“Silently, one by one,
In the infinite meadows of Heaven.
Blossomed the lovely stars,
The forget-me-nots of the angels.”
Henry Wadsworth Longfellow (1847)1
How do the Stars Move?
It looks as if the stars are moving overhead each night, but these apparent movements are instead due to the Earth’s rotation beneath the fixed stars. The turning Earth also explains why the Sun seems to rise and set each day. As the Earth rotates, day turns into night and the stars slide by.
Aside from the Earth’s rotation, there is another more subtle motion of the stars that was discovered long ago, by the Greek astronomer Hipparchus around 150 BC. The appearance of a nova stella, a new star, at a place in the sky that no star had been seen before, inspired him to compile an accurate star catalogue of 850 bright stars. The listing of their brightness and position could, he thought, be used to determine if a star brightened or moved in later years.
When comparing his measurements of the stellar locations with those of his predecessors, Hipparchus discovered that some of these stars were apparently moving by the same amount and in the same direction as time went on. He suspected that all of the stars were slowly and steadily moving together, which Ptolemy confirmed about two hundred years later. The entire celestial sphere seemed to be twisting eastward at the rate of one angular degree per century, but this apparent shift in position is also due to the Earth’s movement.
The Earth not only spins; it gyrates like a very large and slow, wobbling top. As shown by Isaac Newton, the gravitational pull of the Moon and the Sun on the Earth’s elongated shape causes a slow circular twist of the Earth’s axis of rotation in space. It completes one circuit, every 26,000 years.2
So despite eons of stellar observation in antiquity, there was not a shred of evidence to contradict the belief that the stars are rooted in the sky. They always appeared at the same place in the celestial sphere, and never changed their apparent separations on it. That’s why we can identify long-lived patterns amongst groups of stars, the constellations.
In contrast, it seems like everything on the Earth moves. Streams flow downhill, waves rise and fall on the sea, clouds drift across the blue sky, and tree leaves quiver in the wind. We all rise from bed to begin our daily movement across the land, and the entire planet spins on its axis and moves around the Sun.
And when you stop to think about it, the stars ought to move. Without motion, there would be nothing to keep the stars apart and suspended in space. Their mutual gravitation would eventually pull them into a single mass. There is not one star that is completely at rest.
We now realize that a star’s motion in space manifests itself in two ways, depending on the method used to observe it (Fig. 5.1). One component of the motion is the “sideways” velocity directed perpendicular or transverse to the line of sight. The radial velocity is the other part of the motion. It is the component moving toward or away from us in the direction of the star. When a star is moving straight at you, there is no perpendicular motion, and if the star is moving directly across your line of sight, the radial motion is reduced to zero. When both velocity components are known, we can determine the speed and direction of the star in three dimensions.
Figure 5.1 A star moves The transverse velocity of a star, V⊥, which is perpendicular to the line of sight, can be inferred from measurements of a star’s distance, D, and proper motion, µ. The radial velocity, Vr. directed along the line of sight, can be determined from the Doppler shift of the star’s spectral lines. When these two velocity components are combined, the relative speed in space is obtained.
The Proper Motions of the Stars
The English astronomer Edmond Halley discovered that the stars do move on their own accord, but to do this he had to compare his observations with those made long before he was born. In 1718 he noticed that his determinations of the locations of a few bright stars differed from those measured by the Greek astronomer Hipparchus around 150 BC and recorded by Ptolemy in his Almagest in the second century.3 So it took about 1,800 years before anyone noticed that a star could move in the sky.
In 1760 the German astronomer Johann Tobias Mayer fully confirmed Halley’s discovery of stellar motion.4 Mayer was not looking for stellar motion. He wanted to improve the way mariners and surveyors were using observations of stars to get their bearings, and to measure a person’s location on the Earth. Since these measurements depended on the accuracy of the stellar observations, Mayer was investigating instrumental and atmospheric effects that could influence them. When he compared his own newly-acquired measurements of the positions of stars in the sky with those made by the Danish astronomer Ole Rømer only half a century before, Mayer found differences that could only be attributed to stellar movements.
