Astronomy and Space Science: Galactic Astronomy
Astronomy and Space Science: Galactic Astronomy
Astronomy and Space Science: Galactic Astronomy
The fundamental goal of galactic astronomy is to determine the scale and structure of the group of stars around Earth. The most obvious indication of this structure is the Milky Way (also called the galaxy, from the Greek for “milk”): a broad band of stars and cloudy illumination that stretches across the sky. It is a striking phenomenon and appears in many premodern mythologies.
Historical Background and Scientific Foundations
Early Western astronomy, beginning with the Greeks, paid surprisingly little attention to the Milky Way; Aristotle (384–322 BC), in fact, dismissed it as an atmospheric effect. It was not perfect, like the stars, nor did it move like the planets, making it unimportant to most early astronomers. The details of the Milky Way's size and shape, and even the existence of other galaxies like it, were determined only in the mid-twentieth century. The study of our galaxy has always been complicated by the fact that we are inside it, and the story of galactic astronomy is largely the story of how astronomers solved the problem of what we would look like from afar.
Historical Background and Scientific Foundations
The first step to understanding our galaxy came in 1610 when the Italian astronomer Galileo Galilei (1564–1642) improved the design of the recently invented telescope and turned it toward the night sky. His new instrument revealed that the Milky Way was, he said, “in fact nothing but a congeries of innumerable stars grouped together in clusters.”
The technical term for Galileo's observation is that he “resolved” the Milky Way—saw that it was not inherently cloudy, but was rather a collection of smaller objects. He also used the telescope to inspect other cloudy patches in the sky called nebulae (from the Latin nebula, meaning “cloud”). He found that, like the Milky Way, many of these nebulae could be resolved into groups of stars. Inaugurating a debate that would last for centuries, Galileo then concluded that all nebulae could be resolved into stars: “[O]ne may deduce another doctrine—that the nebulae and Milky Way do not exist in the sky but are a pure sensation of our eyes, in the sense that, if our vision were so acute as to distinguish between those minute stars, there would be neither nebulae nor a Milky Way upon the sky.”
Galileo also hoped his telescope would untangle another problem: how far away the stars were, and thus help determine the size of the universe. This had been a pressing issue since the Polish astronomer Nicolaus Copernicus (1473–1543) postulated that Earth was in motion. If that were true, then astronomers should have been able to observe a phenomenon known as stellar parallax—the apparent motion of a nearby object relative to a distant one as observed by an observer who is himself in motion. An observer on a moving Earth should be able to see nearby stars moving annually relative to distant ones. But this was not seen, meaning either that Copernicus was wrong or that the universe was much larger than anyone had imagined (since parallax decreases with the distance of the observed objects).
The English physicist and mathematician Isaac Newton (1642–1727) and others in the seventeenth century tried a different method to estimate the distances to the stars: They used a technique now known as standard candles, in which an astronomer uses a source of known brightness (absolute magnitude) and compares it to the same or similar object at an unknown distance. By looking at how bright the source appears at a distance (relative magnitude), and using the laws of optics, they were able to calculate how far away the object is.
The cosmology of the French philosopher René Descartes (1596–1650) postulated that the fixed stars were simply suns like our own at a great distance; Newton used this assumption to treat the sun as a standard candle (that is, that other stars were about the same absolute magnitude as the sun). This was very difficult to do accurately, but Newton's results suggested that the closest stars were millions of times as far away as the sun from Earth. This was vastly greater than anyone had ever imagined, and easily explained why stellar parallax had not been detected.
Newton also had to confront a puzzle derived from his own achievements in physics: If gravity were indeed universal, how do the fixed stars remain fixed? Why aren't they drawn to each other gravitationally? His solution was that divine action kept the stars in place—similar to his solution of the apparent instability of the solar system. These conclusions show Newton's commitment to a theology in which God regularly intervenes in the world, and also that he saw no precise border between scientific and theological problems.
Newton's defense of a static stellar system was called into doubt by his contemporary, the English astronomer Edmond Halley (1656–1742). In 1718 Halley discovered that a number of stars had moved an observable distance from positions at which they had been measured in antiquity. These changes, called the proper motions of the stars, seemed to indicate that the stars were not fixed, as had been assumed. Later German-born English astronomer William Herschel (1738–1822) realized that some of these motions could be a result of the sun's motion through a group of stars and tried to calculate the direction of that motion. He found that the sun was indeed moving toward a point (the solar apex) in the constellation Hercules.
The eighteenth century saw a series of developments that set up one of the fundamental tools of galactic astronomy: analogical reasoning between nebulae and the Milky Way. Thomas Wright (1711–1786) of Durham, a self-taught itinerant teacher in England, enters the story through a misreading. In 1750 Wright tried to explain the appearance of the Milky Way by postulating that our sun was part of a hollow sphere (or ring) of stars. If one looked toward the center of the sphere, one would see relatively few stars, but if one looked at a tangent to the sphere's surface, one would see a dense grouping of stars not unlike the Milky Way.
