Have you ever ever questioned what the celebs are fabricated from? It’s not like we will leap 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 would possibly do. For one, people can’t at the moment journey to different stars. Supposing this was someway potential, it nonetheless wouldn’t be the very best concept to strategy the floor of a star: their temperatures are nicely over a thousand levels. How, then, do we all know what the celebs are fabricated from? We all know this due to the work of Cecilia Payne-Gaposchkin.
This 12 months, 2025, marks the 100 12 months 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 fabricated from [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 had been fabricated from the identical parts as Earth, like silicon and iron, and in the identical proportions too. Her outcomes had been initially rejected by the neighborhood. It was not till later that impartial research confirmed her groundbreaking outcomes, which have since paved the way in which for astrophysics as we research it at present.
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; nevertheless, she didn’t obtain a level as Cambridge didn’t award levels to girls on the time. Regardless of this setback she moved to the USA in 1923 to hold out her analysis at Harvard below the supervision of Harlow Shapley[2].
Cecilia studied the spectra of stars. A spectrum is what you see if 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 mild 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 mild. The spectrum we observe from a star is the sunshine that it has emitted, which has handed by way of its ambiance, traversed the vacancy of area, 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 by way of the ambiance of the star. Their atmospheres, like Earth’s, include 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 much 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 as a result of 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 subsequently take up particular colors of sunshine, and the mixed absorption sample we observe can be completely different relying on what kinds of atoms are current. These absorption options are referred to as spectral traces, as they seem as darkish traces within the steady spectrum of a star. A sketch of a spectrum is proven in Determine 2 beneath. By cross matching the patterns of spectral traces produced by particular person atoms, we will be taught what’s current within the stellar ambiance from its spectrum. On this sense, a spectrum presents a novel puzzle, and it’s as much as astronomers to resolve 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 strain play an enormous position in figuring out the patterns of traces we see. When it’s sizzling sufficient, atoms can lose electrons. That is referred to as ionisation. The fingerprint of an ionised atom seems fully completely different to that of its impartial kind. The idea of ionisation was the important thing concept explored in Cecilia’s thesis. Her work was revolutionary as a result of she used the just lately developed concepts of Saha (1920) and Fowler and Milne (1923 and 1924) to indicate how the situations 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 traces in every. From the spectra, Cecilia might infer the temperature, strain 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 nice 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 traces, hand drawn by Fraunhofer in 1817. Every line represents mild 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 principally of hydrogen and helium: there are roughly one million occasions 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 celebs was completely different to that of the planets blew open the sphere of astrophysics, and our view of the Universe. Planets and stars at the moment are essentially completely different objects, not the identical issues which are 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 sizzling? This was the primary time an in depth research of a selected matter in astrophysics was carried out 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 principally 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 had been fabricated from the identical parts in the identical proportions as noticed on Earth. Her outcomes had been rejected by many, comparable to the nice astronomer Henry Norris Russell. Even Cecilia felt obliged to state that her outcomes had been “virtually actually not actual”.
Little did each Cecilia and the remainder of the astronomical neighborhood realise that she was in actual fact right. Impartial research that came about 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. As we speak, utilizing way more superior instruments, we will inform not solely what stars are fabricated from, but additionally how briskly they’re spinning, how sizzling they’re at their floor, and likewise whether or not there are hidden companions orbiting them.
Spectroscopy is used every single day by the astronomers right here at Armagh Observatory. The origins of the spectra we analyse vary from essentially 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 an entire image of the celebs and the Universe. All of this is able to not have been potential 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, exhausting work, and a optimistic perspective, something is feasible.
References:
Spears, S. (2022), The First Girl of Harvard’s Observatory Hill: Cecilia Payne-Gaposchkin, American Astronomical Society,
Payne, Cecilia H. (1925), Stellar Atmospheres; a Contribution to the Observational Research of Excessive Temperature within the Reversing Layers of Stars (PhD thesis). Radcliffe School
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 appearing director of Armagh Observatory in 1959.
