100 years ago, Cecilia Payne discovered what the stars are made of

Inside the Sun, the most powerful source of energy for several light-years in all directions, an incredible process occurs. Deep within its core, hundreds of thousands of kilometers beneath the edge of its photosphere, temperatures exceed a critical threshold of four million degrees, rising up to a maximum of 15 million K in its center. Under these conditions, hydrogen atoms — and specifically, the nuclei of hydrogen atoms — smash into one another, causing their quantum wavefunctions to overlap. Although these collisions are extremely frequent, they most often simply bounce off of one another, failing to create a meaningful, energy-liberating reaction.

But every once in a while, this results in a nuclear fusion reaction, where heavy isotopes (like deuterium or tritium) or even heavier elements (like helium-3 or helium-4) result. These heavier isotopes and elements are more energetically stable than bare protons are on their own, and so as a result of these reactions, energy is liberated. We take for granted, today, that this is the process that not only powers the Sun, but nearly all of the stars (i.e., every star on the main sequence) in the Universe.

It seems hard to imagine it now, but just 100 years ago, we not only didn’t know about this process at all, but we didn’t even know what the Sun (and all stars) are made of: hydrogen and helium. How did we find out? That’s the work of astronomer Cecilia Payne, whose PhD dissertation celebrates its 100th anniversary this year: in 2025. Here’s how this brilliant scientist showed us what the most common luminous object in the Universe is made of, and with it, how stars themselves actually work.

In November of 1969, the Apollo 12 spacecraft left Earth for the Moon. During its journey, the disk of the Earth was seen partially blocking the Sun, which is when this photograph was taken. From portions of the Moon’s surface, during a penumbral eclipse, a similar sight would arise, as the Earth will partially, but not completely, block out the Sun’s disk. Our knowledge of the composition of the Sun is only 100 years old, as of 2025.

From the time of Newton, we’ve known that the Sun had to be very, very massive: around 300,000 times as massive as planet Earth. Because the Sun is known to be 93 million miles (150 million km) away from us, we were able to infer how much energy it emits just by measuring how much energy we receive here on Earth, which we were able to do some 200+ years ago. Making that measurement and extrapolating the Sun’s total energy, in all directions, leads to a figure for the Sun’s total power output: a whopping 4 × 10²⁶ watts, or around 10 quadrillion times the amount of energy-over-time released by the most energetic power plants ever constructed on Earth.

However, there was a big open question that plagued astronomers, physicists, and all scientists who cared about such matters: where did the Sun get its energy from? With such an incredibly large power output, an amount that was virtually unfathomable to reach via conventional means, it was a question that many investigated over the 17th, 18th, and 19th centuries, without a satisfactory answer ever having been reached during that time. In fact, near the end of the 19th century, it was no less a towering figure than the great William Thomson, also known as Lord Kelvin, who sought to tackle that burning question.

A cross-section of the Wealden Dome, in the south of England, which required hundreds of millions of years just to explain the erosion features observed, with fossils of past life found in the different layers. The chalk deposits on either side, absent in the center, provide evidence for an incredibly long geological timescale required to produce this structure: longer than any contemporary explanation for the Sun’s energy could have provided in the late 19th century. This was noted by none other than Charles Darwin in the mid-1800s, and would present a puzzle that would not be resolved until the process powering the Sun, nuclear fusion, became understood.

Earlier in the 1800s, Charles Darwin had undertaken some remarkable work: not just within biology for the theory of evolution, for which he’s best known, but also in the study of planet Earth itself, geologically. From an evolutionary standpoint, Darwin had concluded that the Earth required at least hundreds of millions of years to produce the diversity of life that we see today, but from a geological standpoint, Darwin had observed something remarkable: the geological erosion of a domed structure that showed various layers of sedimentary rock within it. It was clear that the layers on either side of the eroded dome matched, and Darwin sought to understand both how these layers formed and how long it took to form them.

Erosion had played a major role, and while the process of erosion might have occurred relatively quickly, the formation of these various sediments, including a key set of chalk deposits, suggested that even hundreds of millions of years was too short of a time; perhaps more like a billion or even a few billion years would have been necessary. However, if the Earth were truly that old, how could the Sun have sustained its power output for such a fantastically long duration? That’s the question that Kelvin — the same Lord Kelvin who discovered the existence of absolute zero — considered, coming up with three possibilities:

1. The Sun was burning some type of fuel, like a combustion reaction does on Earth.
2. The Sun could be feeding on raw material present within the Solar System, as though it were absorbing fuel as it burned on and on.
3. Or the Sun was gravitationally contracting, and the conversion of gravitational energy into the heat and light from the Sun was what was powering it.