Wright's book was reviewed in a German periodical read by philosopher Immanuel Kant (1724–1804). Kant thought Wright had proposed a theory that the Milky Way was shaped like a solid disk, with the sun embedded somewhere within it. Kant was convinced that this not only explained the appearance of the night sky, but also had important cosmological consequences. He was aware of astronomical observations noting that many small nebulae appeared to be elliptical. Kant theorized that a disk of stars would appear elliptical if seen at an angle. Thus, he argued, the nebulae could be vast systems of stars like our own Milky Way. This bold claim came to be called the “island universe” theory, which postulated that the universe was an infinite sea populated with patches of stars separated by vast distances.
Careful observations of nebulae remained few until the late eighteenth century. The first major collection of nebular observations was actually intended for a very different purpose. French astronomer Charles Messier (1730–1817) searched fanatically for comets and was frustrated by nebulae and other objects that were easily confused with them. To alleviate this problem he composed in 1784 what is now known as the Messier catalogue, a list of 103 nebulae and similar features of the night sky against which comet hunters could check their observations. Many astronomical objects are still referred to by their Messier number (e.g., the Crab Nebula is M1).
Messier's accidental contribution to the study of nebulae inspired William Herschel to investigate them systematically. Herschel, a German musician who had immigrated to England, eventually taking a job as an organist in Bath, was an unlikely figure for epochal contributions to celestial science. Self-taught in astronomy and natural philosophy, Herschel learned to make his own precision telescopes and developed novel observing techniques. Assisted by his sister Caroline, a gifted observer in her own right, he charted the night sky in extraordinary detail.
Unlike many of his contemporaries, Herschel was interested in nebulae themselves: by 1820 he had charted 5,000. With his powerful telescope he found he could resolve some into systems of stars (clusters), and he carefully considered the island universe hypothesis that these clusters were similar to our own Milky Way. However, he became convinced that some nebulae were not distant groups of stars, but rather were made up of a “shining fluid.” He eventually concluded that most, if not all, of the nebulae were clouds of this fluid in the process of collapsing into stars. In this his thinking was similar to the nebular theory of Pierre Simon, marquis de Laplace (1749–1827), a French mathematician of extraordinary skill. These two possibilities—that nebulae were island universes or that nebulae were prestellar clouds—formed the core debate that would dominate galactic astronomy throughout the nineteenth century.
Herschel also conducted a number of pioneering studies of the Milky Way. He was interested in what he called the “construction of the heavens”: the arrangement of stars in three dimensions, particularly with respect to the Milky Way. He developed a technique he called “stargaging,” which involved combining some basic assumptions about the stars with precise observation. He assumed that the sun could be a standard candle for most stars, and extended this assumption of uniformity to the distribution of stars in space—he assumed stars were evenly spread throughout.
With these ideas, he then counted the number of stars visible in various directions, and inferred the depth of the Milky Way in that direction from that number (a higher number of stars meant the edge of the galaxy was distant, a low number meant it was close). He concluded that the sun was inside the Milky Way, probably near the center, and that the galaxy itself was vaguely lenticular (shaped like a double-convex lens). His placement of the sun within the Milky Way was quite significant, and many astronomers thought it implied that the galaxy was the entire universe, thus rejecting the island universe theory. Astronomers sharing this rejection used the terms “galaxy” and “universe” interchangeably.
With his standard-candle assumption Herschel was able to calculate roughly how far away the edges of the galaxy were. He judged that the most distant stars he could see were about 1019 miles away, a staggering distance. It was known by this time that light traveled at a finite speed and Herschel commented that the vast distance meant the light he was seeing in his telescope had begun its journey millions of years ago. This casual challenge to biblical chronology drew no special attention, as literal readings of Genesis would not become fashionable until the middle of the nineteenth century.
The assumptions that Herschel used to map the galaxy were realized to be erroneous even within his lifetime, but his achievements in trying to deduce the structure of our stellar system were remarkable nonetheless. William Herschel passed on his passion for astronomy to his son, John (1792–1871), who extended his father's survey of the nebulae to the southern hemisphere, discovering 2,306 nebulae and clusters while in South Africa between 1834 and 1838.
The implications of the massive body of observations compiled by the Herschels remained unclear in the mid-nineteenth century. Astronomers were still undecided about the resolvability of nebulae in general. Some nebulae could be resolved into stars with a telescope, some could not. Some remained cloudy in small telescopes, but could be resolved with very powerful telescopes. This raised a difficult philosophical question: If a given nebula could not be resolved, was that because it was a “true nebula”—i.e., a cloud of luminous material—or because the telescope being used was insufficiently powerful? How many nebulae did astronomers have to resolve before they concluded that all nebulae were star clusters?
These questions had serious implications for important theories: The nebular theory and its progressivist ideas required the existence of true nebulae, whereas the island universe theory expected all the nebulae to be distant stellar systems. Were there many galaxies, or just one? It was unclear how this dispute could be settled satisfactorily.
The issue was complicated further by the difficulty of representing the appearances of nebulae: Stellar photography was impossible until the 1870s due to Earth's rotation and the long exposure times required. This meant that astronomers like Herschel who wanted to show others what they had seen through their telescope had to draw their resolved (or unresolved) nebulae. Unfortunately nebulae are by nature cloudy and ill-defined, making them exceptionally difficult to draw with pencil or pen; there was serious debate about whether they could be rendered accurately at all. What might seem like a trivial task—accurately recording what had been seen—became a controversial and intractable problem.
