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Astronomy Edwin Hubble Essay Research Paper THE (стр. 1 из 2)

Astronomy: Edwin Hubble Essay, Research Paper

THE JOURNAL OF THE ROYAL

ASTRONOMICAL SOCIETY OF CANADA

JOURNAL DE LA SOCI?T? ROYALE

D ASTRONOMIE DU CANADA

Vol. 83, No.6 December 1989 Whole No. 621

EDWIN HUBBLE 1889-1953

By Allan Sandage

The Observatories of the Carnegie Institution, Pasadena, California, U.S.A.

(Received September 22, 1989)

Hubble’s role. This year marks the centennial of the birth of Edwin Hubble. There can be no doubt that future historians, writing about the scientific advances of this age will describe the 20th century as epoch-changing in giving us the first correct view of how the universe is organized. The principal cosmological problem of discovering the large scale content of the universe was solved observationally between 1920 and 1936. Hubble was a major figure in this development. Knowledge that galaxies mark the space and provide the means to measure it was gained by the first convincing analysis of new data on the nature of the nebulae – knowledge that came directly from the sky rather than by dialectic discussion or revelation.

In Hubble’s time, the centre of observational work on the new astrophysics, and later on what we know as cosmology, was the Mount Wilson Observatory. The two largest telescopes in the world were there and could be regularly used on these problems. With his appointment to the Mount Wilson staff in 1919, Hubble had continuing access to both the 60-inch and the 100-inch Hooker reflectors.

He also had a most remarkable ability to cut to the core of unsolved problems concerning the nature of the nebulae. He would invariably proceed to the essence of a problem without stopping at the many lovely resting places that usually accompany the road to solutions, becoming the leading astronomer in the 1920s concerned with problems of the nebulae. In the 12 years from 1824 to 1936 he had set down the foundations upon which observational cosmology rests. From his central role in the solution of so grand a problem, Hubble has become a legend. But because part of his life has also become a myth, it is only from a study of his published papers that can we obtain a reasonable understanding of his enormous influence on the development of cosmology.

Hubble’s name is attached to many things of everyday astronomical life. There is Hubble’s zone of avoidance, the Hubble galaxy type, the Hubble sequence, the Hubble luminosity law for reflection nebulae, the Hubble luminosity profile for E galaxies, the Hubble constant, the Hubble time, the Hubble diagram, the Hubble redshift-distance relation, the Hubble radius for the universe, and now the Hubble Space Telescope. It seems appropriate in this centennial year to celebrate the memory of a scientist whom some have called the greatest astronomer (in changing paradigms) since the times of Galileo, Kepler and Newton. What did he do, and how did he do it?

Characteristics. It will be difficult for historians to write an accurate personal biography of Hubble using some of the extant archive sources. Known facts contradict part of the recollections set out in materials in the Hubble collection in the Huntington Library in San Marino, California, making it difficult to know which other parts are accurate. After his death, Mrs. Hubble, who survived him by 26 years, organized archive materials, also in the Huntington collection, around commentaries of his own, some of which glorify him in ways larger than life. In view of this, the authoritative historical essay by Osterbrock, Brashear, and Gwinn (1990) on Hubble’s education and career to about 1922 should be consulted. Their history is based on sources that are, as far as possible, independent of personal recollections, largely from letters and documents in the archives of the Yerkes and the Lick observatories.

But interesting as the personal aspects of the life of great scientists are in understanding how they arrive at solutions, the solutions themselves must be independent of the personality. Otherwise, the results have no objective reality. Yet the internal excitement in arriving at solutions is never this cold within the personality itself. Every scientist lives in a world of imagination. The grander the problem, the more wonderful must be the imagination. And Hubble lived with an ineffable problem – the discovery of the structure of the World on the largest scale. From this work, by him and by others of his generation, it is widely believed that some glimpse of a “creation event of the universe” became available to science by an objective method, not, as in other times, by metaphysics or speculation.

Hubble must have understood more clearly than anyone what he was dealing with and what he had accomplished. The realization that an understanding had finally been reached of the way the universe is organized, and above all, the discovery of its expansion, must have influenced in some way how Hubble lived with everyday events. His apparent aloofness, often mentioned by his critics, would, one suspects, not be so unusual in those who themselves could have been among the first to have had such a vision.

But we, in fact, know little of Hubble’s inner world. He was remarkably silent about the meaning of what he and Humason had found with their velocity-distance relation. Neither in his personal conversations nor in his writings did he discuss its implications for ideas about either the evolution of the universe from a primitive state or its meaning concerning a “creation event”. What little we can find of his thoughts about these matters – so common in the modern literature – must be inferred from his published papers.

Four central accomplishments. From 1922 to 1936 Hubble solved four of the central problems in cosmology, any one of which would have guaranteed him a position of the first rank in history.

(a). From 1922 to 1926 Hubble proposed a classification system for nebulae, both galactic (diffuse) and extragalactic. The galaxy classification system has become the Hubble morphological sequence of galaxy types.

(b). With his discovery of Cepheids in NGC 6822 in 1924, with parallel work in M33 and M31, Hubble settled decisively the question of the nature of the galaxies, whose correct solution, to be sure, had previously been given using what many believed to be inconclusive arguments, by Curtis, Lundmark, and ?pik.

