Episodes in the Development of the Hubble Galaxy Classification

Abstract

In March, David Block telephoned from Johannesburg with an interesting invita-tion to comment on the provenance of Hubble’s famous tuning fork galaxy classification diagram. Over the years he had often noticed that the diagram is not in Hubble’s definitive 1926 classification paper (Astrophysical Journal, 64, 321), and that it had only appeared in his semi-popular book “Realm of the Nebulae”, but with no antecedents mentioned. And then he made the central observation that a skeleton diagram, albeit turned 90 degrees, had first appeared in the 1929 edition of James Jeans’ “Astronomy and Cosmogony”. Could I comment? In Coda I, the question is addressed: Is there a duality of the classification depending on wavelength? No one can argue that there is no difference. The point is that the wavelength dependence of the morphology that is surely present at some level, is either dominantly serious for some problems but not for others. There also is a comment on the current thrust for new classifications by adding ever increasing detail. In Coda II, an insistance is made that there must be a strict separation between the pure morphologist and the theoretician. “Imagination” or “genius” or “intuition” provides the elusive opening with which to break the hermeneutical circle. Sherlock Holmes said it right in many places. “It is a capital mistake to theorize before you have all the evidence. It biases the judgement.” The discovery of a classification system, seeming so simple at the beginning is, in fact, extraordinarily complicated.