From his studies of star clusters, Harlow Shapley deduced the size of the Milky Way galaxy and Earth’s location within it.
Over the centuries, the question of humans’ place in the universe has been a very fruitful area of inquiry for both science and religion. After giving the matter long and careful thought, the ancient Greek philosopher Aristotle concluded that Earth lay in the center of the universe and that the universe visible to humans ended at the sphere of stars that lay just beyond the planets. He considered the possibility that Earth might move around the Sun but could find no evidence to support this idea. It thus seemed that the universe was a cozy abode for humans and was focused on humans at its center, as the Greeks supposed their gods had intended. Some nineteen hundred years later, Nicolaus Copernicus established the Sun as the center of the universe. As it seemed that the Sun was largely here for our benefit, human beings could still consider themselves special because of their location in the universe.
At about this same time, Christian scholars began to suggest that God, being infinite, must have a domain far more extensive than Earth and the heavens that surround it. This was not a new idea, given that Christian cosmological understandings from late antiquity had generally accepted Ptolemy’s claim that the Earth was but a small point in a vastly large universe. The writings of early Church fathers, such as St. Augustine, largely adopted this Ptolemaic cosmological assertion. The English astronomer Thomas Diggs supposed that the stars were distributed throughout an infinite universe and that myriad planets orbited these stars and were inhabited by the creations of God.
Several astronomers attempted to discover how stars were distributed throughout space by doing “star counts.” These efforts culminated in the work of Jacobus Cornelius Kapteyn. In its simplest form, the star-counting method requires counting the number of stars seen in a patch of the sky. Some stars will look faint and others bright. If it is assumed that all stars would be equally bright if they were all placed at the same distance from Earth, then the faint stars are faint only because they are farther away. After one has repeated this process for many patches across the sky, one can construct a model of our galaxy. Suppose one had the means to place tiny sparks in the surrounding air and they would stay put. Using the data from the star counters, the model would be shaped rather like a grindstone, a squat cylinder many times wider than it is thick. The Sun would be about in the center. In order to establish a scale for the model, one might assume that all the stars are as bright as the Sun. One must then deduce how far away they must be in order to look as dim as they do in the sky.
All stars do not have the same brightness as the Sun, however. Some are brighter, but most are fainter. In order to improve the model, a better method is needed to estimate distances to stars. This was Kapteyn’s specialty. He began by using parallax
Knowing the distances to many stars, one can now calculate the brightness of different kinds of stars and use that information to estimate how far away distant stars of a given type are from Earth. Kapteyn’s “grindstone” galaxy, also called “Kapteyn’s universe,”
Harlow Shapley took a different approach. Scattered throughout the sky are ball-shaped groups of stars called globular clusters.
Shapley supposed the Cepheids to be the same as the variable stars he found in globular clusters. He measured their periods and used Leavitt’s results to calculate how bright these cluster variables should be. Comparing this value with how bright they looked, Shapley could then calculate how far away they must be.
If globular clusters were placed on the grindstone galaxy model, one would find that none of them fit into the grindstone. Instead, the clusters would float above and below the grindstone and would occupy a roughly spherical volume. Two other aspects of the model would be striking: First, the sphere of globular clusters would be ten times the size of Kapteyn’s grindstone; second, the center of the cloud of clusters would be well off to one side of the grindstone. Assuming that the globular clusters reflected the structure of our galaxy, Shapley found the Milky Way to be 300,000 light-years across, with its center 50,000 light-years from the Sun in the direction of the constellation Sagittarius. Later, Shapley would write in his book The View from a Distant Star (1963), “It was a shocking thought—this sudden realization that the center of our universe was not where we stood but far off in space, that our heliocentric picture of the universe must be replaced by a strange sort of eccentric universe.” Shapley’s summarized his results on January 8, 1918, in a letter to Sir Arthur Stanley Eddington, a well-known English astronomer and physicist.
The full impact of Shapley’s work was years in coming because it took that long to separate truth from error. The relevant questions were brought into focus by the “great debate” held between Shapley and another astronomer, Heber Doust Curtis, in April of 1920. One important issue was that of the nebula. “Nebula” is Latin for cloud, a meaning it retains in the word “nebulous.” In 1920, astronomers were mostly concerned with two kinds of nebulas: irregular patches of glowing gas near hot stars and glowing patches with a somewhat spiral shape.