Placing a chunk of sodium metal in contact with water results in a violent, and often explosive, reaction. This is due to the sodium donating an electron to hydrogen ions in the water, which leads to the emission of heat and the creation of hydrogen gas. When that gas combines with the atmosphere’s oxygen in the presence of heat, a combustion reaction occurs. Combustion is a chemical-based reaction: an insufficient process for powering the stars over known stellar lifetimes.

Alas, all three explanations only led to spectacular failures. Kelvin analyzed and considered them one at a time, including the timescales on which they would successfully result in a power output of 4 × 10²⁶ watts: the observed power of the Sun.

1.) If the Sun was burning some type of fuel, then at some point, that process will reach completion. The most combustible fuel sources known to Kelvin were hydrogen, hydrocarbon molecules, and TNT. All of these fuel sources combine with oxygen to release tremendous amounts of (chemical) energy when burned. Indeed, if the Sun were made entirely out of one of these fuels, there would be enough material for the Sun to produce that incredible amount of power, 4 × 10²⁶ watts, for merely tens of thousands of years. That’s not at all long enough to explain the long history of life on planet Earth. As a result, Kelvin ruled this option out.

2.) On the other hand, just because there isn’t enough material in the Sun for a chemical reaction, such as combustion, to sustain it for the necessary timescales that biology and geology required, doesn’t mean that the entire Solar System doesn’t have that much material. In principle, it could be possible to continuously add some type of fuel to the Sun to keep it burning. It was well-­known that comets and asteroids abound in our Solar System, and so long as there was enough new (unburned) fuel being added to the Sun at a roughly steady rate, its lifetime could be extended by large amounts. Unfortunately, you can’t add arbitrary amounts of mass to the Sun, because at some point, it would increase to a value that began altering the orbits of the planets, which were observed to be consistent for hundreds of years. A simple calculation showed that even just adding that small amount of mass to the Sun — less than a thousandth of a percent over the past few centuries — would have a measurable effect, and that the steady, observed elliptical orbits ruled this option out.

In addition to the two groups of Trojan asteroids (green, originally named “Greeks” and “Trojans”), there are also the Hildas: a set of asteroids orbiting in a 3:2 resonance with Jupiter. The Trojans (and Greeks), as well as the Hildas, are safely shepherded into these quasi-stable orbits which will not cross Earth’s. If we attempt to have enough of these asteroids fall into the Sun to provide an ongoing source of fuel for it, such large additions of mass, over the Solar System’s history, would have destabilized the orbits of the inner planets.

This left only the third and final option in Kelvin’s mind: the option that the Sun gets its energy from gravitational contraction.

3.) Let’s assume that this is true: that the Sun generates its energy from gravitationally contracting over time. This would simply be an example of turning gravitational potential energy into another form of energy, just as raising a ball to a certain height on Earth and then releasing it will cause it to pick up speed (and gain kinetic energy) as it falls, where that energy then gets converted to heat, sound, and deformation energy when it collides with Earth’s surface and eventually comes to rest. Kelvin reasoned that the very same type of initial energy, gravitational potential energy, could cause molecular clouds of gas to heat up as they contract and become denser.

If a massive collection of material contracts, they will then be smaller, denser, and more spherical than they were back when they were simply diffuse gas clouds, and because a spherical collection of matter can only radiate energy away through its surface — a surface of finite area that’s only proportional to its radius squared, as opposed to mass, which is proportional to its radius cubed — it will take a very long time indeed for such a collection to radiate the entirety of its internal heat away. Kelvin was the foremost expert in the world on the mechanics of how this would happen, as the Kelvin­-Helmholtz mechanism is named after his work on this subject. For an object such as the Sun, Kelvin calculated, its lifetime for emitting the needed amounts of power, 4 × 10²⁶ watts, would be on the order of tens of millions of years: somewhere between 20 and 40 million years, and certainly no more than 100 million years, to be precise.

This image shows Sirius A and B, a bluer and brighter star than our Sun and a white dwarf star, respectively, as imaged by the Hubble Space Telescope. While Sirius B is indeed emitting light powered by gravitational contraction and the emission of its energy through a small surface area, Sirius A, a main sequence star, is powered by an entirely different process: nuclear fusion in its core.

Of course, this figure was far too short to line up with the timescales that biology and geology required for the living and structural entities we find on Earth’s surface, and Darwin himself noticed. The overall solution, whatever it was, would remain obscure for the remainder of both Darwin’s and Kelvin’s lives.

