How a Harvard maverick forever changed our concept of the stars

Astronomy, regarded as the oldest science, has always held a special place in humanity’s quest for understanding. Since ancient times, the night sky served as a fundamental laboratory for observation and inquiry. Before the advent of written language, people constructed stone circles to mark celestial events like the summer solstice and recorded lunar cycles on bone, weaving tales of the cosmos into their myths. As time progressed, humanity’s perception of the universe transformed dramatically—with the pivotal Copernican revolution in the 16th century shifting our understanding of Earth’s position in the cosmos. Yet, amid this legacy of exploration and learning, it was a woman who first unveiled the true essence of the stars.

Cecilia Payne-Gaposchkin, at just 25, made a groundbreaking revelation in 1925 regarding the composition of stars, identifying them primarily as hydrogen and helium, interspersed with trace amounts of other elements. This work represented one of the initial applications of the burgeoning field of quantum physics to astrophysical observations, igniting significant debate. Prevailing beliefs suggested that stars were analogous to hot versions of Earth, composed largely of iron, silicon, and other heavy elements. Payne-Gaposchkin’s findings challenged these entrenched ideas, urging prominent astronomers to rethink their conceptualizations of the universe.

Recognition of her insights took time, but eventually, the astronomical community embraced her findings.

“You can’t overstate the impact,” remarks astronomer David Charbonneau from Harvard University. By elucidating the fundamental makeup of stars, Payne-Gaposchkin opened pathways to understanding stellar formation and evolution, the origin of chemical elements, and the cosmos’s inception itself. “That has revolutionized our picture of the cosmos,” he added.

In the Era of Quantum Physics

Born in 1900 in England—the same year Max Planck began to unravel the quantum realm through his research on black-body radiation—Payne-Gaposchkin entered a world ripe with scientific advancement. The rediscovery of Gregor Mendel’s rules of inheritance was fostering the emergence of a new field: genetics. During this period, improvements in sanitation and medicine led to a dramatic decline in child mortality rates. By the mid-20th century, it was widely accepted that atoms comprised the universe – a notion that was only contentious until her early years.

Payne-Gaposchkin once expressed a youthful exuberance in her pursuit of knowledge, noting in her 1979 autobiography, The Dyer’s Hand, that she felt a sense of urgency to conduct research lest all the wonders of nature be uncovered before she could contribute.

She arrived at the University of Cambridge in 1919, where the field of atomic physics was still in its infancy, grappling with the intricacies of atomic structure and light interaction.

The realization—dating back centuries—that light disperses through a prism into a spectrum was foundational. In the early 1800s, William Hyde Wollaston illustrated this by revealing dark gaps in the sunlight spectrum that went unexplained until German scientists Robert Bunsen and Gustav Kirchhoff discovered that these gaps showed the unique signatures of specific chemical elements.

This phenomenon, linked to the quantum nature of atoms, arises because electrons exist in quantized energy states around an atom’s nucleus. The absorption of photons of specific energies allows electrons to move between these energy levels, producing a characteristic spectrum—a sort of chemical fingerprint for elements.

At the Cavendish Laboratory, a premier physics institution, Payne-Gaposchkin learned from luminaries like J.J. Thomson and Ernest Rutherford. The lab became a cradle of new ideas, particularly when physicist Niels Bohr introduced a new quantum understanding of the hydrogen atom. Payne-Gaposchkin was captivated, embracing the revolutionary concepts that would soon direct her gaze towards the stars.

Bridging Atomic Physics and Astronomy

After securing her position as a young female astronomer during a time when opportunities for women were limited primarily to teaching, Payne-Gaposchkin was offered a role at the Harvard Observatory in the U.S., where women had played crucial roles as “astronomical computers.” Funded by a fellowship dedicated to women in astronomy, she intended to conduct research for a year; however, this single year would extend into a lifelong career.

The story resonates with many individuals like Franciele Kruczkiewicz, an astrochemist from Leiden University, who found inspiration in Payne-Gaposchkin’s journey as she, too, pursued her dreams in a foreign land. The Harvard Observatory had been historically prolific in generating vast amounts of astronomical data through the use of glass plates sensitive to light, which were later utilized to capture stellar spectra.

Prior to Payne-Gaposchkin’s arrival, her predecessors had meticulously documented this spectral information. Among them was Annie Jump Cannon, who, in forming a classification system for stars based on their spectra, laid groundwork that remains relevant today. Yet, Payne-Gaposchkin, equipped with her atomic physics training and the observatory’s extensive data, set out to verify a new hypothesis.

At extreme temperatures, atoms tend to ionize, causing electrons to escape their nuclei and producing overlapping spectral lines. This phenomenon complicates the interpretation of stellar spectra, a fact not thoroughly considered until discovered by a distant astrophysicist, Meghnad Saha, who developed a formula linking the temperature and pressure of gases to the ionization of atoms.

While Payne-Gaposchkin was deepening her understanding of physics at Cavendish, Saha’s work laid essential groundwork for connecting stellar spectral observations to the temperature and pressure conditions within stars. He had hoped that someone like Payne-Gaposchkin would leverage this data for further discovery. In her subsequent two years at Harvard, Payne-Gaposchkin effectively brought Saha’s theoretical insights into practical application through meticulous research.

By applying Saha’s thermal ionization theories, she determined that the spectral classifications reflected temperature distinctions rather than compositional differences among stars. Subsequently, she utilized Saha’s equations to quantify the relative abundances of elements within stars and published her pivotal Ph.D. thesis in 1925, concluding that hydrogen and helium dominated the cosmic materials.

The simplest elements were the primary constituents of the universe.

The Enduring Impact of Payne-Gaposchkin

Much discussion surrounds the initial resistance encountered by Payne-Gaposchkin in the scientific community, specifically regarding how her accomplishments were overshadowed by her male contemporaries. Notably, the findings she established would be rediscovered by Henry Norris Russell, who received significant credit shortly thereafter. Many contemporary astronomers, including Kruczkiewicz, lament that they learned of her contributions only peripherally, often through media portrayals instead of direct academic teachings.

Nevertheless, as recognition grows, the significance of her contributions to both stellar composition and later works on variable stars and galactic structures is increasingly acknowledged as foundational to modern astrophysics. Kruczkiewicz affirms Payne-Gaposchkin as one of the pioneers of astrochemistry, citing her groundbreaking identification of universal composition.

“I consider her one of the first astrochemists, as she effectively mapped out the universe’s makeup,” Kruczkiewicz states. Emma Chapman, an astrophysicist at the University of Nottingham, notes her own work concerning primordial stars is indebted to Payne-Gaposchkin’s realization that the universe is rich in light elements. “Her contributions were vital in helping us understand not only what stars are but also how they differ from the Earth itself,” she emphasizes.

Payne-Gaposchkin’s discoveries are held in the same esteem as significant milestones in astrophysics, including the discovery of cosmic microwave background radiation and the identification of exoplanets. Yet, despite her monumental contributions, she would never receive a Nobel Prize, leaving many to ponder how history might differ had her gender been different.

Ultimately, Payne-Gaposchkin made history as Harvard’s first female full professor and later chaired its astronomy department. In her own words, she stated, “The truth will prevail in the end; nonsense will fall away under the force of an intellectual law of gravitation.”