James Keeler declared that astrophysics seeks to ascertain the nature of heavenly bodies rather than their locations. This field applies principles from classical mechanics, electromagnetism, and thermodynamics to celestial objects.
William Hyde Wollaston and Joseph von Fraunhofer independently discovered dark lines in the solar spectrum during the nineteenth century. These regions showed less or no light when decomposing sunlight into its component parts.
George Ellery Hale and James Keeler established The Astrophysical Journal in 1895 alongside ten associate editors from Europe and United States. This journal helped bridge the gap between astronomy and physics at a time when astronomers measured positions while physicists studied matter on Earth.
Annie Jump Cannon expanded her work by 1924 into nine volumes containing over a quarter of a million stars. Her team originally prepared a catalog of over 10,000 stars grouped into thirteen spectral types starting in 1885.
Cecilia Helena Payne wrote an influential doctoral dissertation at Radcliffe College in 1925 applying Saha's ionization theory to stellar atmospheres. She discovered that hydrogen and helium were principal components of stars rather than Earth composition.