Some observers thought the key to solving the resolution problem was to build more powerful telescopes, resolve more nebulae, and create an overwhelming body of data. The exemplar of this method was William Parsons, the third Earl of Rosse (1800–1867), an Irish nobleman who used his personal fortune to build in 1845 what would remain the largest telescope in the world for 70 years. This was the “Leviathan,” a scope 54 feet (16 m) long with a 72-inch (188 cm) aperture and a 4-ton (3,629 kg) mirror. By 1848 Rosse had observed 50 of the brightest nebulae in John Herschel's 1833 catalogue, and resolved them all. He found that many could be resolved into stellar systems of definite spiral structure, as well. Rosse's apparently unstoppable success in resolving every nebula he observed convinced many astronomers that there were no true nebulae.
The key to solving this problem came from a technique initially developed outside astronomy: spectroscopy. This technique analyzed the distinctive colors given off by elements heated to combustion, allowing scientists to determine the composition of an object solely by viewing it through a spectroscope. The founders of spectroscopy were able to detect elements such as sodium and iron in sunlight, and the wealthy British amateur astronomer William Huggins (1824–1910) found similar materials in distant stars. He then turned his spectroscope on a nebula and observed something striking. Instead of seeing the continuous spectrum always found in the sun and stars, he saw only the bright lines characteristic of rarefied gases. Huggins' observations indicated strongly that some nebulae were indeed “true nebulae,” clouds of gases and not distant clusters of stars. Thus by the late nineteenth century the resolution debate arrived at an unexpected conclusion: Both true nebulae and systems of stars existed.
By the end of the century, proponents of the island universe theory focused on the spiral nebulae discovered by Rosse. They argued that these spirals were the predicted stellar systems similar to our own Milky Way but at great distances. This raised the question of the structure of the Milky Way, since the island universe theory expected our galaxy to be similar to the spirals.
Unfortunately the appearance of the Milky Way is quite complex and is too difficult to allow an immediate analogy to be drawn between it and the spirals. The Milky Way seems to have no stars in its center and strange pockets of stellar density and vacuity (such as the Coalsack). There are certainly no obvious features that make one think of a spiral, and the search for the Milky Way's spiral structure was entirely guided by the island universe theory's analogy to spirals in the night sky.
Defenders of the theory needed to explain why our galaxy looked so different from the spirals. One possible explanation was that something obscured parts of the galaxy from view. It was difficult to know how to search for matter that could not be seen, so astronomers again reasoned by analogy with the spirals. In 1917 the American astronomer Heber Curtis (1872–1942) observed that spirals seen edge-on had dark bands across them, indicating that there could be obscuring material in our own galaxy.
Two years later American astronomer Edward Emerson Barnard (1857–1923) published extensive observations and his interpretations of them as dark clouds in interstellar space. He argued that what appeared as merely voids of stars were actually clouds of dust, persuading many astronomers that our ability to see our own galaxy's structure was significantly impaired. This was deemed support for the island universe theory, although it provided no direct evidence, only the weakening of certain objections against it.
In addition to observations of where stars and nebulae appeared in the sky, astronomers also tried to determine the structure of the galaxy from their movement. Proper motions of stars had been known since the seventeenth century, but had two important limitations: they only indicated motions across one's line of sight (i.e., perpendicular to the direction one looks), and could only be observed for objects very close by or moving very fast.
Astronomers' knowledge about the motions of the stars had expanded dramatically with the detection of the Doppler shift, which, in this context, describes how the color of light coming from a moving object will change relative to the observer. An object moving toward the observer will appear slightly bluer (blue shifted), and an object moving away from the observer will appear slightly redder (red shifted). These changes are very slight, and an object must be moving at very high speeds before they are noticeable, so it was not until the late-nineteenth century that precision spectroscopy was able to detect the Doppler shift in stars and other celestial bodies.
This technique is very powerful in that it can detect motion at any distance, but has the opposite limitation of proper motion observations: It can only measure movement along our line of sight. By combining the proper motion and Doppler shift of an object, however, one can determine an object's complete direction and speed. So by the first few years of the twentieth century, a group of stars whose motions astronomers could describe accurately allowed the beginning of a coherent picture of motion within the Milky Way.
The pioneer in analyzing these motions was the Dutch astronomer Jacobus Cornelius Kapteyn (1851–1922). Kapteyn was an astronomer without a telescope.
He had neither the climate nor the money for his own observations, so he focused on international efforts to provide needed data and on inexpensive mathematical investigations. He deduced some patterns in stellar motions whose significance would not be realized for some time: In the 1890s he noticed a correlation between spectral type and proper motion—yellow stars had greater proper motion than either red or blue stars. Then in 1904 he announced the discovery of a “star streaming” effect, where it seemed that our local stellar population consisted of two interspersed streams of stars moving in opposite directions. It was not clear exactly what that implied, but it did indicate strongly that the Milky Way had complex internal motions. Kapteyn's work indicated that the Milky Way was probably a fat disk a few thousand light years across, with the sun somewhere near the center.