It turns out that gravitational contraction does play a role in the shining of many massive objects: of white dwarfs and neutron stars, the stellar corpses of our Universe. They can indeed shine for extremely long periods of time and do get their energy from gravitational contraction, but are much, much smaller than stars like our Sun, and have much smaller surface areas to radiate their energy away through.

Kelvin’s estimated age of the Sun, and of all stars for that matter, were far too small to account for what was observed from other lines of evidence. It wouldn’t be until several generations later that we learned of the process of nuclear fusion that powered the Sun, as we didn’t even discover the existence of the atomic nucleus until the 20th century. However, it must be realized that, in the time of Kelvin and Darwin, we didn’t even know what the Sun, or any star, was made out of. The conventional wisdom at the time, believe it or not, was that the Sun was made out of pretty much the same elements that the Earth is. Although that might seem a bit absurd to you, consider the following piece of evidence.

This figure shows the apparent color of light as sorted by wavelength in the top row, followed by the emission line features that fall into the visible part of the spectrum for elements like sodium, hydrogen, calcium, magnesium, and neon. At different ionizations, these lines appear and/or disappear.

Every element or atom in Mendeleev’s periodic table — which was discovered prior to the important work of both Darwin and Kelvin — has a characteristic spectrum inherent to that species of element itself. When each species of atom is heated up, the electrons within that atom get excited, and then transition back down to lower-energy states, with those atomic transitions leading to a specific set of emission lines. When a background, multi-spectral light is shone on those atoms, they absorb energy at those very same wavelengths: the same emission features, when an excited atom de-excites, are present as absorption features when a ground-state atom gets excited.

This led to an incredible piece of reasoning: if we could observe the Sun at all of these different, individual wavelengths together, we could figure out what elements were present in its outermost layers, simply by observing its absorption features. This technique is known as spectroscopy: where we break the light up from any source into its individual wavelengths for further study. We can perform spectroscopic analysis not only of the Sun, but for any star that we like, simply by collecting a sufficient amount of light and using a device — a prism, a spectrograph, a charge-coupled device, etc. — to break that light up into its different component wavelengths. When we do this to our Sun with modern technology, we can obtain a spectrum like the one shown below.

The visible light spectrum of the Sun, which helps us understand not only its temperature and ionization, but the abundances of the elements present. The long, thick lines are hydrogen and helium, but every other line is from a heavy element that must have been created in a previous-generation star, rather than the hot Big Bang.

Basically, the elements that we see in the Sun are indeed the same elements that we find on Earth. But what is it, exactly, that causes those lines to appear with the relative strengths that they appear? For example, you may notice that some of these absorption lines are very narrow, while some of them are very, very deep and broad. Take a closer look at the strongest absorption line in the visible spectrum, which you can find in about the seventh row down, about ¾ths of the way to the right-hand side: occurring at a wavelength of 6563 Ångströms.

What determines the strength of these lines, as well as the relative weakness of the lines surrounding it? It turns out that there are two factors, one of which is obvious: the more of an element you have, the stronger the absorption line is going to be. The particular wavelength of that strongest absorption feature — 6563 Å — corresponds to a well-known Hydrogen line: Balmer alpha.

But there is a second factor that must be understood in order to get the strength of these lines right: the level of ionization of the atoms present. Different atoms lose an electron (or multiple electrons) at different energies. So not only do different elements each have a characteristic spectrum associated with them, they can exist in a number of different ionized states (missing one electron, or two, or three, etc.) that each have their own, unique spectrum.

This graph shows the first ionization energy of each atom in the periodic table by atomic number. This is how much energy it takes to strip the most loosely-held electron off of the atom entirely and ionize it. Elements in the first group of the periodic table, particularly lithium, sodium, potassium, rubidium, and so on, lose their first electron much more easily than any other elements. At even higher temperatures and energies, multiple electrons can be stripped off of an atom, creating a state of greater ionization.

Because energy is the only thing that determines the ionization state(s) of atoms, this means that different temperatures will result in different relative levels of ionization, and therefore, different relative levels of absorption. So when we’re looking at stars — like the Sun — we know that they come in a huge variety of different types, as a look through any telescope or binoculars will immediately show you, if it isn’t clear to your naked eye. These stars, very notably, come in strikingly different colors, which tells us that — at least at their surfaces — they exist at vastly different temperatures from one another. Because hot objects all emit the same type of (blackbody) radiation, when we see stars of different colors, we’re really detecting a temperature difference between them: blue stars are hotter and red stars are cooler.

