Hubble, Edwin Powell

(b. Marshfield, Missouri, 20 November 1889; d. San Marino, California, 28 September 1953)

observational astronomy, cosmology.

Hubble was the founder of modern extragalactic astronomy and the first to provide observational evidence for the expansion of the universe. The son of John Powell Hubble, a lawyer, and the former Virginia Lee James, he spent his early years in Kentucky and attended high school in Chicago, where his father was in the insurance business. At school he excelled both in his studies and in athletics. He won a scholarship to the University of Chicago, where he came under the influence of the eminent physicist R. A. Millikan and of the astronomer G. E. Hale, who inspired in him a love of astronomy. Hubble received a B.S. in mathematics and astronomy and also made his mark on the campus as a heavyweight boxer (he was six feet, two inches tall). A sports promoter wanted to train him to fight Jack Johnson, the world champion, but instead Hubble went to Queen’s College, Oxford, in 1910 as a Rhodes scholar from Illinois.

At Oxford, Hubble first thought of reading mathematics; but after studying some of the final examination papers, he concluded that they were too specialized for his liking and instead decided to read jurisprudence. He took his B.A. in that subject in 1912. Hubble had a great love of England and was interested in the common law of the country from which his ancestors had emigrated in the seventeenth century. While at Oxford he was awarded a blue for track events and boxed in an exhibition match with the French champion, Georges Carpentier.

In 1913 Hubble returned to the United States, was admitted to the bar, and opened a law office at Louisville, Kentucky. After a short while he abandoned this career and in 1914 went to the Yerkes Observatory of the University of Chicago, where he was an assistant and a graduate student under E. B. Frost. He was awarded the Ph.D. in 1917 for a thesis entitled “Photographic Investigations of Faint Nebulae,” in which he considered the classification of nebular types and concluded that planetary nebulae are probably within our sidereal system and the great spirals outside; but these questions, he said, could be decided only by instruments more powerful than those currently available.

Hubble’s powers as an observer attracted the attention of Hale during a visit to Yerkes; Hale offered him a post at the Mount Wilson Observatory, where the sixty-inch reflector was then in operation and the 100-inch under construction. Meanwhile the United States had entered World War I, and Hubble had immediately enlisted as a private in the infantry. He therefore telegraphed Hale that he would accept his offer as soon as he was demobilized. He served with the American Expeditionary Force in France and rose to the rank of major. After the Armistice he remained with the authmu of 1919. On his return to the United States in October, he joined Hale on Mount Wilson, as he had promised. At last, at the age of thirty, he settled down to the work that was to bring him fame.

Hubble’s earliest investigations at Mount Wilson were made with the sixty-inch telescope and concerned galactic nebulae. In one of his earliest papers, “A General Study of Diffuse Galactic Nebulae,” he suggested a classification system based upon fundamental differences between galactic and nongalacticnebulae. He discovered many new planetary nebulae and variable stars, but the most important result of his early researches concerned the origin of the radiation from diffuse galactic nebulae. Hubble showed that they were made luminous by certain stars assoiated with them, the nebulosity consisting of clouds of atoms and dust not hot enough to be selfluminous. He discovered a relation between the luminosity of a diffuse galactic nebula and the magnitudes of the associated stars and showed that the gases were excited and made luminous by neighboring blue stars of high surface temperature.

The Hooker 100-inch telescope came into operational use at about the time Hubble arrived on Mount Wilson. This was a most fortunate circumstance, for the crucial contributions made to cosmology by Hubble required the full light-gathering power and resolution of this instrument. From about 1922 he turned his attention more and more to objects that we now regard as lying beyond our own stellar system.

Hubble’s first great discovery was made when he recognized a Cepheid variable star in the outer regions of Messier 31, the great nebula in Andromeda in a plate that the he took on 5 October 1923. This proved to be the long-sought means of settling the problem of the status of the spiral nebulae that had puzzled astronomers for three-quarters of a century. The use of Cepheid variable stars as distance indicators had been suggested more than ten years earlier by Henrietta Leavitt of the Harvard College Observatory, and they had been used with great effect by Harlow Shapley to determine the distance and dimensions of the globular star clusters that surround the Milky Way. Hubble’s discovery was the first sure indication that the Andromeda nebula lies far outside our own stellar system.