With significant successes in charting the motions of stars, astronomers turned the same techniques to the nebulae. Doppler shift measurements of gaseous (“true”) nebulae showed that in general they had the same velocities of stars, which suggested that they were part of the gravitational system of the Milky Way. In 1915 Vesto Slipher (1875–1969) made the same measurements for spiral nebulae and found that they had very high velocities, usually away from the sun. This implied that they were not part of the Milky Way, and was support for the idea that they were distant galaxies. Slipher also conducted spectroscopic observations of end-on spiral nebulae that suggested they were rotating around their centers.
Scientific and Cultural Preconceptions
There were two major factors that complicated scientists' attempts to figure out the arrangement of our galaxy and the reality of distant galaxies. The first was a reluctance to accept the truly enormous distances involved. Copernicus's cosmology seemed to suggest that stars needed to be millions of miles away, and Herschel's work proposed the even more bizarre idea that the Milky Way was so large it would take millions of years for light to cross it. These distances were so large, and the scale so much greater than anything anyone had worked with before that they seemed absurd—people had trouble accepting they could be right. Similarly, very few people were willing to accept that the Milky Way itself was much larger than had been previously thought until the evidence became overwhelming. This problem was also difficult in cosmology, where there was significant resistance to the idea that the universe could be infinite.
The second factor was the problem of uniformity. Figuring out the distances to other stars, and therefore the structure of the galaxy, usually required scientists to decide whether distant stars were like our own sun. If stars were uniform (all similar in size and brightness), then certain conclusions could be drawn about distance and structure. But if stars were not, the conclusions drawn would be quite misleading. The early days of galactic astronomy usually involved scientists being either overly confident in uniformity or overly critical of uniformity, both of which led them astray. It was not until astronomers realized that there were several different types of stars that significant progress was made.
The idea that the spiral nebulae were galaxies like our own Milky Way was dealt a near-fatal blow by the Dutch-born American astronomer Adriaan van Maanen (1884–1946) in 1916. He observed rotations in spiral nebulae perpendicular to view by examining particular parts of a spiral nebula several years apart and measuring how far they had moved. Van Maanen estimated a rotational period (how long it took a star to do a full orbit) in the spiral of 85,000 years. The speed of rotation (e.g., in miles per second), however, depended on how far away the spiral was: Since objects look smaller the farther away they are, the same angular size actually corresponds to a very large object far away, or a small object up close.
At the time there was no way to measure the distance, so astronomers estimated that the nebula was about the size of the Milky Way (as predicted by the island universe theory). If this were true, then the deduced speed of rotation would be so fast that the galaxy would rip itself apart, and might even be faster than the speed of light—an impossibility under the theory of relativity. The implication was that the spiral nebula cannot be as large as the Milky Way. It must be instead close and small, apparently refuting the island universe theory.
Many astronomers still held to the island universe theory despite van Maanen's evidence, sometimes for aesthetic reasons, sometimes for its utility as a working hypothesis. This is an excellent example of a common phenomenon in science: An idea or theory survives despite observations that should immediately disprove it. This does not mean that astronomers at the time were dishonest or unskilled, but rather that empirical evidence does not automatically trump sophisticated and intriguing theories.
In the late 1920s the American astronomer Edwin Hubble (1889–1953) measured galactic distances that flatly contradicted van Maanen's observation of rotation in the spirals. It seemed that the two bodies of observations implied completely different facts about the universe. To resolve this difficulty, Hubble and two of his colleagues replicated all of van Maanen's measurements and found no evidence of detectable rotation. Van Maanen essentially withdrew with previous claims, and it became accepted that the rotations had been illusory. It is intriguing that his observations, which seemed so straightforward and empirical, could also be so wrong, and the incident is a useful warning in trusting naively in one set of results.
By the 1920s many of the basic facts about the Milky Way remained inaccessible due to a lack of knowledge of the distance to various celestial objects (most importantly, the spiral nebulae). In 1838 the German astronomer Friedrich Bessel (1784–1846) became the first to use the parallax method to measure the precise distance to another star, finding that the star 61 Cygni was about 11 light-years away (1 light-year, commonly abbreviated ly, is the distance light travels in one year: about 5.8 trillion miles/9.5 trillion km). This measurement gave the first precise indication of the spacing of stars within our galaxy, but could not be easily extended past about 300 ly.
Newton's and Herschel's attempts to use the sun as a standard candle failed because they had no way of knowing the absolute magnitude of distant stars. Some astronomers in the late nineteenth and early twentieth centuries tried to use novae (stars that appear suddenly or become dramatically brighter) as standard candles since they were visible in some spiral nebulae, but they failed because of a lack of information about what novae were and how bright they could get.
The search for a reliable standard candle finally made significant progress in the 1910s. The new Hertz-sprung-Russell diagram indicated that while the absolute magnitude of red stars varied widely, most blue stars had very similar absolute magnitudes, making them candidates for standard candles. Thus an astronomer could, in theory, measure the distance to any object (such as a star cluster) that contained a blue star. In practice this was sometimes difficult, and not particularly precise, but it allowed measurement of otherwise unavailable distances.
