classification de Hubble

[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]
[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]