Controversy on this question had previously culminated in the famous Shapley-Curtis debate held before the National Academy of Sciences on 26 April 1920, neither side convincing the other. Curtis had argued that “the spirals are not intragalactic objects but island universes, like our own galaxy, and that the spirals, as external galaxies, indicate to us a greater universe into which we may penetrate to distances of ten million to a hundred million light years.” Shapley rejected this conclusion. He maintained that there was no reason “for modifying the tentative hypothesis that the spirals are not composed of typical stars at all, but are truly nebulous objects,” The strongest argument for this view was evidence obtained by Adriaan van Maanen that Messier 101 rotated through 0.02 seconds of arc in a year and that Messier 33 and 81 rotated at comparable rates. These large angular velocities implied relativly small distances, of the order of a few thousand light-years. (The spurious nature of van Maanen’s measurements was finally established in 1935, when it was conclusively shown by Hubble that they arose from obscure systematic errors and did not indicate motion in the nebulae concerned.)

By the end of 1924 Hubble had found thirty-six variable stars in M 31, twelve of which were Cepheids. From the latter he derived a distance of the order of 285,000 parsecs, or about 900,000 lightyears, whereas the maximum diameter of the Milky Way stellar system was known to be in the order of 100,000 light-years. The public announcement of Hubble’s discovery was made at a meeting of the American Astronomical Society in Washington, D.C., at the end of December 1924. Hubble was not present; but Joel Stebbins recalled many years later that when Hubble’s paper had been read, the entire Society knew that the debate had come to an end, that the island-universe concept of the distribution of matter in space had been established, and that an era of enlightenment in cosmology had begun. Both Shapley and curtis were present.

The way was now open for a new attack on the cosmological problem which had hitherto been the concern of theoretical investigators. Two lines of research were possible for the observer to pursue, and Hubble was a pioneer in both. On the one hand, he studied the contents and general structure of galoxies. On the other, he investigated their distribution in space and their motion. Both approaches were strongly motivated by his belief that galaxies are the structural units of matter that together constitute the astronomical universe as a whole.

Hubble was the first to introduce a significant classification system for galaxies. He presented this at the meeting of the International Astronomical Union at Cambridge, England, in 1925 and it was published the next year in the Astrophysical Journal. This system is the basis of the classification still used. Hubble found that most galaxies showed evidence of rotational symmetry about a dominating central nucleus, although a minority, amounting to not more than 3 percent of those he studied, lacked both these features. He called the two types “regular” and “irregular,” respectively. He found that the regular galaxies fell into two main classes—“spirals” and “ellipticals”—and that each class contained a regular sequence of forms. One end of the elliptical sequence was found to be similar to one end of the spiral sequence. The spirals were subdivided into two parallel subsequences, normal and barred. The classification was essentially empirical and independent of any assumptions concerning the evolution of galaxies.

In addition to studying the shapes of galaxies, Hubble explored their contents and brightness patterns. In the nearer galaxies he discovered and studied almost every kind of intrisically bright object known in our own system: novae globular clusters, gaseous nebulae, super-giant blue stars, red long-period variables, Cepheids, and so on.

Despite the advance in knowledge in the last forty years, Hubble’s claim to have introduced order into the apparent confusion of nebular forms and to have shown that galaxies are closely related members of a singly family stands. It must be regarded as one of his most significant achievements.