A second, more precise method resulted from the work of Henrietta Leavitt (1868–1921) at the Harvard College Observatory. One of several women assigned to examine photographs of variable stars, Leavitt was tracking a kind of variable star called a Cepheid, which can be characterized by how long it takes for its brightness to return to its peak, and found a number of them in the Small Magellanic Cloud. In 1912, she found a convincing mathematical relationship between the periods of variations and the Cepheids's peak brightness; that is, once she measured the period of a Cepheid she could deduce its absolute magnitude and therefore its distance.
Initially she was only able to find relative distances (e.g., this star is seven times as far away as that star), but statistical methods soon allowed this period-luminosity relationship to provide exact distances. Now astronomers could measure the distance to any object containing a Cepheid variable, with impressive precision. These two standard candle techniques set the stage for the developments that finally established the size of the Milky Way and determined whether the spiral nebulae were galaxies in their own right.
These issues were the center of what was called the “Great Debate” between the American astronomers Harlow Shapley (1885–1972) and Heber Curtis (1872–1942). Shapley was appointed director of the Harvard College Observatory in 1921, but several years earlier he had been a new Ph.D. investigating globular clusters at Mt. Wilson Observatory in Los Angeles. These were spherical groups of thousands of stars that had been known since the seventeenth century. Globular clusters were found to group in one half of the sky, near the densest star fields of the Milky Way, and it had been suggested that they marked the galaxy's boundary.
Shapley followed up on this hypothesis by trying to establish the distance to the clusters, hoping to establish the Milky Way's size. He searched for Cepheids in the clusters, finding about a dozen and measuring their distance. For clusters without detected Cepheids he made a bold assumption: that all globular clusters were about the same size. Then, using the clusters he had already measured as a baseline, he used simple geometry to deduce the distance to other clusters. This assumption of uniformity is in some sense impossible to prove, but was nonetheless fundamental to the study of the galaxy. Without it, astronomers simply could not have progressed.
Shapley presented his results in 1917, concluding that the Milky Way must be 300,000 ly across: a 1,000-fold increase over the conventionally accepted value, and displacing the sun far from the center. In addition to dramatically increasing the size of our galaxy (sometimes called the big galaxy theory), Shapley felt he had soundly disproved the island universe theory, since if the spirals were as gigantic as he had measured the Milky Way to be, and were at the distances expected, they would appear much larger on the sky.
In 1920 Heber Curtis, an expert on spiral nebulae and a defender of the island universe theory, vigorously attacked Shapley's conclusions. He argued that Shapley's methods (blue stars and Cepheids as standard candles, and the assumption of uniform size of globular clusters) were simply not strong enough to make such radical claims about the galaxy and the universe. Note that Curtis was not necessarily saying Shapley's results were wrong, but rather that the conclusions he drew from them were too dramatic. Curtis was a conservative thinker and expected science to advance by careful small steps, whereas Shapley saw himself as a revolutionary who could overthrow current ideas in one blow. (He compared himself to Copernicus.) Both were respected, and astronomers had a difficult time deciding between their arguments.
These issues were resolved by the work of the American astronomer Edwin Hubble at the Mt. Wilson observatory. In the mid—1920s he found Cepheid variables in spiral nebulae and used them to calculate, for the first time, an accurate distance to the spirals. He used Leavitt's period-luminosity relation to place the Andromeda Nebula (a prominent spiral) at about one million ly away, much farther than anyone had imagined. Hubble had provided the long-awaited evidence for the island universe theory: the spiral nebulae were indeed galaxies like our own at vast distances. He later also found an intriguing relationship between the distances to other galaxies and the speed with which they were moving away from the Milky Way, a discovery that would have important cosmological implications.
The technique of using standard candles was critical to the establishment of galactic astronomy, and remains an important tool in the science today. It relies on a property of light called the inverse square law, which states that a light source becomes dimmer in proportion to the square of its distance from the observer. This means that if the distance to a light source is doubled, it will appear four times as dim. This simple mathematical law means that the brightness of any star (its relative magnitude or apparent magnitude) is determined both by how bright it really is (its absolute magnitude) and its distance from Earth. Therefore, if astronomers know either a star's absolute magnitude or its distance, they can determine the other. An object whose absolute magnitude is known is called a standard candle because its distance can be easily determined. If they know neither magnitude nor distance, however, astronomers are out of luck.
Early attempts to solve this problem used the sun as a standard candle by assuming all stars were similar to it.
This assumed that all stars were uniform in brightness and size. It also assumed that there was no obscuring dust or clouds in between Earth and the star that might make it appear dimmer. It was recognized early on that neither assumption was well founded. Only when astronomers realized that there were different types of stars that varied greatly in brightness did the search for standard candles gain some speed. It was observed that blue stars did not range very much in brightness and could therefore function as crude standard candles. (The distances were uncertain by as much as a factor of two, however!) Standard candles techniques are also critical in cosmology, where supernovae are used instead of stars due to the great distances involved.
