving up or down between these levels creates the spectral lines that we’re talking about. To go up a level, an electron has to take in energy. This is usually in the form of light. And to move down a level, the electron has to give off energy, give off light.  And the amount of energy that is transferred corresponds to a specific color.  For example, red light intrinsically has less energy than blue light. And different amounts of energies lead to different spectral lines at different colors. 

 

Now, as we mentioned, every element has a different arrangement of electrons in these energy levels, which means that every element is associated with different amounts of energy needed for the electrons to move around, and that leads to the fact that each element has its own fingerprint of spectral lines. This is what scientists can use to figure out what an object is made out of. By looking at a star through a spectrometer, scientists see these patterns of lines and can work backwards to learn about the composition of the star. If the pattern of lines for one element is stronger than the pattern of lines for another, then that tells scientists that there is more of the first element than the second.

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And this was exactly what Cecilia did. She looked at hundreds of strips of light and little narrow spectral lines in them to figure out which elements were in the stars. By looking at the strengths of the patterns of lines, Cecilia was able to determine the composition of the stars, how much of each element is there. Through this, she found that stars are mostly made up of hydrogen and helium.  

 

[pause]

 

This was highly unexpected. At that time, scientists believed that our Sun and the stars had the same composition as the Earth. For example, they thought that the Sun should be made up of just as much carbon and nitrogen as is on Earth. The Sun being mostly hydrogen and helium was inconceivable. When Cecilia brought her findings to her advisor, Harlow Shapley, he told her not to publish them because he wasn’t sure if her research was correct. And he was influenced not just by the science ideas of the day but also by his former advisor, a very prominent astronomer named Henry Norris Russell.

 

Discouraged from confidently presenting her results, Cecilia ended up having to write in her thesis instead:

“The outstanding discrepancies between the astrophysical and terrestrial abundances are displayed for hydrogen and helium. The enormous abundance derived for these elements in the stellar atmosphere is almost certainly not real.”

Even though she was confident in her data, Cecilia was pressured into publishing and presenting her results as most likely being wrong. At least, because she presented her results the way her advisor wanted, she was able to successfully get her degree. This made her the first woman  to earn a PhD in astronomy from Radcliffe College, the women’s college of Harvard University at the time. 

 

As it turns out, a couple years later, Cecilia’s results were revealed to have been correct all along! In fact, Henry Norris Russell, who had been a part of the influences keeping her from confidently standing by her results – he released a research paper of his own stating exactly what Cecilia had found earlier. Stars are indeed primarily hydrogen and helium! Russell cited Cecilia’s thesis work as evidence for his claim, but Cecilia was not an author on this paper.
 

Sadly, after making this important discovery and building an impressive research career, Cecilia was still not given the proper recognition for her expertise and contributions. She was paid very poorly for most of her career. And even though she fulfilled the role and responsibilities of instructor and professor, she was simply known as “Shapley’s technical assistant”. Because she was a woman, she was often passed up for job promotions in favor of men with similar qualifications. It took over 30 years after she had joined Harvard College Observatory for her to gain a professorship. However, following that, she was appointed head of the department of astronomy. This made Cecilia the first woman to lead a department at Harvard. 

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The experience Cecilia went through because of her incredible discovery is a reminder that truth lies within the data. 

 

Now, data, of course, can be misinterpreted, and we have to be careful and make every effort to come to the correct conclusions from our data. In Cecilia’s case, the data and her conclusions were correct! It was revealing to her an incredible fact about the universe. We know today that stars in our universe are made out of ~73% hydrogen and ~25% helium. The remaining ~2% is for all the other elements.

 

Cecilia reflects on her experiences in her autobiography and talks about what she learned about herself and research:

“I was to blame for not having pressed my point. I had given into authority when I believed I was right. That is another example of How Not To Do Research. I note it here as a warning to the young. If you are sure of your facts, you should defend your position.”

Whenever I think of Cecilia’s experiences, I’m also reminded of the importance of encouraging a safer and more inclusive culture in scientific research, where data and conclusions are valued from all researchers, regardless of gender, or what stage of career they’re in, or any other part of their identities.  Cecilia’s job was to do good research, take sound data, and draw logical conclusions – which she did.  She should not have had to bear the burden of making the senior scientists around her – those who were meant to be guiding her – she shouldn’t have had the burden of making them listen to her.

 

But at least, as it turned out, not everything had been found out about the Universe before Cecilia had a chance to explore it. Not only did she make her own discoveries, she kindled a whole new understanding about the makeup of the Universe that is still being studied today.

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That’s all for now folks!  Tune in next time for Part 2 of Hydrogen Hunters, where we chat with Dr. Antara Basu-Zych, a NASA astrophysicist currently exploring the Universe with Hydrogen, about how she weaved her path in today’s astro community.  

“This was actually where I learned what modern astronomers do – they're not just looking through telescopes and making qualitative assessments, that they actually take data on these CCD detectors – and this is actually where I learned what spectroscopy was.”

In the meantime, join us at universeunplugged.org/diaries for fun extras and more on spectroscopy!  I’m Rutuparna Das, and thanks for listening to NASA’s Universe of Learning’s Diaries of the Cosmos!