During the late 1920’s Hubble’s main preoccupation was to determine a reliable extragalactic distance scale to the limits of observation. This was the essential preliminary to any serious investigation of the distribution of galaxies in space and its bearing on the cosmological problem. The philosophy underlying his approach to this problem had previously been summarized by him in his first detailed paper on an extragalatic system (NGC 6822), the distance of which was obtained by the Ceoheid criterion. Hubble’s use of the Cepheid period-luminosity law (which enabled him to regard these stars as distance indicators) was based on an appeal to the principle of the uniformity of nature. “This principle,” he wrote, “is the fundamental assumption in all extrapolations beyond the limits of known and observable data, and speculations which follow its guide are legitimate until they become self-contradictory.”

On this basis, Hubble proceeded to estimate the distance of galaxies beyond the “local group” in which Cepheids could be detected with the 100-inch telescope. He argued that with increasing distance one could expect the Cepheids to fade out first, then the irregular variables, then the blue giants, until only the very brightest of stars would be seen. He found that the data, although somewhat meager, indicated that the very brightest stars in late-type spirls are of about the same absolute luminosity. This upper limit of stellar luminosity appeared to be about 50,000 times that of the sun. The “brightest star” criterion of distance enabled Hubble to extend the extragalactic distance scale to about 6,000,000 lightyears. In view of the criticism to which this criterion has been subjected since Hubble’s day, it should be noted that he was fully aware that a risk was involved in regarding the images in question as individual stars; but he pointed out that, regardless of their real nature, the objects selected a brightest stars appeared to represent strictly comparable bodies. (In 1958 Allan Sandage showed that they are bright couds of ionized hydrogen.)

To extend the distance scale farther, Hubble used information gained from the fact stars could be detected in some of the spirals in the great Virgo cluster. Analysis of this large sample collection provided average characteristics of galaxies which could be used as statistical criteria of distance for more remote galaxies. For measurements of the depths of space, Hubble concentrated on the brightest members of clusters of galaxies. He regarded the clusters as so similar that the mean luminosity of the ten brightest members or even the individual luminosity of, say the fifth-brightest member formed a convenient measure of distance. In this way he build up his distance scale to 250 million light-years.

By 1929 Hubble had obtained distances for eighteen isolated galaxies and for four members of the Virgo cluster. In that year he used this somewhat restricted body of data to make the most remarkable of all his discoveries and the one that made his name famous far beyond the ranks of professional astronomers. This was what is now known as Hubble’s law of proportionality of distance and radial velocity of galaxies. Since 1912, when V. M. Slipher at the Lowell Observatory had measured the radial velocity of a galaxy (M 31) for the time by observing the Doppler displacement of its spectral lines, velocities had been obtained of some forty-six galaxies, forty-one by Slipher himself. Attempts to correlate these velocities with other properties of the galaxies concerned, in particular their apparent diameters, had been made by Carl Wirtz, Lundmark, and others; but no definite, generally acceptable result had been obtained. In 1917 W. de Sitter had constructed, on the basis of Einstein’s cosmological equations, an ideal world-model (of vanishingly small average density) which predicted red shifts, indicative of recessional motion, in distant light sources; but no such systematic effect seemed to emerge from the empirical data. Hubble’s new approach to the problem, based on his determinations of distance, clarified an obscure situation. For distances out to about 6,000,000 light-years he obtained a good approximation to a straight line in the graphical plot of velocity against distance. Owing to the tendency of individual proper motions to mask the systematic effect in the case of the nearer galaxies, Hubble’s straight-line graph depended essentially on the data obtained from galaxies in the Virgo cluster. These indicated that over the observed range of distance, velocities increased at the rate of roughly 100 miles a second for every million light-years of distance (500 kilometers a second for every million parsecs).

Further progress depended on the extension of the observations to greater distances and fainter galaxies. The spectroscopic part of the work was undertaken by Milton L. Humason, Hubble’s colleague at Mount Wilson. Within two years, with the aid of a new type of fast lens suitable for the difficult task of photographing the exceedingly faint spectra of remote galaxies, Hubble’s law was extended to a distance of over 100 million light-years, the straight-line relationship between velocity and distance being maintained. This result has come to be generally regarded as the outstanding discovery in twentieth-century astronomy. It made as great a change in man’s conception of the universe as the Copernican revolution 400 years before. For, instead of an overall static picture of the cosmos, it seemed that the universe must be regarded as expanding, the rate of the mutual recession of its parts increasing with their relative distance.