The motion that Halley and Mayer detected is the “sideways” component of velocity directed perpendicular or transverse to the line of sight. It produces an angular change in position known as proper motion. The term suggests that the motion belongs to the star, and is proper in that regard. But the observed movement might nevertheless be attributed to either the star’s transverse motion in one direction, the Sun’s movement in the opposite one, or some combination of both motions.
When thinking about this uncertainty, Mayer noted that if the Sun was moving toward some region in space, all the stars which appear in that region would seem to be gradually separating from each other, while those in the opposite part of the sky would seem to be joining up. It is similar, he supposed, to walking through a forest in which the trees in front of you appear to move to your sides as you approach them, and those behind you seem to merge together. It wasn’t until 1783 that William Herschel found such a pattern from the proper motions of just seven stars, and concluded that the Sun was moving toward the constellation Hercules.5
More than a century later, the Dutch astronomer Jacobus C. Kapteyn, of the University of Gröningen, found that nearby stars are apparently moving in two preferred directions.6 They seemed to be traveling in a pair of large intermingled streams that pass through each other while moving in opposite directions in the Milky Way. The two star streams were found wherever Kapteyn and his colleagues looked, and they spent a lot of time looking. Altogether, the proper motions of some 2,400 stars were measured for Kapteyn’s 1905 report on star steaming.
Telescopes lofted above the Earth’s obscuring atmosphere have now been used to detect much smaller star-location changes than can be detected from the ground, with or without a telescope. Instruments aboard the HIPPARCOS satellite have determined the accurate positions and proper motions of more than 100 thousand stars.
Proper motion belongs to a star, but it isn’t a velocity. It is the angular rate at which a star moves across the sky over the years or centuries, and it does not by itself determine the speed of motion. To convert a star’s proper motion into a velocity, you have to know the star’s distance, and no one knew the distance of any star other than the Sun until more than a century after Halley’s proper-motion discovery.
Assuming that all stars move through space at roughly the same speed, those closest to Earth should display the largest proper motion over a given length of time. A nearby bird flying overhead similarly travels rapidly across a great angle, while a high-altitude one moving with the same speed barely creeps across the sky. That is how a duck hunter estimates the distance of a duck — by its angular speed.
So the nearest stars ought to exhibit the largest proper motion, provided they all move at about the same speed. That is why Wilhelm Bessel choose the “flying star” 61 Cygni to make the first measurement of a star’s distance in 1838.7 It had the largest proper motion known for a star at the time.
But lets now take a brief diversion and look at Halley’s diverse interests.
A Glimpse at Edmond Halley’s Interesting Life
Edmond Halley was born November 8, 1656 in the outskirts of London. He began his education at St. Paul’s School, London, where he excelled in the study of classics and mathematics, and in the summer of 1673 he entered Queen’s College at Oxford University. While at Oxford, Halley assisted John Flamsteed at the Royal Observatory, which was under construction at Greenwich with the support of King Charles II for the purposes of navigation by the ships of the seafaring nation.
Influenced by Flamsteed’s compilation of the locations of northern stars in the sky, Halley proposed to do the same for the Southern Hemisphere. With financial assistance from his wealthy father and the King, and a letter of introduction to the East India Company, he sailed in 1676 to the South Atlantic island of Saint Helena, where he catalogued the accurate positions of 341 southern stars. When Halley returned to England in 1678, he published his observations,8 was awarded a degree from Oxford University, and was elected a Fellow of the Royal Society at the age of 22.