Curtis championed Kapteyn’s view of our galaxy. He further supposed that spiral nebulas
A close associate of Shapley, Adriaan van Maanen, attempted to measure the speed at which spiral nebulas rotate. He found that they rotated in a fashion predicted by some of the leading theorists of the day. Shapley pointed out that if spiral nebulas are very far away, van Maanen should not have seen evidence of their rotation over the few years for which photographs were available. It was later proved that van Maanen’s measurements were in error. Accurate measurements were beyond the capabilities of the equipment available to him.
Another important factor was interstellar dust. A handful of dust particles scattered throughout a volume of cubic kilometers is not enough to change it from a good vacuum, but thousands of light-years of such dusty vacuum is enough to dim, or even block, starlight. Shapley looked for evidence of dust, but, by chance, he looked in a direction in which there was little. The chief effect dust was to have on Shapley’s measurements was to explain why he did not see globular clusters close to the plane of the Milky Way, which is dusty. The effect of dust was far greater on Kapteyn’s results. The Sun appeared to be near the center of our galaxy only because he could see into the dust about the same distance in all directions. Also, because his view was limited by dust, “Kapteyn’s universe” was only a fraction of the real Milky Way galaxy.
The final piece fell into place when Shapley discovered an error in the calibration of the distances to variable stars. Shapley had originally supposed that our galaxy was an “island universe”
Modern measurements place the diameter of the visible part of our galaxy at 80,000 light-years. This is roughly three times Kapteyn’s number and only one-fourth of Shapley’s original number. Shapley’s philosophical legacy remains, however: The Sun is not at the center of our galaxy. The center lies 25,000 light-years away in the direction of the constellation Sagittarius. The Milky Way is vast beyond anything Kapteyn had imagined.
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Berendzen, Richard, Richard Hart, and Daniel Seeley. Man Discovers the Galaxies. New York: Columbia University Press, 1984. Excellent book, written in an engaging, nontechnical style, is filled with historical photographs and drawings. Presents biographies of all the principals, includes extensive bibliographies in each section, and even offers thought questions for those who would match wits with people of the past. Covers the period from Thomas Wright (1750) to Albert Einstein and the expanding universe. -
Ferris, Timothy. Galaxies. New York: Tabori & Chang, 1982. Outstanding volume for anyone wanting a modern view of galaxies. Presents a magnificent collection of some of the finest pictures of galaxies ever published accompanied by lucid text. Begins with objects in our galaxy and moves outward to the rest of the universe. Includes helpful diagrams and glossary. -
_______. The Red Limit: The Search for the Edge of the Universe. Rev. ed. New York: Harper Perennial, 2002. Discusses the history of the major discoveries in astronomy, paying particular attention to the individuals who made the discoveries. Comprehensible, accurate discussion of astronomy written in an engaging style for readers who have no familiarity with modern cosmological ideas. Includes extensive glossary, selected bibliography, and index. -
Harrison, Edward R. Masks of the Universe: Changing Ideas on the Nature of the Cosmos. 2d ed. New York: Cambridge University Press, 2003. Fascinating book, aimed at lay readers, discusses the various notions held about our galaxy and the universe from the dawn of time up to the late twentieth century. Shows how culture and science influence each other and places Shapley’s work within the panorama of the ten-thousand-year-long reach for the stars. -
Shapley, Harlow. Galaxies. 3d ed. Cambridge, Mass.: Harvard University Press, 1972. Reviews the work of Shapley and others on galaxies and summarizes what was known then about our galaxy, neighboring galaxies, and the expanding universe. Intended for lay readers, but contains some technical material. Includes many helpful pictures and charts. -
_______. Through Rugged Ways to the Stars. New York: Charles Scribner’s Sons, 1969. “Informal” autobiography based on interviews conducted with Shapley for an oral history project. Includes chapters on finding the center of the Milky Way and on the great debate between Shapley and Curtis. Also discusses Shapley’s studies of ants, which he performed while waiting for nightfall. For the general reader. -
_______. The View from a Distant Star: Man’s Future in the Universe. New York: Basic Books, 1963. Collection of many of Shapley’s lectures and articles. One chapter deals with his discovery of the vastness of the Milky Way. Other topics covered are humans’ place in the universe, the educational system, astrology, and water dowsing. For the general reader.
Kapteyn Discovers Two Star Streams in the Galaxy
Slipher Obtains the Spectrum of a Distant Galaxy
Leavitt Discovers How to Measure Galactic Distances
Hertzsprung Uses Cepheid Variables to Calculate Distances to the Stars
Hubble Determines the Distance to the Andromeda Nebula
Hubble Shows That Other Galaxies Are Independent Systems