Hubble’s discovery stimulated much theoretical work in relativistic cosmology and aroused great interest in fundamental papers on expanding world models by A. Friedmann and G. Lemaître that had been written several years before but had attracted little attention. The interpretation of the straight line in Hubble’s graph of velocity against distance and of its slope were eagerly discussed. The constant ratio of velocity to distance is now usually denoted by the letter H and is called Hubble’s constant. It has the dimensions of an inverse time—its reciprocal, according to Hubble’s original determination, being approximately two (since revised to about ten) billion years. If the galaxies recede uniformly from each other, as was suggested by E. A. Milne in 1932, this could be interpreted as the age of the universe; but, whatever the true law of recessional motion may be, Hubble’s constant is generally regarded as a fundamental parameter in theoretical cosmology.

In the early 1930’s Humason obtained red shifts indicating velocities of recession up to about one-seventh the velocity of light. This was remarkably high for astronomical objects; and Hubble tended to prefer the neutral term “red shift” to “velocity of recession,” since he believed that, although no other explanation could compete with the Doppler interpretation of the spectra, it was possible that some hitherto unrecognized principle of physics may be responsible for the effects observed. This became a central problem for him in the course of the 1930’s and was one of the objectives of his detailed investigations of the distribution of galaxies. These investigations were of two kinds: surveys of large areas of the sky penetrating to moderate depths, and surveys of selected small areas to the limits of observability.

Hubble’s study of the large-scale distribution of galaxies over the sky produced two important results. At first sight, this distribution appeared to be far from isotropic. No galaxies were found along the central region of the Milky Way, and outside the zone of avoidance the number of galaxies observed appeared to increase with galactic latitude. Hubble showed that these observations could be explained as the effect of an absorbing layer of diffuse matter surrounding the main plane of the Milky Way, and that when this effect was taken into account there were no significant major departures from isotropy in the distribution of galaxies. These conclusions were of great significance for the structure of our own galaxy and also for cosmology because they strengthened the case for regarding the system of galaxies as constituting the general framework of the universe.

In regard to the distribution of galaxies in depth, a preliminary reconnaissance by Hubble indicated that this was uniform. Guided by this information, surveys were made by him and by N. U. Mayall to determine the total number of galaxies in a square degree of the sky brighter than certain limiting magnitudes—for instance, nineteenth or twentieth magnitude. The analysis of these surveys presented Hubble with a difficult theoretical problem, and he enlisted the support of R. C. Tolman, a distinguished theoretical physicist and relativity expert at the California Institute of Technology, Pasadena. The crux of the problem concerned the statistical relationship between apparent brightness and distance; but the apparent brightness of a remote galaxy, corrected for all “local” effects such as the dimming due to interstellar absorption of light in our own system, depends not only on the intrinsic brightness of the galaxy but also on its red shift; and the effect of this is greater for the more remote, and therefore fainter, galaxies. (Moreover, the intrinsic brightness of a remote galaxy when the light left it may not be the same statistically as at later epochs.) The red shift, whatever its cause, diminishes the energy of the light from a galaxy and makes it appear fainter than would be the case otherwise. Moreover, the true absolute magnitude (the bolometric magnitude) depends on the total radiation of all wavelengths, whereas the magnitude registered on the photographic plate is confined to certain parts of the spectrum; and the red shift complicates the problem of converting from photographically determined apparent magnitudes to bolometric magnitudes.

As a result of his investigations with Tolman, Hubble was inclined, from about 1936, to reject the Doppler-effect interpretation of the red shifts and to regard the galaxies as stationary. He claimed that uniformity of distribution in depth was compatible with this assumption. On the other hand, if the galaxies are receding, uniformity in depth can be reconciled with the observations only if there is also a positive curvature of space, the required radius being about 500 million light-years, which was actually less than the range of the 100-inch reflector for normal galaxies. Theoretical cosmologists, notably G. C. McVittie in the late 1930’s and Otto Heckmann in the early 1940’s, criticized Hubble’s analysis and rejected his conclusions but respected his observational achievements.