During his sea voyage, Halley also mapped the prevailing trade winds over the oceans, and identified solar heating as the cause of atmospheric motions. He subsequently built and personally tested a diving bell for underwater exploration, traveled in an English war vessel to make magnetic charts of the Atlantic Ocean, and was one of the first to examine the statistics of mortality and age, which allowed the English government to sell life annuities at a price determined by the age of the purchaser. In other words, Edmond Halley was a curious and intelligent fellow who was captivated by the natural world and liked to find out how it worked.
Religion played an important part of Halley’s life, as it did for just about everyone in England at the time. As an example, he was denied the post of Savilian Professor of Astronomy at the University of Oxford because he was not an orthodox Christian. But he was appointed to the similar Geometry Professorship after the death of his religious enemies, including the Archbishop of Canterbury.
Like Newton, Halley seems to have denied the strict equality of Christ and God, or the Son and the Father, and he similarly avoided strict interpretations of the Bible. As an example, Halley proposed to the Royal Society that the great flood of Noah’s time may have been due to a comet (Fig. 5.2), and perhaps not the direct result of God opening the flood gates of Heaven because of humanity’s misdeeds.9
Figure 5.2 The eve of the deluge The arrival of a comet foretells the great flood in Noah’s time. (John Martin made this painting in 1840; collection of Her Majesty the Queen.)
Nevertheless, Halley retained a strong faith in an all-wise and all-powerful God.10 In the Latin Ode he prefixed to Newton’s Principia at its publication, Halley acclaimed his achievement as an insight to God the Creator of the Universe. For both Halley and Newton, the goal of astronomy was to comprehend Nature, and to thereby gain access to the Divine and knowledge of God.11
To return to stellar motion, there is another component of their movement that is directed along the line of sight to them.
Radial Velocities of Stars
If a star is headed straight toward or away from you, its speed is as difficult to judge as that of an approaching car in the opposite lane of a highway. You cannot detect any change in location as the car or star moves along your line of sight.
In order to measure this radial component of stellar motion, visible starlight has to be dispersed into its different colors and dark features detected within them. The wavelengths of these spectral lines are well known for a non-moving star, and the radial velocity component of a moving star is determined from the way its observed line wavelengths have been shortened or lengthened by its movement.
The Austrian scientist Christian Doppler first suggested such a wavelength shift for sound waves, when he also proposed a change in the colors of stars produced by their motion relative to the Earth.12
The change in the wavelength of radiation due to the relative radial motion between a star and its observer, along the line of sight, is now called the Doppler shift. If the relative motion is toward the observer, the radiation waves tighten up and shift to shorter wavelengths, and when that motion is away the waves stretch out with longer lengths (Fig. 5.3). The greater the radial velocities along the line of sight in either direction, the bigger the wavelength-shift.
The first successful measurements of the radial line-of-sight velocities of stars did not occur until the 1890s at the Potsdam Astrophysical Observatory in Germany, when Hermann Vogel and Julius Scheiner used photography with a refractor only 11-inches (0.28 meters) in diameter to obtain the long exposures needed to record their spectra. For several stars they obtained radial velocities of up to 30 kilometers per second with respect to the Sun, with an uncertainty of about a tenth that value.13 That was nearly two centuries after Edmond Halley’s discovery in 1718 of the transverse component of a star’s motion.
Once much larger telescopes were constructed and the relevant instruments had been devised, the number of measurements of stellar motions increased dramatically. During the 20th century, many astronomers dedicated their lives to measuring the positions, proper motions, and radial velocities of tens of thousands of stars, resulting in extensive catalogues of these quantities.
The Sun’s Motion
The observed transverse and radial velocities of stars are relative measurements. They can be due to the Sun’s motion, the star’s motion, or some combination of both of them. To separate the stellar and solar motions, the detected movements are compared to the mean of all of those observed. The mean result is attributed to the Sun’s motion with respect to the nearby stars, and the motion of individual stars is inferred from their departures from this mean.14
Figure 5.3 Doppler effect A stationary source of radiation (top) emits regularly spaced light waves that get stretched out if the source moves away from the observer (bottom). The size of the wavelength change from the stationary to moving condition provides a measurement of the relative speed of the source’s motion along the line of sight.
