Extra-Galactic Nebulae

A scientific publication written in the early 20th century. This work primarily focuses on the detailed classification and statistical investigation of extra-galactic nebulae, which are celestial formations distinct from those within our Milky Way galaxy. Hubble delves into the characteristics and distribution of these nebulae, making pivotal observations that contributed to our understanding of the cosmos. The opening of ''Extra-galactic Nebulae'' presents an abstract and introduction to the study, emphasizing a statistical analysis of 400 extra-galactic nebulae, observed and classified according to their visual magnitudes and structural forms. The classification reveals that approximately 97% of the nebulae exhibit regular forms, reflecting rotational symmetry around dominating nuclei, while a small fraction appears irregular in structure. Hubble discusses the relationships between luminosity, apparent size, and distance, providing a foundation for the subsequent detailed classification of different types of nebulae, which includes elliptical, spiral, and irregular categories, suggesting that these classifications may correspond to different evolutionary phases of astronomical bodies.