One of the curiously baffling problems concerning galaxies that engaged Hubble’s attention related to the sense of rotation of spiral arms. According to some theoretical astronomers, notably Bertil Lindblad, these arms opened up in the same sense as they rotated about the nucleus, whereas other astronomers believed that they trailed. The question was difficult to resolve, because if a galaxy is seen at the right orientation to observe the arms clearly, it is not easy to tell which is the near and which the far side. With his intimate knowledge of galaxies, Hubble selected as a favorable test object NGC 3190 and in 1941 obtained the necessary spectroscopic and photographic material with the 100-inch reflector. He concluded that there was no reason to doubt that this spiral trails its arms. In the last year of his life, radio and optical evidence was forth-coming that the same situation prevails in our own galaxy.

In 1942 war again caused Hubble to divert his energies from astronomy. He had long been aware of the dangers that threatened the free world and was chairman of the Southern California Joint Fight for Freedom Committee. After the United States entered the war, he sought active service in the army but was asked instead by the U.S. War Department to become chief of ballistics and director of the Supersonic Wind Tunnel Laboratory at the Aberdeen Proving Ground, Maryland. He remained there until 1946 and was awarded the Medal of Merit for his services.

After the war Hubble devoted much time to plans relating to the Hale 200-inch telescope. He became chairman of the Research Committee for the Mount Wilson and Palomar Observatories and was largely responsible for planning the details of the Palomar Observatory Sky Survey that was made with the forty-eight-inch Schmidt telescope. Toward the end of 1949 the 200-inch was at last available for full-time observation, and Hubble was the first to use it. The first major advance after its introduction was Baade’s discovery that all extragalactic distances had been underdetermined by a factor of about two. One of the reasons for this conclusion went back to Hubble’s discovery in 1932 that the globular clusters in M 31 appeared to be, on the average, four times fainter than those in our own galaxy.

During the last years of his life Hubble suffered from a heart ailment. He died suddenly in 1953 from a cerebral thrombosis while preparing to go to Mount Palomar for four nights of observing.

A man of wide interests, Hubble was elected a trustee of the Huntington Library and Art Gallery in 1938. He bequeathed his valuable collection of early books in the history of science to Mount Wilson Observatory. He was a skilled dry-fly fisherman and fished in the Rocky Mountains and also on the banks of the Test, near Stockbridge, Hampshire, where he and his wife (the former Grace Burke, whom he married in 1924) used to stay with English friends.

Hubble’s great achievements in astronomy were widely recognized during his lifetime by the many honors conferred upon him. He gave the Halley lecture at Oxford in 1934, the Silliman lectures at Yale in 1935, and the Rhodes lectures at Oxford in 1936. In 1948 he was elected an honorary fellow of Queen’s College, Oxford, in recognition of his notable contributions to astronomy.

Hubble’s work was characterized not only by his acuity as an observer but also by boldness of imagination and the ability to select the essential elements in an investigation. In his careful assessment of evidence he was no doubt influenced by his early legal training. He was universally respected by astromers, and on his death N. U. Mayall expressed their feelings when he wrote: “It is tempting to think that Hubble may have been to the observable region of the universe what the Herschels were to the Milky Way and what Galileo was to the solar system.”

BIBLIOGRAPHY

I. Original Works. Hubble’s Halley lecture, delivered at Oxford in 1934, was published as Red Shifts in the Spectra of Nebulae (Oxford, 1934). His Silliman lectures, delivered at Yale University in 1935, appeared as The Realm of the Nebulae (Oxford, 1936). His Rhodes memorial lectures, delivered at Oxford University in 1936, were published as The Observational Approach to Cosmology (Oxford, 1937). His Penrose memorial lecture was published as “Explorations in Space: The Cosmological Program for the Palomar Telescopes,” in Proceedings of the American Philosophical Society, 95 (1951), 461–470; and his George Darwin lecture as “The Law of Red-Shifts,” in Monthly Notices of the Royal Astronomical Society, 113 (1953), 658–666.