The speed with which the Sun moves along its path among the stars can only be measured from the radial motions that have been extracted from the Doppler shifts of the stars’ spectral lines. These results had to be obtained using spectral photography to increase the effective observing time of the dispersed starlight. The American astronomer William Wallace Campbell pioneered such measurements in the late 19th and early 20th centuries. He derived a speed of about 20 kilometers per second for the solar motion with respect to the nearby stars, which became widely accepted.15
The nearby stars are not moving much faster than the Sun with respect to their neighbors, but they are all revolving about ten times as fast around a common, distant and massive center. This brings us to the discovery that our Milky Way is much larger than it was once thought to be, and that all its stars are whirling about a remote location.
The Sun is Immersed within the Milky Way
On a clear moonless night, we can look up and see a hazy, faint, luminous band of light that stretches across the sky from one horizon to the other; it is known as the Milky Way (Fig. 5.4). According to ancient Greek myth, the goddess Hera, Queen of Heaven, spilled milk from her breasts into the sky. The Romans called the spilt milk the Via Lactea, or the “Milky Way.” It is also called our Galaxy, with a capital G, derived from the Greek word galakt- for “milk.”
Figure 5.4 The Milky Way A panoramic telescopic view of the Milky Way, the luminous concentration of bright stars and dark intervening dust clouds that extend in a band across the night sky. (Courtesy of the Lund Observatory, Sweden.)
For at least two thousand years, it was supposed that the Milky Way consists of stars, rather than misty white clouds. In the first century, the Roman poet Ovid, for example, wrote:
“A way there is in Heaven’s expanded plain,
Which, when the skies are clear, is seen [from] below,
And mortals, by the name Milky, know.
The ground-work is of stars....”16
As soon as telescopes were invented, astronomers confirmed that the luminous parts of the Milky Way are mainly due to multitudes of stars too distant and faint to be resolved with the unaided eye. As Galileo Galilei noted in 1610:
“The Galaxy is nothing else than a congeries of innumerable stars distributed in clusters. To whatever region of it you direct your spyglass [telescope], an immense number of stars immediately offer themselves to view. … What is even more remarkable — the stars [celestial regions] that have been called “nebulous” by every single astronomer up to this day are swarms of small stars placed exceedingly closely together.”17
By the 18th century, all stars were thought to be distant suns, and the flat shape of the Milky Way was attributed to the rotation of a vast collection of stars. In 1750, for example, the English astronomer Thomas Wright, born in County Durham, speculated that the visible, luminous arc of the Milky Way is a small part of either a vast ring or an enormous spherical shell of stars, centered on a Divine Presence, our God (Fig. 5.5).18 The Sun and other stars were supposed to be in circular motion around this common Divine Center, the place from which God’s infinite and eternal power emanates and directs the motion of the stars. Wright futher speculated that there might be other star systems, similar to our Milky Way, with their own divine centers.
In 1755 the German philosopher Immanuel Kant attributed the flattened shape of the Milky Way to its formation from a large, collapsing, rotating nebula all in accordance with the plan of the “Great Master Builder,”19 much like the origin of our Solar System from a considerably smaller nebula. He improved on Wright’s model by replacing his ring of stars with a spinning disk shape, and supposed that the stars in the Milky Way are all revolving about a very distant center, with even the nearest stars so far away from us that their motion could not be detected.
Figure 5.5 Model of the Milky Way Thomas Wright proposed that the Milky Way is composed of a large number of stars arranged in a layer, with the Sun placed at its center (right). It was supposed to be a small segment of a much larger spherical shell of stars, with a Divine Presence signified by a central eye (left). (Adapted from Thomas Wright, An Original Theory or New Hypothesis of the Universe, 1750, Reproduced by Science History Publications, New York 1971, page 139 plate XXV.)