Modern Cultural Connections
Impact on Science
One remaining mystery was that the Andromeda Nebula (now the Andromeda Galaxy) appeared to be much smaller than the Milky Way. This was explained when the German-American astronomer Walter Baade (1893–1960) found that galaxies actually held two quite different populations of stars (referred to as type I and type II) and that Cepheids in the two populations had different magnitudes. His discovery required a recalibration of the period-luminosity relation, which doubled the calculated distances to other galaxies and indicated that they were indeed as large our own Milky Way.
The Cepheid standard candle technique demonstrated that the spiral nebulae were indeed stellar systems similar to our own at tremendous distances—the fundamental claim of the island universe theory. However, it was still unclear whether the Milky Way was a spiral like so many of its neighbors. By the mid—1930s, there was still no consensus on the structure of the galaxy. The Dutch-born American astrophysicist Bart Bok (1906–1983) proposed a detailed spiral model of the galaxy, arguing that we were unable to see the spiral arms because the sun was located near the galactic plane, which was seeded with many obscuring clouds. This was still only a defense of our ignorance, though, and not evidence for spiral structure.
Baade's investigations of stellar populations in the 1940s determined that spiral galaxies tend to have concentrations of bright blue stars in their arms; he suggested that astronomers could chart the Milky Way's spiral arms by looking for those stars. At the same time, in the Netherlands, Jan Oort (1900–1992) realized that Bok's obscuring interstellar clouds would not affect radio waves, and tried to track the so-called hyperfine radio emission from hydrogen clouds. Oort and his colleagues were working in very difficult conditions, however (using a radio antenna left by the German army), and made little progress. The hyperfine line was eventually first observed in 1951 at Harvard. In that year the spiral arms of the Milky Way were mapped out in detail by both the blue star method (by W.W. Morgan [1906–1994] in the United States) and by radio methods (in the Netherlands and Australia), finally establishing that our galaxy was similar in structure to the spiral nebulae observed long before.
By the end of the twentieth century astronomers agreed on the size, structure, and composition of the Milky Way. It was accepted that there is a large number (estimated at 1011) of other galaxies in the universe, and that they are generally similar despite the recognition of several broad types. The most important issue for twenty-first-century galactic astronomy is the how galaxies developed and evolved in the early universe, a problem critical for understanding the evolution of the universe as a whole.
Astronomers continue to debate the nature of objects in the early universe such as “active galaxies” that are tremendously energetic and the effect of large-scale phenomena such as galactic collisions. The difficulty of studying the Milky Way from within it remains, though, and astronomers understand distant galaxies far better than our own.
Impact on Society
The concept of island universes held important social significance. As far back as the ancient Greeks, speculation about whether life existed elsewhere was rampant.
At stake was whether human beings were unique. And as often happens, people disagreed about whether the truth of the island universe theory would have a positive or negative impact on the way humans thought about themselves.
The writings of Kant and Descartes revealed their belief that island universes would show that universe to be the result of a divine plan, and that the laws of nature had been set up to create life-supporting stars and planets. Most thinkers through the nineteenth century agreed that the universe was probably full of life, and that this was a tribute to a far-sighted deity. However, in the nineteenth century the rise of religious fundamentalism made some suspicious of the island universe theory. After all, the Bible makes no reference to other planets or other forms of intelligent life. Although everyone agreed that the uniqueness or ubiquity of intelligent life was an incredibly important fact about the universe, there was no consensus about exactly what that meant.
Modern Cultural Connections
Knowledge about galaxies is one of astronomy's most important tools for understanding the universe as a whole, as well as humanity's place in it. Galaxies are the yardstick by which astronomers determine the large-scale structure of the universe and its evolution. The crucial insights of galactic astronomy led to the initial proposal of big bang cosmologies, as well as the realization in the early 2000s that the universe is expanding at ever-accelerating rates.
IN CONTEXT: EDWIN P. HUBBLE AND THE RIDDLE OF THE NEBULAE
Hubble contributed to many of the decisive events in twentieth-century astronomy. He revolutionized our understanding of the nature of galaxies and the structure of the universe, and gave astronomers the foundations underlying modern cosmology.
Hubble was born in Missouri in 1889, the son of a lawyer. He studied astronomy at the University of Chicago with George Ellery Hale (1868–1938), and then attended Oxford as a Rhodes scholar. He received his law degree there and briefly practiced law. At one point he considered becoming a professional boxer, but eventually his interest in astronomy drove him back to Chicago, where he enrolled as a graduate student. When the United States entered World War I in 1917 Hubble immediately volunteered to fight. After the war he accepted a position at Mt. Wilson observatory in 1919, which had the world's best telescope. His research established him as the elder statesman of astronomy after World War II, and he remained an influential figure until his death in 1953.
Hubble's early research involved trying to classify nebulae, hoping to make it clear which types were internal to our galaxy and which were external. Once he had access to the powerful 100-inch (254-cm) telescope at Mt. Wilson he was able to make dramatic improvements on his earlier work. While studying the Andromeda Nebula in 1923, he found a Cepheid variable that allowed the first-ever calculation of the distance to a spiral nebula: 900,000 light years, far outside the boundaries of the Milky Way.
