Joseph von Fraunhofer: The Glassmaker Who Ignited Astrophysics

By Kitty Ferguson

The celestial bodies—the sun, moon, and myriad stars—have captivated human imagination since time immemorial. The quest to measure their distances began in earnest during the 3rd and 4th centuries B.C. Even today, astronomers and astrophysicists utilize advanced telescopes and computational tools to explore the cosmos and seek answers to age-old questions.

However, one pivotal discovery in this quest emerged not from sophisticated technology or astronomical observations, but rather from the humble experiments of Joseph von Fraunhofer, a Bavarian glassmaker and researcher, who, 200 years ago, identified dark lines within the sunlight spectrum. Unbeknownst to him, this revelation would enable future scientists to gauge star distances and herald one of the most significant scientific breakthroughs—the acknowledgment of an expanding universe.

Born on March 6, 1787, in Straubing, Bavaria, Joseph was the youngest of eleven children, hailing from a lineage involved in glass production. After the early death of his parents, he apprenticed with the glassmaker Philipp Anton Weichselberger in Munich. Unfortunately, this apprenticeship proved to be a challenging experience, as Weichselberger imposed menial tasks and limited Joseph's access to the scientific literature he yearned to explore.

After two arduous years, fate intervened when Weichselberger's house collapsed, trapping Joseph inside. His dramatic rescue drew the attention of Prince-elector Maximilian IV, who subsequently took Joseph under his wing, providing him with the opportunity to study mathematics and optics. Maximilian also financially supported him to leave the unfulfilling apprenticeship and pursue his passion for optics.

Joseph’s quest for knowledge led him to the Optical Institute in Benediktbeuern, where he excelled in producing telescope lenses and surveying instruments. By the age of 20, he had authored his first significant scientific paper.

In 1814, while investigating light refraction, Fraunhofer sought to improve the accuracy of glass measurements. He recognized that a prism could transform white light into a spectrum of colors—a phenomenon known since ancient times. Isaac Newton had previously demonstrated that white light consists of various colors, with longer wavelengths appearing red and shorter wavelengths appearing violet.

Fraunhofer's work revealed that when white light passes through a prism, different wavelengths bend at varying degrees due to the medium's refractive index. This property led to the discovery that the colors in a spectrum are not sharply defined but blend into one another. His curiosity was piqued when he observed a distinct bright line in the spectrum produced by alcohol and sulfur flames. He then repeated the experiment using sunlight, replacing the light source with a narrow slit and measuring the results with a theodolite telescope.

He reported, “While searching for the bright line found in artificial light, I encountered instead a multitude of dark lines of varying thickness, some entirely black.” These lines remained constant under various experimental adjustments, indicating they were inherent to the solar light itself.

Fraunhofer published his findings, noting that the solar spectrum contained numerous dark lines, which he labeled, ultimately identifying 574 distinct lines. His research extended to bright stars, revealing similar dark lines that differed slightly in arrangement, leading him to conclude that these features were characteristics of the stars themselves, conveying information about their composition.

Despite his entrepreneurial success at the Institute, Fraunhofer's contributions went largely unrecognized during his lifetime. In 1821, after much debate regarding his lack of formal education, he was appointed an "extraordinary visiting member" of the Royal Bavarian Academy of Sciences. He received an honorary doctorate from the University of Erlangen in 1822 and was knighted by King Maximilian I Joseph in 1824.

Although he appeared vigorous, Fraunhofer's health deteriorated due to exposure to toxic materials in glass production, leading to his death from tuberculosis in June 1826 at the age of 39. His friend, Utzschneider, aptly remarked, “He brought us closer to the stars,” a testament to Fraunhofer's lasting impact on astronomy.

As scientific advancements unfolded in the mid-19th century, researchers like Gustav Kirchhoff and Robert Bunsen confirmed that Fraunhofer's lines represented the signatures of elements present in the sun's atmosphere. By analyzing these spectral lines, astronomers could determine the chemical composition of distant stars.

The concept of classifying stars based on their spectral lines began in the 1860s, leading to the understanding that these lines could reveal their absolute magnitudes. This knowledge was crucial for calculating distances to stars, ultimately supporting Edwin Hubble's groundbreaking discovery in the late 1920s that the universe is expanding.

Fraunhofer's innovative lenses and telescopes were unparalleled in their time, facilitating a deeper understanding of the cosmos. His experiments provided the foundation for measuring stellar distances, a vital step toward comprehending the vastness of the universe.

In conclusion, Joseph von Fraunhofer's legacy lies in his ability to uncover the hidden secrets of starlight, providing humanity with the tools to navigate the cosmos and ultimately understand our place within it.

• Aller, Lawrence H. Atoms, Stars and Nebulae. Cambridge University Press, 3rd Edition (1991).
• Danielson, D. The Book of the Cosmos: Imagining the Universe from Heraclitus to Hawking. Perseus Publishing (2000).
• Jackson, M. Spectrum of Belief: Joseph von Fraunhofer and the Craft of Precision Optics. The MIT Press (2000).
• Wolfgang, J. Fraunhofer in Benediktbeuern Glassworks and Workshop. Burton, Van Iersel & Whitney GmbH (2008).

Kitty Ferguson is the author of nine books of popular science, including Measuring the Universe.