Moreover, Kant speculated, some of the fuzzy nebulae observed in the Milky Way may represent separate “island Universes,” or Milky Ways, similarly composed of innumerable stars too distant to be discerned individually. Although these concepts were probably not that influential in Kant’s time, since his publisher went bankrupt and only a few copies of his book reached the public, his basic ideas were revived and accepted in subsequent centuries, when astronomers realized just how prescient he was.
It was the German-born English astronomer William Herschel who provided the first observational verification of these imaginative speculations, just a few decades after Wright and Kant proposed them. He constructed the biggest telescopes of the time, with the largest mirrors and greatest light-gathering power, in order to fathom the distribution of the stars, gauge their depth in space, and establish the shape of the stellar Milky Way. As Herschel put it, he was determining the “Construction of the Heavens.”
Herschel established the stellar distribution by counting the number of stars he could see in different directions. Assuming that the stars are distributed fairly uniformly in the space they occupy, and that his telescope could penetrate to the faintest stars at the boundary of the stellar system, then the number of stars in each field of view would indicate the extent of the system in that direction. In other words, the greater the number of stars seen along a given line of sight, the larger the distance to the outer edge of the Milky Way.
After counting the stars in thousands of directions, and avoiding obvious star clusters, Herschel found in 1785 that the Sun is located at the center of a flattened disk of stars with a disk diameter that is 5 times its thickness.20 But since the distances of the stars were not known, no one knew how big the stellar disk was.
Wright, Kant, and Herschel were all correct in supposing that the Sun is one star of many, all immersed in the flattened disk of the Milky Way, but Herschel was wrong about its center. Determinations of the distances to globular star clusters were eventually used to infer the great extent of the stellar system, and to demonstrate that the Sun is not located at its center.
Harlow Shapley, the Wonder of the Whole Natural World
Harlow Shapley was a country boy, born November 2, 1885 on a farm near Nashville, Missouri. At the age of 15, he became a crime reporter for the Daily Sun in Chanute, Kansas, a rough oil-mining town, and went on as a police reporter for the Times at Joplin, Missouri, an even tougher town. After saving his money, he entered the Presbyterian Carthage College Institution with only a fifth-grade education. Harlow then completed six years of high school training in one year and a half and graduated in 1907.
Shapley then went on to Missouri University at Columbia where he intended to study at its new school of journalism. But when he got there, Shapley found that the opening of the much-advertised school of journalism had been put off for another year. As he tells it, when he looked into the university catalogue, he could not even pronounce the first entry, archeology, and so he picked the second one, astronomy, beginning this career almost by accident. A few years later, he discovered that the Thaw fellowship in astronomy was available at Princeton University, and continued his studies there; mainly for financial reasons.21 Such unexpected turns of events have probably played important roles in most of our lives.
At Princeton, Shapley became the first graduate student of Henry Norris Russell, and embarked on a four-year investigation of eclipsing binary stars. Then in 1914, Harlow married Martha Betz, who he had met at the University of Missouri, and they traveled to his first astronomical job at the Mount Wilson Observatory in California. In the next few years at Mount Wilson he completed his legendary determinations of the distances and distributions of the globular star clusters, which displaced the Sun from the center of our stellar system and greatly enlarged its known extent.
During his years of nighttime observing from Mount Wilson, Shapley frequently spent his days observing ants. He found that trail-runner ants move back and forth along well-defined paths with speeds that increase with their temperature, and even published the result in the Proceedings of the National Academy of Sciences.22 The ants, he noted, are highly civilized, altruistic, and loyal to their home, but have little hope of venturing outside their trail and escaping from their rut of uniformity. A similar fate, he thought, threatens many graduate students.