(c). From 1926 to 1936 the distribution of galaxies, averaged over many solid angles, was determined to be homogeneous in distance. The test was made by showing that the coefficient of the log N(m) count distribution with magnitude has a value of about 0.6 at bright magnitudes. This proved that galaxies truly mark a space which is significant to the universe itself. Galaxy counts to the magnitude limit of the Mount Wilson 100-inch telescope were then used to attempt a measurement of the radius of curvature of space by finding deviations of the coefficient from the Euclidean value of faint magnitudes.

(d). The linear velocity-distance relation was set out in a discovery paper in 1929, followed by a series of papers with Humason between 1931 and 1936 that verified and extended the relation to large (i.e. 60,000 km s[-1] redshifts. This discovery lead to the notion of the expanding universe which is the centre-piece (being the necessary condition) for the cosmological models of the present day.

The published papers.

(a) The Hubble classification sequence.

No satisfactory classification system for galaxies existed before 1926, at which time two similar systems appeared in the literature, following discussions at the 1925 meeting of the International Astronomical Union in Cambridge, England. Before this time a purely descriptive system set up by Wolf (1908) had been used, but the classification showed no continuity between the Wolf types and was generally considered to be in need of revision. Hubble (1920) in his Ph.D. publication had remarked that the Wolf classification “while admittedly formal, offers an excellent scheme for temporary filing until a significant system shall be constructed” (emphasis added here), and later in the same publication “[the Wolf system] is wholly empirical and probably without physical significance, yet offers the best available system of filing away data and will later be on great service when a significant order is established” (again emphasis added here).

One cannot help but note that the construction of these two quotations shows that Hubble already possessed the confidence to accomplish what lay ahead when he would enter the field. This surety of language characterized much of his later writings – a surety which 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 the form of the luminosity function, provided that the objects are distributed homogeneously in distance.

With this knowledge, the early aim of Hubble’s work on counts was to determine the numerical value of the coefficient of the magnitude term. In the first discussion in his remarkable 1926 paper, Hubble shows that the data then known were consistent with the required value of 0.6, indicating homogeneity (his equation 10 from Table XVII of Hubble 1926b). He used data from the standard sources of counts then available, including the classical work on galaxy distribution by Seares (1925) – a generally neglected major discussion of what is now known as Hubble’s zone of avoidance.

But it was clear that the data could be fundamentally improved and carried to fainter magnitudes by using the enormous power of the 100-inch reflector. Building on the experience of his Ph.D. work, Hubble began a massive observing programme to do just that. The results began to appear in a series of papers that was to culminate in 1936 in the attempt to measure the curvature of space.

In his first paper, Hubble (1931) gives no hint of the direction which the problem would take toward the curvature determination when he teamed with R.C. Tolman in 1935. The 1931 announcement was simply an abstract of preliminary results from his new survey of galaxy counts made with the Mount Wilson telescopes.

The detailed paper on the distribution appeared three years later (Hubble 1934a). As in the Cepheid work ten years earlier, this paper was so thoroughly convincing that it brought the problem of the mean galaxy distribution, which by then was more than 100 years old, to a close. The paper has become a classic. Its power lies in the large amount of new data presented, and in Hubble’s straightforward, seemingly simple analysis of them – a trait characteristic of much of Hubble’s work. After presenting the data and the technical methods of reducing the material to “uniform plate conditions”, Hubble treats (1) the distribution in galactic latitude outlining the “zone of avoidance”, recovering Seares’ (1925) prior result, (2) the extinction in the poles (the famous cosecant distribution of the counts which has so confused modern discussions; see Noonan 1971 for a critique), (3) the tendency to cluster, based on the nature of the count residuals, field-to-field, the residuals being Gaussian in log N(m) rather than in N(m) itself, (4) the space density of galaxies, (5) the mean mass of galaxies, and (6) the mean density of matter in space of the order of 10[-30] g cm[-3]. Curiously, no mention of space curvature was made in this paper nor in the account of his Halley lecture (Hubble 1934b), although it was to be the major theme from then on.

Hubble’s interest in what Gauss and Karl Schwarzschild called experimental geometry can be traced to his collaboration with Tolman that must have begun in 1934. Their joint paper (Hubble and Tolman 1935), sets out how galaxy counts, conceptually, could be used to find the curvature of space by direct measurement. The principle is to determine if the volume encompassed within various “distances”, appropriately defined, increases at the rate of r[3], or more rapidly or more slowly than this Euclidean value.

The observational problem is complicated by the delicate corrections required to the data for the effects of redshifts, etc. But the grandeur of the conception and the carrying out of the programme still provokes the modern reader, despite the fact that the attempt failed because of large errors in the magnitude scales and what we now know to be the overwhelming effects of galaxy evolution in the look-back times.

The technical aspects of the methods need not be discussed here (cf. Sandage 1988 for that) nor the criticisms of them. More useful is a chronicle of Hubble’s progress in the curvature programme following his initial collaboration with Tolman. The problem still remains as a principal goal of observational cosmology. But because of the effects of galaxy evolution, galaxy counts are no longer considered to be the main source of data with which to solve it. Rather, we now attempt, in one way or another, to measure the deceleration of the expansion from which spatial density can be derived and hence the curvature from Einstein’s relativity equations.