Have you ever ever questioned what the celebrities are product of? It’s not like we are able to bounce on a spaceship, go to them, and, utilizing a particular bucket and spade, take a pattern from their floor to carry again to the lab for evaluation, as a geologist or biologist may do. For one, people can not at present journey to different stars. Supposing this was by some means attainable, it nonetheless wouldn’t be one of the best thought to method the floor of a star: their temperatures are properly over a thousand levels. How, then, do we all know what the celebrities are product of? We all know this due to the work of Cecilia Payne-Gaposchkin.
This yr, 2025, marks the 100 yr anniversary of the publication of the PhD thesis of Cecilia Payne[1]. In her revolutionary work, Cecilia analysed the sunshine of over 100 stars and measured, for the primary time, what they’re product of [1,2]. She discovered that they’re primarily composed of hydrogen and helium. This discovering contradicted what was believed on the time, because it was then accepted that stars have been product of the identical parts as Earth, like silicon and iron, and in the identical proportions too. Her outcomes have been initially rejected by the group. It was not till later that impartial research confirmed her groundbreaking outcomes, which have since paved the best way for astrophysics as we examine it right now.
Determine 1: Cecilia Payne. Picture Credit score: Smithsonian Establishment Archives.
Cecilia was born in England in 1900. Whereas learning physics and chemistry in Cambridge College, she attended a lecture given by well-known astronomer Arthur Eddington. This lecture fascinated her, and, from then on, she knew that astrophysics was her calling. She completed her research; nonetheless, she didn’t obtain a level as Cambridge didn’t award levels to girls on the time. Regardless of this setback she moved to america in 1923 to hold out her analysis at Harvard beneath the supervision of Harlow Shapley[2].
Cecilia studied the spectra of stars. A spectrum is what you see whenever you cut up up the sunshine of a star. For instance, a rainbow is technically the spectrum of the Solar, because the raindrops within the sky cut up up its white gentle into its element colors. Consider the quilt artwork of the Darkish Aspect of the Moon by Pink Floyd, besides on this case, they used a glass prism as an alternative of a raindrop to separate up the Solar’s gentle. The spectrum we observe from a star is the sunshine that it has emitted, which has handed via its ambiance, traversed the vacancy of house, and reached our telescopes, the place it has been cut up up (as Pink Floyd did in 1973 for his or her album cowl) and recorded in our detectors.
The important thing stage of this journey happens when the sunshine passes via the ambiance of the star. Their atmospheres, like Earth’s, encompass many, many atoms. An atom is a tiny particle consisting of a nucleus of protons and neutrons that’s orbited by electrons. The electrons orbit the nucleus just like how the planets within the Photo voltaic System orbit the Solar. What makes atoms fascinating is that every one absorbs and re-emits very particular wavelengths, or colors, of sunshine. That is because of the completely different numbers of protons, neutrons, and electrons in every atom. In that sense, every atom has its personal distinctive fingerprint that we see once we observe its spectrum.
The ambiance of a star will due to this fact take in particular colors of sunshine, and the mixed absorption sample we observe will probably be completely different relying on what kinds of atoms are current. These absorption options are known as spectral strains, as they seem as darkish strains within the steady spectrum of a star. A sketch of a spectrum is proven in Determine 2 under. By cross matching the patterns of spectral strains produced by particular person atoms, we are able to study what’s current within the stellar ambiance from its spectrum. On this sense, a spectrum presents a singular puzzle, and it’s as much as astronomers to unravel it.
Determine 2: The spectrum we observe from a star is a mixture of the completely different atoms current in its ambiance. Picture Credit score: Ciarán Furey.
In a star’s ambiance, the temperature and stress play an enormous position in figuring out the patterns of strains we see. When it’s scorching sufficient, atoms can lose electrons. That is known as ionisation. The fingerprint of an ionised atom appears to be like fully completely different to that of its impartial type. The idea of ionisation was the important thing thought explored in Cecilia’s thesis. Her work was revolutionary as a result of she used the not too long ago developed concepts of Saha (1920) and Fowler and Milne (1923 and 1924) to point out how the circumstances in a star’s ambiance affect the absorption patterns we observe in its spectrum.