The death of Edward C. Pickering in 1919 resulted in a search for his successor as Director of the Harvard College Observatory. The ambitious Shapley had already informed Harvard authorities of his interest in the position at least a year before Pickering’s demise, and upon hearing of the event he promptly wrote letters requesting support for his replacing Pickering to his Princeton mentor and to George Ellery Hale, the Director of the Mount Wilson Observatory where he then worked. Russell replied that he would not recommend Shapley to take Pickering’s position, and that “you would make the mistake of your life if you tried to fill it.” Hale recommended that Shapley should never attempt to take an active part in seeking the post, but the next year he supported Shapley’s application for it, based on his knowledge, ability, industry and daring.23
In 1920, Russell turned down an offer for the position, and Shapley was then considered an important candidate.24 After alternative choices were considered, Shapley was offered a temporary position beginning in April of 1921, and soon became Professor of Astronomy at Harvard University and Director of the Harvard College Observatory, a post he held for thirty-one years.
When remembering Shapley, Harvard Astronomy Professor Chuck Whitney wrote: “I have never seen a quicker mind, a more agile sense of humor, or a more complete absence of what usually passes for humility.”25 Harlow seems to have been gifted, ambitious, and hard working, with a tendency to avoid recognition of other astronomers, even those whose findings were directly related to his own.
In the late 1940s and early 1950s, conservative congressmen thought his outspoken liberal views and distrust of authority were dangerous and even subversive, including the infamous Senator Joseph McCarthy who listed Shapley as a Communist — which he was not.
Harlow Shapley was not a devout man in any traditional religious sense, but he had respect for religious movements. He rarely mentioned God, and does not appear to have either believed in a Deity one might pray to or to consult the Bible for inspiration. In his view, belief in the supernatural had to be tempered with rational thought, and living things were not in need of divine interventions. Shapley nevertheless thought the Divine might be found in Nature, writing: “It is a religious attitude to recognize the wonder of the whole natural world … to avow reverence for all things that exit, all that is touched by cosmic evolution, and reserve the greatest reverence of all for existence itself.”26
Shapley was much taken by the relatively recent discoveries that heavy elements are synthesized within stars. “Every baby born, every saint and sinner, and every common man and common beast breaths some of these former elements of the stars,” he exclaimed. “These elements have already participated in the “snorts, sighs, bellows, shrieks, cheers, and spoken prayers of the prehistoric and historic past.”27
The Harvard astronomer endorsed a religion that includes our newfound knowledge of the observable Universe, with its abundant mysteries that still lie beyond our grasp, and urged everyone to participate in the search for the unseen and unknown.28
In his later life Shapley played an important role in the founding of IRAS, or the Institute for Religion in an Age of Science, an institution that survives to this day. One of the institute’s objectives is to combine a scientific understanding of the natural world with the goals and hopes of humanity expressed in religion. These purposes resonated with Shapley’s proposals that traditional Christianity could be enriched and vitalized by including the discoveries of modern science and that religion might also ennoble the concepts of science.
To illustrate an overlap of science with religious concerns, Shapley quoted four pages of an address in 1951 by Pope Pius XII to the Vatican Academy of Sciences, which endorsed scientific observations of ongoing change throughout the living and non-living Cosmos.29 But Shapley didn’t include a complete account of the Pontiff ’s remarks, which were entitled: “The Proofs for the Existence of God in the light of Modern Natural Science.”
The Pontiff was specifically referring to the expanding Universe that could be extrapolated back in time to a beginning. If there was a beginning, he argued, then there had to be a Creator God, as First Cause. This would explain the origin of the outward motion of spiral nebulae [galaxies] and all subsequent transformations in the ever-changing Universe.
Shapley wrote several essays in the 1960s that touched on the interface of astronomy and religion. At this time in his life, when he was more than 75 years old, Shapley also participated in considerations of religion in the scientific age, edited the book Science Ponders Religion, and proudly received a Doctor of Divinity from the Meadville-Lombard Theological School affiliated with the University of Chicago.
As Shapley’s Princeton mentor Henry Norris Russell had also mentioned, a transcendent God is the only reason why there is any Universe.30 Science, Russell noticed, answers only the question of how things come to pass, but not why things are so.