At the time of his death Hubble was preparing an atlas of photographs to illustrate his revised classification of the galaxies based on a careful study of the magnificent set of plates that he had accumulated between 1919 and 1948 with the sixth-inch and 100-inch telescopes at Mount Wilson Observatory. The details of his classification were not completed when he died; and responsibility for publication was taken by Allan Sandage, who worked with Hubble in the last years of Hubble’s life. Sandage has explained his role in this publication in the following statement: “I have acted mainly as an editor, not as an editor of a manuscript but rather an editor of a set of ideas and conclusions that were implied in the notes.” The work was published by Sandage as The Hubble Atlas of Galaxies (Washington, D. C., 1961).

Most of Hubble’s original papers were published in Astrophysical Journal and were also issued as Contributions From the Mount Wilson Solar Observatory.

II. Secondary Literature. Among the numerous biographical notices the most informative are the following: Walter S. Adams, “Dr, Edwin P. Hubble,” in Observatory, 74 (1954), 32–35; M. L. Humason, “Edwin Hubble,” in Monthly Notices of the Royal Astronomical Society, 114 (1954), 291–295; N. U. Mayall, “Edwin Hubble—Observational cosmologist,” in Sky and Telescope13 (1954), 78–81, 85; and H. P. Robertson, “Edwin Powell Hubble: 1889–1953,” in Publications of the Astronomical Society of the Pacific, 66 (1954), 120–125.

G. J. Whitrow

Edwin Powell Hubble

The American astronomer Edwin Powell Hubble (1889-1953) established the scale of the universe and laid the observational basis for the cosmological theory of the expanding universe.

Edwin Hubble was born on Nov. 20, 1889, in Marshfield, Mo., where his father, a lawyer, was in the insurance business. Hubble received scholarship aid to go to the University of Chicago. He chose law for a career, and after receiving his bachelor's degree in 1910, he went as a Rhodes scholar to Oxford University, England. In 1913 he returned to the United States, was admitted to the bar in Kentucky, and practiced law for about a year in Louisville.

Quite suddenly, Hubble decided that he would devote his life to astronomy, and in 1914 he left for the University of Chicago's Yerkes Observatory in Williams Bay, Wis. In 1917 he completed his doctorate and enlisted in the infantry. He served in France as a line officer in the American Expeditionary Force.

Early Work at Mount Wilson

As a student at Chicago, Hubble had attracted the attention of the well-known astronomer G. E. Hale, and after the war Hale offered him a staff position at Mount Wilson Observatory near Pasadena, Calif. Except for the period 1942-1946, when Hubble was with the Ordnance Department in Aberdeen, Md., he was connected with the Mount Wilson Observatory for the rest of his life.

Hubble's early observations at Mount Wilson were made with its 60-inch reflecting telescope and concentrated on objects within our own galaxy, for example, novae, nebulous stars, and variable stars. Gradually he began to observe more distant objects. To determine the distances of the spiral nebulae (galaxies), he used Cepheid variable stars. This method derived from Henrietta S. Leavitt's 1912 discovery that the period of variation in the intensity of these stars is directly related to their absolute magnitude, so that by measuring the former, one may easily determine the latter. By knowing the star's absolute magnitude and measuring its apparent magnitude, its (relative) distance may be readily calculated from the inverse-square law.