After all, this is — as Annie Jump Cannon figured out — why we classify stars the way we do in modern times, with the hottest, bluest stars (O-type stars) at one end and the coolest, reddest stars (M-type stars) at the other. But this was not how we always classified stars. There’s a hint in the naming scheme, because if you had always classified stars by temperature, you might expect the order to go something like “ABCDEFG” instead of “OBAFGKM,” right?

As you might have suspected, there’s a story here. Back before this modern classification scheme, we instead looked at the relative strengths of absorption lines in a star, and classified them by what spectral lines did or did not show up. The pattern, when we looked at these stars, is far from obvious.

The spectra of the various stars here, sorted by their Morgan-Keenan spectral classification, shows how ionization and temperature both affect the appearance and disappearance of various absorption lines, with purple lines of ionized calcium, a yellow pair of sodium line, and low-temperature features of titanium oxide (TiO) appearing prominently.

Different lines appear and disappear at certain temperatures, as atoms in their ground state are unable to make certain atomic transitions, while completely ionized atoms have no absorption lines! So when you measure an absorption line in a star, you need to understand what its temperature is (and hence its ionization properties are) in order to rightfully conclude what the relative abundances of the elements are within it. If we go back to the Sun’s spectrum, with the knowledge of what the different atoms are, their atomic spectra, and their ionization energies/properties, what do we learn from that?

That, in fact, the elements that are found on the Sun are pretty much the same as the elements found on Earth, with two major exceptions: Helium and Hydrogen were both vastly more abundant than they are on Earth. Helium was many thousands of times richer on the Sun than it is here on Earth, and Hydrogen was about one million times more abundant on the Sun, making it the most common element there by far. It was only this combined understanding — of how color-and-temperature were related, how ionization was impacted by temperature, and how the strength of absorption lines were a function of ionization — that enabled us to figure out the relative abundances of the elements in a star.

And do you know who the scientist was who put all of that together? It was a 25 year-old woman who was never, during her lifetime, given the full credit for the discovery that she deserved.

Cecilia Payne, shown here as a young academic, had a remarkable astronomy career spanning more than 40 years at Harvard, and then she continued working at the Smithsonian. Her PhD thesis in astronomy was one of the most revolutionary ever written.

Meet Cecilia Payne (later Cecilia Payne-Gaposchkin), who did this work for her Ph.D. thesis way back in 1925. To understand how profound this work was, astronomer Otto Struve simply called it “undoubtedly the most brilliant Ph.D. thesis ever written in astronomy,” and few would disagree. Payne was just the second woman in history to earn her Ph.D. in astronomy through Harvard College Observatory, and in fact she had to move there to earn her Ph.D., as her original alma mater, Cambridge, didn’t award their first Ph.D. to a woman until 1948. However, Payne proved that her early work was no fluke; she wound up having a remarkable astronomy career, becoming the first female chair of a department at Harvard, the first female tenured professor at Harvard, and an inspiration to generations of astronomers: male and female both.

Unfortunately, the historical record was very unkind to Cecilia Payne for a very long time. Historically, it was Henry Norris Russell (the “Russell” of Hertzsprung-Russell fame) who was often given the credit for the discovery that the Sun is primarily composed of hydrogen. It was Russell himself who dissuaded Payne from publishing her conclusion — calling it “impossible” — only to wind up stating it himself, and taking credit for the discovery, four years later, in 1929.

Cecilia Payne, who had become Cecilia Payne-Gaposchkin by the time this photo was taken, became the first woman to be chair of a department at Harvard, as well as its first woman professor to achieve tenure. She, and her work, have inspired generations of astronomers of all genders.

Although history may have committed a great wrong against Cecilia Payne, we can make sure that astronomers and historians here in the 21st century (and beyond) get it right from now on. The contents of the Sun, and an understanding of how stars are classified — by temperature, ionization, and spectral features — can be understood as a result of Cecilia Payne’s work, for which she and she alone deserves full credit. The strength of the absorption lines you observe, combined with the surface temperature of stars and the known ionization properties of atoms, leave us with one inescapable conclusion: the Sun is a mass of primarily hydrogen (and, secondarily, helium), with all other elements representing only 1-2% of the rest of the star’s mass, total.

It would be years after Cecilia Payne’s work that we would figure out what it was that was indeed powering the Sun at its core (and all main sequence stars, for that matter): the nuclear fusion of these hydrogen nuclei into helium. All of our later, subsequent understanding of how this worked was made possible thanks to the remarkable work of Cecilia Payne, and her incredible insights into the workings and composition of stars, including how they relate to directly observable properties. It’s a pity that we still call the color-magnitude diagram the “Hertzsprung-Russell” diagram, but have little to nothing in astronomy named after Cecilia Payne. Perhaps, in time, we’ll right that historical wrong as well.