Enlarging and Re-Centering the Milky Way
Shapley’s greatest contribution to astronomy was the discovery that globular star clusters could be used to look outside the flattened disk of the Milky Way to establish its dimensions and center. Each of these dense clusters contains hundreds of thousands of stars held together and bound into a spherical shape by their mutual gravitation (Fig. 5.6). Observing them was analogous to flying in an airplane to look down and determine the extent of a city, which cannot be done from inside where nearby buildings hide the distant parts of the city from view.
Figure 5.6 Star cluster Several hundred thousand stars swarm around the center of the globular star cluster NGC 6934, which is estimated to be about 10 billion years old. (A Hubble Space Telescope image courtesy of NASA/ESA.)
Harlow’s doctoral thesis at Princeton University, published in 1913, involved nearly 10,000 observations of eclipsing binary stars, which enabled him to determine the orbital parameters of 90 of them and to increase the number of known orbits by about ten times.31 When he arrived at the Mount Wilson Observatory in 1914, Shapley turned his attention to another type of variable star known as the Cepheids, after their prototype Delta Cephei. This class of variable stars changes periodically between a bright state and a dimmer one and back to a bright condition again. These stars are additionally distinguished by an exceptional luminosity, high temperature, and large mass and size. The Cepheid stars are so big, Shapley showed, that if they were eclipsing binary stars the two hypothetical stars would have to be inside each other. As an alternative, Shapley suggested that the Cepheid variations might arise from pulsations of isolated individual stars.32 As the outer atmosphere of a Cepheid contracts and expands with a regular beat, it acts like a valve that periodically absorbs and releases the outward flow of energy from the star’s central region, leading to its periodic light variation.
While Shapley was investigating eclipsing binary stars back at Princeton, Henrietta Leavitt, a researcher at Harvard College Observatory, discovered a novel way of determining the distances of these luminous Cepheid variable stars. After years of scrutiny, she had determined the period of light variation for 25 variable stars in the Small Magellanic Cloud, which ranged from 1.2 to 127 days, and showed that this variation period increases with the star’s observed brightness. Since all of these Cepheids were located within the remote Small Magellanic Cloud, she could therefore modestly state that: “Since the variables are probably at nearly the same distance from the Earth, their periods are apparently associated with their actual emission of light [or luminosity].”33 The more luminous a Cepheid variable star is, the more slowly it varies and the longer its variation period.
When he arrived at Mount Wilson in 1914, Shapley began a study of the Cepheid variable stars in globular star clusters, and found that for a given star cluster they displayed a period-luminosity relation similar to the one that Leavitt had found for the Cepheids in the Small Magellanic Cloud, and concluded that their periodicities and apparent brightness could be used to establish the relative distances of the star clusters. It remained for Shapley to determine the mean luminosity of nearby Cepheids of reliably known distances, and thereby infer the distances of remote Cepheids in globular clusters.34 This mean luminosity is roughly 10,000 times the luminosity of the Sun.
Since the Cepheids are so highly luminous, they can be detected at relatively large distances when compared with fainter stars. Moreover, the mean luminosity of each star is related to its pulsation period, which provides a way of estimating its distance from its observed brightness and established luminosity.
In these ways, Shapley discovered the enormous distances of the globular star clusters. In October 1917 he wrote to his Princeton mentor Henry Norris Russell of this “peculiar Universe” in which the nearest globular clusters were about 20,000 light years away and the furthest ones something like ten times that amount.35 In the following year he reported that the globular star clusters are typically about 50,000 light years away from the Sun and thus outside the bounds of Kapteyn’s Milky Way Universe.36 [A light year is the distance light travels in one year, which is about 10 million billion, or 1016, meters.]
