Astronomer Walter Baade reconciled the several puzzling discrepancies in measurements of distance to nearby galaxies by proposing that the Cepheid luminosity scale was in error.
The meeting of the International Astronomical Union
The first of these discrepancies had been noted by Edwin Powell Hubble in the 1930’s. Hubble had discovered that the Andromeda galaxy seemed to contain more than one hundred fuzzy objects that he identified as most likely star clusters similar to the globular star clusters of Earth’s Milky Way galaxy. These objects have hundreds of thousands of stars as members and occupy a volume of space that is approximately 50 light-years in diameter. (A light-year is the distance that light travels in one year, approximately 10 billion kilometers).
Hubble noticed that the globular clusters of Andromeda appeared to be, on the average, only one-quarter as luminous as those of the Milky Way. He also noticed that the explosive stars called novas that existed in the Andromeda galaxy never reached the high intrinsic brightnesses that the Milky Way’s local novas exhibited. (Novas are double stars, one member of which is a collapsed, high-density object, which periodically erupt brilliantly when gas from the normal star falls onto the surface of the collapsed star, causing a nuclear detonation.)
In the early 1940’s, Hubble, as well as most of the other astronomers at the Mount Wilson Observatory
As a consequence of these two circumstances, Baade was able to take exquisite photographs of the Andromeda galaxy, far better than any that had been taken before. To Baade’s consternation, these photographs did not show what he was trying to find: the central population of red stars of the galaxy. According to all that was known about stellar populations in the Milky Way, he should easily have seen some of these objects on his Andromeda plates, but they were not there. The white pulsating stars called RR Lyrae stars should have been visible, as should the red-giant stars that populate the old globular clusters of Earth’s Milky Way.
Failing to find any of these expected objects, Baade, tried a different approach. Reasoning that the red-giant stars should be easier to detect on red-light-sensitive photographic emulsions than on the usual blue-sensitive ones, he used the former emulsions in the telescope, finding that they also offered the advantage of allowing longer exposures, up to eight hours in length. The change of emulsion worked: The red plates showed the long-sought-after red giants, which were scattered throughout the images of the Andromeda galaxy, though concentrated mostly toward the central areas.
Baade’s detection of the red giants in Andromeda did not solve any puzzles, however; it merely added to them, because the stars were four times fainter than they should have been. The reason for this strange discrepancy became a puzzle. One possible explanation of the discrepancy was that the galaxy was farther away from Earth than was believed at the time. This explanation was not obvious, however, because it was not known if the stars in different galaxies were all fundamentally similar or different. As a result, knowledge gained about nearby stars could not be reliably extended to those in other galaxies, and it could not merely be assumed that the red giants of Andromeda would be as bright as the red giants of the Milky Way. Therefore, Baade realized gradually that the various types of stellar populations must be better understood before deductions about the galaxies in which they were found could be made.
Until Baade’s work on the Andromeda galaxy and its small companion galaxies, astronomers recognized no clear distinctions between populations of stars. It was known that stars with different characteristics seemed to exist in different star clusters, but the reasons for this segregation were unclear. No order was evident behind the apparent chaos. Astronomers hoped that, in general, stars did not differ too much from place to place, so a reasonable amount of uniformity would be assumable when exploring various parts of the universe.
From 1943, when he first saw the red giant stars in Andromeda, to 1952, when he announced the solution to the puzzle, Baade concentrated on the problem of understanding the differences between stars in different environments. The final clue came from an unexpected source: two dwarf galaxies discovered in the southern skies by the Harvard astronomer Harlow Shapley. Named the Sculptor and Fornax dwarfs, these strange, low-luminosity objects seemed to contain only red stars similar to those of the globular clusters and none of the bright blue stars found in the Milky Way and Andromeda galaxies. Thus, they represented a particular stellar population that was very different from the population of stars near the sun. Baade realized that this separation of stellar populations might be the key to the puzzle of the Andromeda galaxy: If there were two different populations of stars, then the principle of uniformity might not work as a basis for calculating the distance between galaxies. The distance that had been calculated to Andromeda could then be shown to be incorrect.
The distance to Andromeda had been established many years before by Hubble, who used the Cepheid period-luminosity scale. This important correlation had been discovered in 1914 by the Harvard astronomer Henrietta Swan Leavitt, who was studying periodically varying stars called Cepheids, which, because of their pulsations in size, vary in brightness every few days. Leavitt found that the Cepheids in a nearby galaxy named the Small Magellanic Cloud showed a remarkable correlation between their apparent brightnesses and their periods, such that the longest-period stars were the brightest. Leavitt reasoned that, because all the stars in the Small Magellanic Cloud are at about the same distance from the Milky Way, the true luminosities of Cepheid variables had to be related to their periods. Furthermore, if the distance to even one Cepheid could be measured, then the distances to all the other Cepheids could be determined simply by applying Leavitt’s scale.