He carefully observed a large number of galaxies and classified them according to their shapes (elliptical and spiral galaxies making up the vast majority). Additionally, he discovered that distant galaxies had many of the same features as our own. His best-known contribution came in 1929 when he found that the distances to galaxies correlated with the speed with which they were moving away from us. His data formed a linear relationship where a more distant galaxy is moving faster than a closer one—a relationship now known as Hubble's Law. This peculiar result was of great significance to astronomers who had been investigating the cosmological implications of Einstein's theory of general relativity. Hubble's Law implied an expanding universe of the sort proposed by Georges-Henri Lemaître, and today forms the most important evidence for the big bang.
Cosmologists now argue that the formation of galaxies in the early universe was one of the most important factors in determining whether the universe would collapse in on itself or expand wildly apart. Some scientists note that if those early galaxies had developed in an even slightly different way, human beings would likely have never had the chance to evolve. Some claim that this shows that the universe was fine-tuned by an intelligent agent to ensure that life would develop at some point. This idea, called the anthropic principle, remains quite controversial. It suggests that human beings are in some sense the reason for the existence of the universe. Many scientists are uncomfortable with such claims, which they say violates the so-called Copernican principle, which states that human beings should not consider themselves or their location in the universe to be special in any way. Further observational and theoretical developments in galactic astronomy will help answer the question of whether we and our galaxy are in some way special or just one typical group of stars among billions.
Primary Source Connection
The following article was written by Peter N. Spotts, a science and technology writer for the Christian Science Monitor. Founded in 1908, the Christian Science Monitor is an international newspaper based in Boston, Massachusetts. The article describes the Galactic Zoo, a joint venture among American and British scientists to recruit help using the Internet for classifying about one million galaxies.
IDENTIFYING GALAXIES: EVERYMAN'S TASK
Astronomers are recruiting ordinary people around the world to help classify 1 million galaxies
The Galactic Zoo is open. And by all accounts, it's doing a land-office business.
Astronomers in Britain and the United States are enlisting people from around the world to help them sort through images of 1 million galaxies from the Sloan Digital Sky Survey, the most ambitious effort to date to map galaxies and quasars in the night sky.
The team's goal is simple: to group these enormous collections of planets, stars, dust, and gas by their shapes—elliptical blobs, stunning pinwheel-like spirals, and an odd assortment of other forms.
As simple as the task sounds, it's far too labor intensive for any individual astronomer, or even large team of astronomers, to tackle in a reasonable amount of time. Yet the payback for science could be huge, the researchers say. The information will help shed light on questions about how galaxies form and evolve. It could help test a recent challenge to a basic assumption about how the universe works. And the information could eventually be used to develop software that would allow computers to efficiently sort through even larger collections of galaxies from more-extensive sky surveys.
The project, dubbed Galaxy Zoo, certainly appeals to amateur astronomers, the project's scientists say. But it's open to anyone with a curiosity about the cosmos and an eye for sometimes-subtle detail. Art majors are welcome. The only nonhuman tools required: a computer and an Internet connection.
Like most scientists, astronomers typically tend to show people what they do, explains Chris Lintott, a junior research fellow at Britain's Oxford University and one of the organizers of the effort. “In this case, we really need them. We can't do this without the public.”
Research groups with other aims have said much the same. They've used the Internet to gather large ensembles of people to crunch numbers on home computers for a range of aims: finding radio signals from extraterrestrials, unraveling the secrets of proteins, modeling climate change—even solving large math problems and running particle-physics simulations. But these projects only love the public for its hardware. Once the programs are set up, the software can run with no additional intervention from the home-computer owner.
Harnessing the Human Eye
Galaxy Zoo's main tool is the human eye and human judgment. No computer can match those when it comes to the kind of pattern recognition this project requires, Dr. Lintott says.
The project stemmed from a Oxford graduate student's desperation. The student, Kevin Schawinski, was trying to perform this type of analysis on 50,000 galaxies in the Sloan collection en route to his PhD. But another conclusion emerged from that work: “After 50,000 galaxies, you never want to see one again,” Lintott says. Yet both scientists also saw enormous value in completing the analysis for the entire Sloan sample.
And while the team initially thought participants would post a new result once every one or two seconds, “We've gotten 20 times that,” says Alex Szalay, a professor of physics and astronomy at The Johns Hopkins University and an architect of the Sloan Digital Sky Survey's database.
They drew their inspiration for Galaxy Zoo from NASA's Stardust mission, which captured and returned samples of interstellar dust to an eager horde of scientists back on Earth. It also captured other bits of cosmic flotsam and jetsam in a special dust-gathering gel. Stardust scientists asked home-computer users to look at online images of the gel to help identify the tracks that signaled dust.
“Looking at galaxies is much more interesting than looking at dust!” Lintott quips.
A Clue for a ‘Universal’ Riddle
Researchers are looking at immediate uses for the data people provide. For Lintott, elliptical galaxies and how stars form in them are of keen interest. Theories of galaxy formation suggest that elliptical galaxies should form late in the universe's history; “they should just be forming now,” he says. But some of the oldest stars in the universe are found in these galaxies. One approach is to look at elliptical galaxies nearby that are forming stars to see if they hold clues to this paradox. This effort to classify galaxies will help him select ones for study, Lintott adds.