Cecilia analysed the spectra of over 100 stars, learning greater than 100 spectral strains in every. From the spectra, Cecilia might infer the temperature, stress and ionisation state within the star’s ambiance, and, from this, she decided the abundance of the weather. She used this methodology to calculate the abundances within the ambiance of the Solar. The Solar’s spectrum is proven in Determine 3. This picture was hand drawn by the good physicist Joseph von Fraunhofer greater than 100 years earlier than Cecilia carried out her pioneering analysis.
Determine 3: The spectrum of the Solar and its spectral strains, hand drawn by Fraunhofer in 1817. Every line represents gentle that has been absorbed by completely different atoms within the ambiance of the Solar. Picture Credit score: Deutches Museum.
The bar chart in Determine 4 exhibits her abundance determinations of various atoms within the Solar’s ambiance. Her findings present that the Solar is made largely of hydrogen and helium: there are roughly one million instances extra of those atoms than there are, say, silicon and iron atoms. Then again, atoms like silicon, oxygen, and iron are the dominant species within the Earth’s crust. Realising that the composition of the celebrities was completely different to that of the planets blew open the sphere of astrophysics, and our view of the Universe. Planets and stars are actually basically completely different objects, not the identical issues which might be at completely different temperatures. If stars usually are not just like the Earth, then what are they? What’s going on of their interiors? How are they so scorching? This was the primary time an intensive examine of a particular matter in astrophysics was performed on the Harvard Observatory, that means that, in 1925, Cecilia was the primary particular person to ever obtain a PhD certificates within the topic from Harvard.
Determine 4: The abundances of parts within the Solar as decided by Cecilia in her 1925 PhD thesis. She discovered that it’s largely comprised of hydrogen and helium – the 2 lightest parts within the Universe. Picture Credit score: Ciaran Furey.
This discovery got here with a value. Cecilia’s outcome conflicted with frequent perception on the time, the place it was accepted that stars have been product of the identical parts in the identical proportions as noticed on Earth. Her outcomes have been rejected by many, comparable to the good astronomer Henry Norris Russell. Even Cecilia felt obliged to state that her outcomes have been “nearly definitely not actual”.
Little did each Cecilia and the remainder of the astronomical group realise that she was in actual fact appropriate. Unbiased research that passed off after her thesis confirmed her outcomes. For the primary time, she linked the atomic fingerprints seen in stellar spectra to the abundance of parts. This laid the foundations for contemporary stellar spectroscopy. In the present day, utilizing much more superior instruments, we are able to inform not solely what stars are product of, but in addition how briskly they’re spinning, how scorching they’re at their floor, and in addition whether or not there are hidden companions orbiting them.
Spectroscopy is used each day by the astronomers right here at Armagh Observatory. The origins of the spectra we analyse vary from probably the most huge stars within the Universe, to the comets in our photo voltaic system, to very peculiar populations of stars, as we glance to construct a whole image of the celebrities and the Universe. All of this is able to not have been attainable with out the work of Cecilia Payne. Her story is an instance for anybody who thinks they want to pursue a profession in astrophysics: that, with perseverance, arduous work, and a optimistic angle, something is feasible.
References:
Spears, S. (2022), The First Woman of Harvard’s Observatory Hill: Cecilia Payne-Gaposchkin, American Astronomical Society, https://aas.org/posts/information/2022/04/month-astronomical-history-april-2022
Payne, Cecilia H. (1925), Stellar Atmospheres; a Contribution to the Observational Research of Excessive Temperature within the Reversing Layers of Stars (PhD thesis). Radcliffe Faculty
Sobel, D. (2016), The Glass Universe; The Hidden Historical past, of the Ladies Who Took the Measure of the Stars, Fourth Property
[1] Cecilia married a person named Sergei Gaposchkin later in life, so the double-barrelled surname is used interchangeably. On this article, we check with her as Cecilia Payne.
[2] Harlow Shapley was the performing director of Armagh Observatory in 1959.
