Ejnar Hertzsprung was a Danish astronomer born on October 8, 1873. He is best known for his work in stellar classification and for developing the Hertzsprung-Russell diagram, which plots stars according to their brightness and temperature. This diagram has become a fundamental tool in understanding the life cycles of stars. Hertzsprung's contributions to astronomy extended beyond the diagram; he also studied the properties of variable stars and the distances to star clusters. His research helped lay the groundwork for modern astrophysics, influencing how scientists understand the evolution of stars and galaxies. Hertzsprung passed away on October 21, 1967.