In 1923 Hubble definitely recognized a Cepheid variable in the Andromeda Nebula, known to astronomers as M31. Others were soon found in M31 and its companion nebula M33. To obtain his photographs, Hubble used Mount Wilson's 100-inch telescope. Once he had located the variables and determined their periods and apparent magnitudes, he used Leavitt's period-luminosity relationship to determine their distances. He concluded that the great spiral Andromeda Nebula is roughly 900,000 light-years away, a fantastically large distance, placing it clearly outside our own galaxy and proving that, in general, galaxies are islands in the universe. To allow for interstellar absorption, Hubble's distance estimate had to be later reduced to roughly 750,000 light-years, a figure that stood until shortly before Hubble's death.

Hubble continued to determine galactic distances and to study galactic characteristics. By 1925 he had enough observations to propose a scheme for their classification: he imagined concentrated, very luminous, spheroidal galaxies to merge into ellipsoidal ones, which in turn branched into "normal spirals" on the one hand, and "barred spirals" on the other. Hubble tended to avoid drawing evolutionary conclusions from his scheme, but it was clearly very suggestive in that direction. Furthermore, it proved invaluable in statistical studies of the universe. At the time of his death, Hubble was attempting to revise his scheme in order to make it more complete.

Expanding Universe

In the late 1920s Hubble laid the observational groundwork for the most spectacular astronomical discovery of this century: the expanding universe. V. M. Silpher had, over a period of years, made spectroscopic observations on tens of nebulae (galaxies) which indicated, on the basis of the Doppler shifts recorded, that these nebulae were receding from the earth at velocities between roughly 300 and 1,800 kilometers per second. Hubble realized the great importance of Silpher's observations for cosmological theories and organized a plan for measuring both the distances and (radial) velocities of as many galaxies as possible, down to the faintest ones detectable with Mount Wilson's 100-inch telescope.

While an assistant, M. L. Humason photographed galactic spectra and analyzed the observed Doppler shifts. Hubble photographed the galaxies themselves, searched for Cepheid variable stars, and computed the distances to the galaxies. By 1929 Hubble had distance data on Silpher's nebulae and announced what became known as Hubble's law: the velocity of recession of a galaxy is directly proportional to its distance from the earth. By the early 1940s this law had been confirmed for galactic velocities up to roughly 45,000 kilometers per second, corresponding to galactic distances up to roughly 220 million light-years.

During the 1930s Hubble became more and more cautious over the interpretation to be placed on the observed Doppler displacements, preferring to refer to them by the neutral (theory-free) term "red shifts." Thus, if at some future time these red shifts were found to be due, not to recessional velocity, but to some presently unknown physical law, the term "red shift" could still be retained as a description.

Postwar Work

After World War II Hubble devoted a great deal of time to planning the research program of the 200-inch Hale telescope at Mount Palomar; he was almost entirely responsible for conceiving and executing the National Geographic Society-Palomar Observatory Sky Survey carried out with the 48-inch Schmidt telescope. He received many honors, including a number of honorary degrees and medals, as well as membership in the National Academy of Sciences and other honorary societies. For his war research he received the Medal of Merit for 1946. In 1948 he was elected an honorary fellow of Queen's College, Oxford. He died of a coronary thrombosis in San Marino, Calif., on Sept. 28, 1953. In 1990, NASA launched the Hubble Space Telescope, which was named in his honor.

Further Reading

Hubble discusses his own work in The Realm of the Nebulae (1937) and Observational Approach to Cosmology (1937). For brief treatments of his life and work see Bernard Jaffe, Men of Science in America (1944; rev. ed. 1958); Otto Struve and Velta Zebergs, Astronomy of the 20th Century (1962); and Harlow Shapley, Through Rugged Ways to the Stars (1969). □

Edwin Powell Hubble

1889-1953

American Astronomer

Edwin Hubble's contributions opened the way to modern cosmology, the study of the universe. He first realized that many of the nebulae, often seen only as faint fuzzy patches in the sky, were actually separate galaxies like our own Milky Way. He also discovered that the universe is expanding. This eventually led to the theory that it began in a massive explosion known as the Big Bang.