The project was formally announced last Wednesday. Within the first 60 hours, 40,000 people signed up.
Web hits vaulted to 6.5 million. The project's initial array of Internet servers ground to a halt under the load. So far, some 650,000 galaxies have received an initial classification. And while 40,000-plus pairs of eyes seem like plenty to do the job, more are needed. “We need to get 10 to 20 classifications per galaxy,” Dr. Szalay says. This would allow the scientists to more readily determine a galaxy's most likely shape. “We still have a lot of galaxies no one has looked at.”
The project, which also includes researchers from the University of Portsmouth in England, is drawing positive reviews from participants. Writes one member of a British government agency in an e-mail to the project team: “Just had a go at my first few galaxies. A great idea, and genuinely humbling to think that you're looking at something rarely or never seen before.”
Peter N. Spotts
spotts, peter n. “identifying galaxies: everyman's task.” christian science monitor
(July 16, 2007).
Primary Source Connection
Newer observation tools helped researchers discover clues about galaxy formation. Researchers used the Hubble Space Telescope's Ultra Deep Field (HUDF), the 26.9 ft (8.2-meter) Subaru telescope in Hawaii, and the Great Observatories Origins Deep Survey (GOODS) to observe some of the oldest and farthest identified objects in the known universe. These galaxies can help researchers better understand what the universe was like when it was young.
Author David Shiga is an astronomy and space reporter for New Scientist magazine.
FIRST GENERATION OF GALAXIES GLIMPSED FORMING
Astronomers have glimpsed some of the first galaxies in the universe in the process of taking shape, according to two new studies.
Scientists can study the early universe by looking at very distant objects. Their light takes billions of years to travel from them to us, so we see them as they were when the universe was very young.
Astronomers believe galaxies formed when many smaller clumps of stars and gas joined together in a process called hierarchical galaxy formation. But the details of this process in its earliest stages are unknown because no one has been able to observe it directly.
Now, astronomers have found signs that the early stages of this assembly process were going on between 700 and 900 million years after the big bang.
Rychard Bouwens and Garth Illingworth of the University of California in Santa Cruz, US, analysed data from the Hubble Space Telescope's Ultra Deep Field (HUDF) and the Great Observatories Origins Deep Survey (GOODS) to try to identify galaxies from the very early universe.
They found one galaxy whose colours suggest that it dates from just 700 million years after the big bang, along with three others whose ages are less certain but appear to be about as old.
Based on the survey, galaxies appear to be about five times less abundant at this time than the better studied period 200 million years later, the researchers say.
That suggests galaxies were still in the process of forming at the earlier time, the researchers say. Many smaller clumps of stars—which are too faint to detect even with Hubble—are likely present at this time and probably coalesced into proper galaxies 200 million years later.
“We always thought that galaxies would build up hierarchically,” Illingworth told New Scientist. “I think what's nice about this is that we can quantify it. We can actually see this effect happening.”
In a related study, another team of astronomers has identified a galaxy from 750 million years after the big bang, called IOK-1. This is not as far back as other candidates, including the Bouwens and Illingworth galaxies.
Its distance is considered to be more certain, however, making it the farthest galaxy known for which a distance has been firmly established.
Researchers led by Masanori Iye of the National Astronomical Observatory of Japan (NAOJ) in Tokyo used the 8.2-metre [26.9 ft] Subaru telescope in Hawaii to measure how its light spectrum shifted due to the expansion of the universe as the light travelled to Earth.
Iye's team had expected to see six galaxies rather than one at that distance. But unlike the other team, they believe this may simply be due to difficulty observing the galaxies. They believe there is more neutral hydrogen gas present at that era than at later times and that the gas blocks the wavelength of light they use to search for these galaxies.
This would be important if confirmed because researchers are anxious to find out exactly when and how the universe became transparent to light, transforming its neutral hydrogen into ionised hydrogen. This transformation, called reionisation, may have had a profound effect on galaxy formation.
“We are fairly confident that we are seeing that the fraction of neutral hydrogen is actually increasing” farther back in time, Iye says.
Richard Ellis of Caltech in Pasadena, US, says Bouwens and Illingworth's explanation for the smaller number of galaxies detected at early times is more likely to be correct, however.
They see their galaxies at a broad range of wavelengths—many of which would not be affected by neutral hydrogen—and they still see the decline towards earlier times, he says. “Probably Bouwens and Illing-worth are closer to the truth.”
Massimo Stiavelli of the Space Telescope Science Institute (STScI) in Baltimore, Maryland, US, says he thinks both effects could be at play—there could be fewer galaxies present at this time to be seen and on top of that, larger amounts of neutral hydrogen could make it harder to detect those that are present.
“I don't think the results are in contradiction,” he told New Scientist.
Journal reference: Nature (vol 443, p 186 and p 189)
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Spotts, Peter N. “Identifying Galaxies: Everyman's Task.” Christian Science Monitor (July 16, 2007).
Shiga, David. “First Generation of Galaxies Glimpsed Forming.” NewScientist.com (September 13, 2006). http://www.newscientist.com/article/dn10069 (accessed March 10, 2008).