While it wasn't a single discovery, the understanding that stars are primarily composed of hydrogen was developed through the work of many scientists over time. Here are the key figures and discoveries:

* 1860s: Gustav Kirchhoff and Robert Bunsen developed spectroscopy, a method to analyze the light emitted by celestial objects and identify their chemical composition.

* Early 1900s: Henry Norris Russell and Ejnar Hertzsprung developed the Hertzsprung-Russell diagram, which showed the relationship between stars' temperature and luminosity. This diagram revealed that most stars fall into a specific region called the "main sequence" and suggested a common composition.

* 1920s: Cecilia Payne-Gaposchkin used spectroscopy to analyze the light from stars and discovered that hydrogen and helium were the dominant elements in stellar atmospheres. Her PhD thesis, published in 1925, was initially met with skepticism but later recognized as a landmark achievement.

Therefore, it's inaccurate to attribute the discovery solely to one person. Cecilia Payne-Gaposchkin played a crucial role in providing the definitive evidence through her groundbreaking research using spectroscopy. However, it was the work of many scientists and the development of key tools and techniques that laid the foundation for this understanding.