Final answer:
Annie Jump Cannon was responsible for the spectral classification system, having classified around 500,000 stars and contributing substantially to the field of astronomy.
Step-by-step explanation:
The individual who was largely responsible for the spectral classification system and the classification of the stars in the HD catalogue is Annie Jump Cannon. In the 1890s, Annie Jump Cannon, a pioneering astronomer, revised the existing stellar classification system to prioritize temperature, leading to the sequence O, B, A, F, G, K, M. She was hired at the Harvard Observatory, where she became exceptionally skilled at classifying the spectra of stars by visually examining photographic plates and managed to classify around 500,000 stars over her lifetime. While doing so, she made significant discoveries, including identifying 300 variable stars. Her contribution formed the basis of the Harvard Classification Scheme, which is an integral part of astronomic research and study even in the 21st century.