Hubble was born on November 20, 1889, in Marshfield, Missouri. As an undergraduate at the University of Chicago, he earned degrees in both mathematics and astronomy. Despite his interest in science, he had promised his dying father he would study law. He went to Oxford University as a Rhodes Scholar, graduating in 1912. Having found that legal work did not hold his interest, he dissolved his Kentucky practice soon after he established it. He did retain from his Oxford experience a rather affected English accent and a taste for Savile Row tailoring—clothes made by the finest tailors in London.

Resuming his interrupted study of astronomy, Hubble returned to the University of Chicago. He received his Ph.D. in 1917. After serving in World War I, he obtained a staff position at the Mt. Wilson Observatory in California, where he worked with the 100-inch telescope there. At the time it was the most powerful telescope in the world.

It was in studying a type of variable stars called Cepheids that Hubble founded the discipline of extragalactic astronomy. Cepheids are particularly useful objects to observe because the period of their variability is related to their absolute magnitude, or intrinsic brightness. By comparing their absolute magnitude to their brightness as seen from Earth, it is possible to determine their distance. Hubble was able to observe Cepheids within some of the nebulae. These fuzzy objects were assumed to be within our Milky Way galaxy, which was the entirety of the known cosmos. To his surprise, his calculations indicated they were millions of light years away. In fact, he announced in 1924, those nebulae were "island universes," galaxies in their own right.

This was a major paradigm shift for astronomers, but Hubble quickly went about classifying and studying the galaxies he could observe. In doing so, he made another spectacular discovery. Like the falling pitch of a whistle on a train passing by, the light from the galaxies was Doppler-shifted toward the red end of the spectrum. This indicated that the galaxies were speeding away from Earth. What's more, the farther away they were, they faster they were receding. The ratio of their speed to their distance was a number now called Hubble's constant, or H₀.

The importance of this work to cosmology can hardly be overstated. The assumption had always been that the universe was static and eternal. Suddenly the galaxies were found to be flying off in every direction. There was no reason that they should be fleeing the vicinity of Earth in particular. The only logical answer was that everything was receding from everything else. In other words, the universe was expanding, with the galaxies moving apart like raisins in a rising loaf of bread. Working backward from Hubble's constant, it was possible to estimate an age for the universe. Although the value for H₀ has been repeatedly revised and remains in dispute, the universe is generally believed to have been expanding for 10 to 15 billion years.

Hubble continued to study galaxies for the rest of his life, except for a hiatus during World War II, when he enlisted as head of ballistics at the Aberdeen Proving Ground in Maryland. His accomplishments brought him fame; he was popular with tourists as well as Hollywood celebrities, and he received many honors and awards. Only the Nobel Prize eluded him, despite his hiring a publicity agent in the 1940s, because there was no prize in astronomy at the time. Eventually the Nobel Committee decided that astronomy was a branch of physics, but it was too late for Hubble. He died on September 28, 1953, in San Marino, California.

SHERRI CHASIN CALVO

Hubble, Edwin Powell (1889–1953) US astronomer. In 1925, he published his classification of galaxies and his discovery that spiral nebulae were resolvable as independent star systems. Using the telescope at Mount Wilson Observatory, he measured the distance to the Andromeda Galaxy. He attributed the red shift of the spectral lines of galaxies to their recession, and hence to the expanding Universe. In 1929 he proposed a linear relation between the distance of a galaxy from Earth and their velocity of recession (Hubble's law). See also galaxy; nebula

Hubble surname of the American astronomer Edwin Hubble (1889–1953), who studied galaxies and devised a classification scheme for them. The Hubble Space Telescope, an orbiting astronomical observatory launched in 1990, was named for him. The telescope's fine high-resolution images are far better than can be obtained from the earth's surface.
Hubble's constant the ratio of the speed of recession of a galaxy (due to the expansion of the universe) to its distance from the observer. The reciprocal of the constant is called Hubble time and represents the length of time for which the universe has been expanding, and hence the age of the universe.
Hubble's law a law stating that the red shifts in the spectra of distant galaxies (and hence their speeds of recession) are proportional to their distance, proposed by Edwin Hubble.