Thus, Leavitt’s scale seemed in theory to be a tool that could be used to calculate the distance to any galaxy containing even one observable Cepheid. The problem that faced astronomers who wanted to use the period-luminosity scale was the difficulty of measuring the distance to any Cepheid. All were found to be too distant, even the nearest ones in the Milky Way, to calculate their distance with any certainty. Shapley, however, found a way to overcome this difficulty. He showed that the globular clusters contain both Cepheid variables and RR Lyrae stars, so the comparative luminosities of these two kinds of stars could be established.
Fortunately, there were enough RR Lyrae stars near Earth’s Milky Way that it was possible to measure their distances using accurate, geometrical methods. These stars therefore provided a means to calibrate the Cepheids’ period-luminosity scale, enabling the establishment of an extremely important tool for measuring distances to galaxies. It was this method that Hubble had used to measure Andromeda’s distance, which he found to be 900,000 light-years. The method, however, assumed that all Cepheid stars were essentially the same.
Baade realized in 1952 that his division of stellar types into two populations meant that there must be two different types of Cepheids: those that existed in “normal” sunlike populations, which he called population I, and those that existed in globular clusters (and in the Sculptor and Fornax galaxies), which he called population II. If the population II Cepheids were intrinsically four times fainter than the population I Cepheids, then all the discrepancies and puzzles about the Andromeda galaxy would disappear. After careful research, Baade concluded that the most reasonable explanation for his data was that the Cepheid period-luminosity scale was incorrect and that all distances based on it must be underestimated by a factor of two. Thus, by announcing this discovery, Baade doubled the distance to the Andromeda galaxy and doubled the size of the universe.
Baade’s 1952 revision of the extragalactic luminosity scale was more than a simple correction of an error of the past. In addition to the direct impact that it had on cosmological problems of distances and sizes, it also signaled the beginning of the understanding of the physical differences between stars of different types. Astronomers began to understand better the life histories of stars that make up the different populations; with this understanding came a more reliable and sophisticated use of the principle of uniformity. It became clear that astronomers must understand the physical reasons for stellar differences before they can use the principle in other environments, because these reasons may show where the principle must be either modified or abandoned.
The doubling of the size of the universe was important. At the time of Baade’s announcement, cosmology was in a fairly primitive stage, involving many assumptions and very little data. The size scale was one of these data, and it was an essential part of the question of the nature of the cosmos. Also doubled were the sizes of the galaxies. Until 1952, it appeared that the Milky Way galaxy was an unusually large object, almost unique in its size. After Baade corrected the distances, however, it was seen that other galaxies were often as large and luminous as the Milky Way, which thus appeared to be less unusual or special than it had appeared. This realization, since the time of Copernicus, was always a source of discomfort to astronomers.
The understanding of stellar populations and their differences was also important. Allan Sandage
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Baade, Walter. Evolution of Stars and Galaxies. Cambridge, Mass.: Harvard University Press, 1963. Based on a series of lectures given at the Harvard College Observatory; one of the most interesting and readable books on galaxies. -
Hodge, Paul. Galaxies. Cambridge, Mass.: Harvard University Press, 1986. A book for the interested layperson, this volume covers the distance scale, the population types, the nature of galaxies, and cosmology. It is fully illustrated. -
Hubble, Edwin. The Realm of the Nebulae. New York: Dover, 1958. Based on a series of lectures given in 1936 at Yale University, this is Hubble’s only book-length manuscript and his only attempt to put the entirety of his many fundamental discoveries about galaxies and the universe before the public. -
Morrison, David, Sidney Wolff, and Andrew Fraknoi. Abell’s Exploration of the Universe. 7th ed. Philadelphia: Saunders College, 1995. This elementary textbook has the best coverage of historical events up to the 1980’s of any of the many astronomy texts. Abell knew and worked with Baade at Mount Wilson and Palomar observatories and writes about the topic of this essay with authority. -
Osterbrock, Donald E. Walter Baade: A Life in Astrophysics. Princeton, N.J.: Princeton University Press, 2001. Biography of Baade focusing on his career and contributions to the history of astrophysics. Bibliographic references and index. -
Rowan-Robinson, Michael. The Cosmological Distance Ladder. New York: W. H. Freeman, 1985. Although this book is intended primarily for readers familiar with the technical details of modern astronomy, there are several sections that can be read with facility by nonscientists, especially chapter 1, which covers the history of the extragalactic luminosity scale. -
Whitney, Charles A. The Discovery of Our Galaxy. New York: Alfred A. Knopf, 1971. This masterpiece of popular science is one of the best examples of astronomical history ever published. It discusses the entire subject of the recognition of our galaxy and its relationship with other galaxies, covering the Baade distance scale revision particularly deftly.
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