Astronomy Edwin Hubble Essay Research Paper THE — страница 3

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tended (and was intended) to conquer the field by prose as well as by the technical results. It was Hubble’s mastery of the language that gave some of his papers such dominance over prior work by others. Often the problem had in fact been solved, but without the same elegance of style, power of presentation, and excellence of summary possessed by Hubble when he was at his best. Clearly, the lesson for students is learn to write at the same time that you learn to do great science. The barest outline of a new galaxy classification scheme had been given by Hubble, almost as an aside, in his fundamental paper on the nature of diffuse galactic nebulae (Hubble 1922a). The scheme was expanded four years later and illustrated by two photographic panels (Hubble 1926b). Clarifying

comments (Hubble 1927) that answered criticisms by Reynolds (1927a, b), and a slightly expanded explanation set out in Chapter V of The Realm of the Nebulae (Hubble 1936d) was the extent to which Hubble amplified his 1926 discussion of the galaxy classification system. But the Hubble classification sequence has become so widely used that it is a curiosity to note that Hubble, in his reply to the comments by Reynolds, describes his principal announcement of the 1926 system merely as “a preface to some general statistical investigations.” Hubble, nevertheless, took his “preface” seriously. He guarded its priority in a revealing footnote in part I of his 1926 paper. There he comments on a classification system proposed at about the same time by Lundmark (1926, 1927). Some of

Hubble’s complaints, which he rarely made public, were unfounded, showing a sensitivity he generally kept hidden. Some of Hubble’s accusations are addressed in a partially justifiably acerbic reply by Lundmark (1927), also in a footnote, in Lundmark’s near great but largely neglected paper. The quite meagre descriptions of his powerful classification scheme which Hubble gave in the literature were, however, sufficient to teach the system to the community for which it was intended, showing the system’s simplicity, yet its power. Finally, note that the famous tuning fork diagram which summarizes the system at a glance did not appear in Hubble’s 1926 technical paper in The Astrophysical Journal, but rather only in his popular book The Realm of the Nebulae. (b). The

convincing proof that galaxies are island universes. Here the story is too well known, needing no detail in its retelling. Hubble’s discovery of Cepheids in NGC 6822, M33, and M31 was “simply” the final, albeit conclusive, demonstration that the arguments by Lundmark, Curtis, and ?pik that galaxies are external to the Milky Way were correct. But the situation was not, of course, so simple, given van Maanen’s measurements of proper motions. In any investigation, the greatness of any synthesis of data lies in knowing what clues to ignore. All who have ever tried know there is always a plenum of false clues in any work. Hubble’s ability to know which clues to trust, which to discard, and then which to use to tie up the facts to make a case, was superior. By ignoring van

Maanen’s result, Hubble’s demonstration of the presence of Cepheids with their period-luminosity relation was complete and final. Only three papers were written by Hubble on the problem, yet even with the first on NGC 6822 (Hubble 1925) he had brought the debate to a close. The papers on M33 (Hubble 1926a) and M31 (Hubble 1929a) showed the generality of the result, but no one seriously criticised the 1925 initial result. (c). Distribution of Galaxies in Space. With Hubble’s final proof that galaxies are beyond the Milky Way, the major problem then became whether they are fair markers of the universe, or if they are merely part of a hierarchical structure in a next rung up in the organization of matter. The solution rested on the way galaxies are distributed in distance. If

they increase in numbers in proportion to the surveyed volume (with no indication of an edge, as with stars in our galaxy), they would, then, clearly be the basic unit of the distribution. The obvious test could be made using galaxy counts to various magnitude limits. The purpose of the counts would be to find the rate of increase in galaxy numbers with increasing volume. From his work on the galaxy luminosity function (a recurring calibration throughout many of the papers, cf. Hubble 1926b, 1934b, 1936a, 1936b, 1936d) Hubble knew that galaxies have a spread in absolute magnitude. Nevertheless, as long as the luminosity function does not diverge at the faint end, the counts to different limits of magnitude will exhibit a distribution that varies as log N(m) ~ 0.6m, regardless of