Astronomy Edwin Hubble Essay Research Paper THE — страница 6

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believed almost immediately. Despite its astonishing content and its few data points, Hubble must have been quite certain of the result. In the paper immediately preceding Hubble’s, Humason (1929) reported the very large (for the time) redshift for NGC7619 of 3779 km s[-1], far larger than any redshift known before. From this result Hubble must have been certain that a significant phenomenon was at hand. All effort was then made at Mount Wilson to confirm and to extend the astounding possibility that the universe expands. By 1930 Humason (1931) had obtained redshifts of galaxies in clusters whose “velocities” were as high as 20,000 km s[-1]. In perhaps the most important paper on the series, Hubble and Humason (1931) showed beyond doubt (a) the existence of the effect, (b)

that it was linear with distance, and (c) that the brightest members of clusters are predominantly E galaxies (a major discovery related to galaxy and cluster formation). The work was extended to field galaxies soon thereafter (Hubble and Humason 1934), showing the generality of the phenomenon. By 1936 the work had been completed as far as it was to be done with the Mount wilson reflector, reaching redshifts of 40,000 km s[-1] for the Ursa Major No. 2 cluster (Humason 1936, Hubble 1936). Humason began the work again in 1949, using the Palomar 200-inch reflector, reaching 60,000 km s[-1] (Humason, Mayall and Sandage 1956) for the Hydra cluster, but was stopped from going further by the techniques of the time in the presence of the night sky air glow. Hubble (1953) symmarized the

work finally in his Darwin Lecture. (e). Other programmers. The previous subsections have outlined the four major subjects in which Hubble’s results were dominant in the 1930s. But he produced other works of influence as well, the results of which are also part of modern astronomical culture. (1). He solved the problem of the source of radiation and the nature of the spectra of diffuse nebulae, recognizing the difference between emission and reflection nebulae (Hubble 1922a, b), and proving that the source of radiation of reflection nebulae is an associated star. An elegant appreciation of the work is given by Greenstein (1951). (2). The surface brightness profiles of E galaxies were measured accurately for the first time (Hubble 1930), providing the basic model from which

later modifications and extensions of the profile laws would be derived by others. (3). He began the detailed study of the stellar content of the nearby galaxies. Besides the identification and measurements of Cepheids and very bright irregular variables in members of the Local Group, he made the unprecedented identification of globular clusters in M31 (Hubble 1932), starting an activity that occupies many present-day astronomers. (4). He discussed the sense of rotation of the spiral arms in individual galaxies. The most important papers, in which the solution of the problem was set out by identifying the near sides of galaxy images by the dust lane asymmetries, are Hubble (1935, 1943) and a paper with Mayall (Hubble and Mayall 1941). (5). In a most important paper, Baade and

Hubble (1939) found the nature of the Sculptor and Fornax dwarf E galaxies that had been announced by Shapley in 1938. Their discovery of RR Lyrae stars provided Baade with the crucial clue to his eventual population concept (cf. Sandage 1986 for a review). An Assessment. The principal surprises in rereading Hubble’s papers from the vantage point of 1989, after the discoveries of the Gamow, Alpher, Herman 3 K radiation, the development of radio astronomy, the discovery of how to age-date the stars, and the invention of the new cosmology of grand unification, are (1) the nature of Hubble’s methods, and (2) those central items that he hardly discussed but which seem so much a part of the cosmology that he pioneered. (1). Hubble’s methods were largely inductive – nearly pure

Baconian. His usual procedure was to assemble massive data sets from which he generalized to reach conclusions of wide scope that had continuing applications in further advances. Occasionally he did employ analytical methods such as in his analysis of the source of the light from diffuse nebulae (Hubble 1922b), his analysis of the flattening distribution of E galaxies (Hubble 1926b), his use of the Emden gravitationally bound gas sphere in understanding the luminosity of E galaxies (Hubble 1930), and his analysis of the galaxy counts for the space curvature (Hubble 1936c using the formalism of Tolman). But the method used in his most important papers – those papers that convincingly changed a field – was that of nearly pure Baconian induction. His success was remarkable, and