The following year, Shapley showed that the globular star clusters form a vast, roughly spherical system enveloping the plane of the Milky Way on both sides and centered about 65,000 light years away from the Sun in the direction of Sagittarius (Fig. 5.7).37 The diameter of the system is, he reported, some 300,000 light years in the plane of the Milky Way.
Figure 5.7 Edge-on view of the Milky Way The globular star clusters are distributed in a roughly spherical system that envelops the Milky Way and is centered at about 27,700 light-years away from the Sun. The disk and central bulge are shown edge-on in an infrared image that penetrates the interstellar dust that limits an astronomer’s view at optically visible wavelengths to a much smaller Kapteyn Universe, centered on the Sun.
At the time, the accepted maximum extent along the plane of the Milky Way corresponded to a diameter of perhaps one hundredth of Shapley’s value. In 1914, A. S. Eddington had estimated a total extent of perhaps 3,000 light years, and on hearing of Shapley’s awesome feat he wrote him exclaiming that:
“I think it is not too much to say that this marks an epoch in the history of astronomy, when the boundary of our knowledge of the Universe is rolled back to a hundred times its former limit.”38
By looking up, out and beyond the known stellar system, Shapley had increased the total known extent of the Milky Way at least tenfold, and its volume more than a thousand times. It was a remarkable finding applauded by some of the most eminent astronomers of the time. In addition, Shapley removed the center of the known stellar Universe from at or near the Sun, and placed the Solar System far off center in the peripheral outer fringes of the Milky Way.
Shapley’s enlarged and re-centered Milky Way was not initially accepted by some established astronomers of the time. In the early 20th century, the Dutch astronomer Jacobus C. Kapteyn and his colleagues had exploited long photographic exposures to extend star counts to the faintest possible limits, and used measurements of the distances of some of the nearest stars to provide a scale to their model. In 1922 Kapteyn concluded that the stars in the Milky Way reside in a Sun-centered system and are confined within the plane of the Milky Way out to a maximum distance of 30,000 light-years from the Sun.39
As it turned out, astronomers could only discern the nearby parts of the Milky Way when looking directly into it. The more distant invisible parts lie behind an opaque veil of interstellar dust that absorbs the light of distant stars. So looking deep within the Milky Way is something like viewing distant objects on a foggy day. At a certain distance, the total amount of fog you are looking through mounts up to an impenetrable barrier. This meant that Shapley was right about the larger extent and remote center of the Milky Way, and that Kapteyn just couldn’t see that far.
Where is the center of the Milky Way, and how far away is it? According to Harlow Shapley, this invisible center has to be located at the heart of the globular cluster system some tens of thousands of light-years away in the direction of the constellation Sagittarius. Such a remote location would be completely obscured by an intervening curtain of interstellar dust, and therefore hidden from direct observations with visible-light [optical] telescopes.
The discovery of the unseen center had to await the development of new technology at invisible radio and infrared wavelengths, initially for military purposes. Radiation at these longer wavelengths, beyond those normally visible to the eye, is able to penetrate and see through the interstellar dust. In much the same way, radio waves pass through storm clouds to reach your car radio or cell phone, even when it is raining or snowing. Observations at these invisible wavelengths have enabled astronomers to conclude that the Sun and nearby gas and stars are revolving about the center of the Milky Way at a speed of 220 kilometers per second, with an orbital period of about 240 million years at their distance from the center of 27,700 light-years.
The most intense radio emission is also coming from the direction of the constellation Sagittarius.40 This compact radio source coincides with a distant collection of infrared-emitting stars,41 and is no larger than our Solar System.42,43 It is most likely energized by a massive black hole that is a colossal 4 million times the mass of the Sun.44
As we shall next see, hundreds of billions of whirling spiral nebulae are moving away from the Milky Way and each other at speeds that increase with their distance. This discovery involved an extension of Shapley’s observations of the Cepheid variable stars in globular star clusters to Cepheids found in spiral nebulae, and disagreed with Shapley’s belief that they had to be embedded within the Milky Way.45