classification de Hubble
1971 / Hubble's Classification of Non-Galactic Nebulae, 1922-1926 / R. Hart ; R. Berendzen / Journal for the History of Astronomy 2 (1971) p. 109 [lien]
1971 / [Notes] Hubble, Lundmark and the Classification of Non-Galactic Nebulae / R. Hart ; R. Berendzen / Journal for the History of Astronomy 2 (1971) p. 200 [lien]