(Background, above) The varying interrelationships among the many components of matter in our Milky Way Galaxy comprise a sort of "galactic ecosystem." Its evolutionary balance might be as complex as that of life in a tidal pool or a tropical rainforest. Here, stars abound throughout the Lagoon nebula, a rich stellar nursery about 1200 pc from Earth.

(Inset A) An emission nebula, the North American Nebula, glowing amidst a field of stars, its red color produced by the emission of light from vast clouds of hydrogen atoms.

(Inset B) An open cluster, the Jewel Box, containing many young, blue stars.

(Inset C) A typical globular cluster, 47 Tucanae. This true-color image reveals its dominant member stars to be elderly red giants.

(Inset D) A true-color Hubble image of part of the Cygnus Loop--a supernova remnant, the remains of a colossal stellar explosion that occurred about 15,000 years ago.

LEARNING GOALS

Studying this chapter will enable you to:

Describe the overall structure of the Milky Way Galaxy.

Explain the importance of variable stars in determining the size and shape of our Galaxy.

Specify how stars in the Galactic disk differ from those in the Galactic halo.

Explain how and why radio astronomy is useful in mapping the structure of the Galaxy.

Describe the orbital paths of stars in different regions of the Galaxy, and explain how these motions are accounted for by our understanding of how the Galaxy formed.

Discuss some possible explanations for the existence of the spiral arms observed in our own and many other galaxies.

Explain what studies of galactic rotation reveal about the size and mass of our Galaxy.

Explain the role of dark matter in the makeup of our Galaxy, and discuss the possible nature of such matter.

Describe some of the phenomena observed at the center of our Galaxy.

Looking out from Earth on a dark, clear night, we are struck by two aspects of the night sky. The first is a fuzzy band of light--the Milky Way--that stretches across the heavens. From the Northern Hemisphere, it is most easily visible in the summertime, arcing high above the horizon. Its full extent forms a great circle that encompasses the entire celestial sphere. Away from that glowing band, however, our second impression is that the nighttime sky seems more or less the same in all directions. Bunches of stars cluster here and there, but overall, apart from the Milky Way itself, the evening sky looks pretty uniform.

Yet this is only a local impression. Ours is a rather provincial view. When we consider much larger volumes of space, with dimensions far, far greater than the distances between neighboring stars, the spread of stellar and interstellar matter changes. It first becomes patchy and irregular; then, on still larger scales, new structure becomes apparent. Eventually, matter is spread so thin that stars, gas, and dust are all virtually nonexistent. We have now left the Milky Way Galaxy and entered intergalactic space--the vast, dark ocean that separates our own "island" of light and matter from its distant neighbors.

Back A galaxy is a gargantuan collection of stellar and interstellar matter--stars, gas, dust, brown dwarfs, black holes--isolated in space and held together by its own gravity. A fairly typical galaxy--not the one we inhabit, but another, similar system--is shown in Figure 23.1. This is the Andromeda Galaxy, lying about 700 kpc (700 kiloparsecs--700,000 pc, about 2 million light years) from the Earth. We have encountered it several times already in this text. Despite its enormous distance, Andromeda is the nearest major galaxy to our own. We show it here in place of our own because our home--the Milky Way Galaxy, or just "the Galaxy," with a capital G--is too large for us to see in its entirety or to photograph. We live inside it, and we cannot step outside to take a snapshot.

Figure 23.1 (a) The Andromeda Galaxy probably resembles fairly closely the overall layout of our own Milky Way Galaxy. The disk and bulge are clearly visible in this image, which is about 30,000 pc across. (b) More detail within the inner parts of the galaxy. (c) The galaxy's peculiar--and still unexplained--double core; this inset covers a region only 15 pc across.

Andromeda's apparent elongated shape is just a consequence of the angle at which we happen to view it. In fact, this galaxy, like our own, consists of a flattened, circular galactic disk of matter that fattens to a galactic bulge at the center. The disk and bulge are embedded in a roughly spherical ball of faint old stars known as the galactic halo. These three basic galactic regions are indicated in Figure 23.1 (although the halo stars are so faint that they cannot be discerned in this image). Armed with this knowledge, we can begin to understand and interpret the appearance of our own Galaxy from Earth, as illustrated in Figure 23.2. From our perspective within it, the Galactic disk is seen as a band of light stretching across the sky--the Milky Way.

Figure 23.2 Seen from within, the flattened disk of our Galaxy appears as a band of light across the heavens, known as the Milky Way. When we gaze at the Milky Way, we are looking in the plane of our Galaxy's disk; in other directions, our line of sight is out of the plane. The plane of the celestial equator is inclined at an angle of about 60 to the Galactic plane. The inset is (mostly) the Milky Way's plane as detected by IRAS, the Infrared Astronomy Satellite.

Deciphering the structure of the Milky Way Galaxy from Earth is a difficult task--a little like trying to unravel the layout of paths, bushes, and trees in a city park without being able to leave one particular park bench. In some directions, the interpretation of what we see is ambiguous and inconclusive. In others, foreground objects completely obscure our view of what lies beyond, but we cannot move around them to get a better look. As a result, astronomers who study the Milky Way are often guided in their efforts by comparisons with more distant, but more easily observable, systems. In this chapter, we will study the various parts that make up our parent Galaxy and see some of the methods used by astronomers in piecing together this picture.

The brief description we have just given represents the modern picture of our Galaxy. But before the early part of the twentieth century, astronomers had a markedly different view of the stellar system we inhabit. The fact that we live in just one of many enormous "islands" of matter separated by even larger tracts of apparently empty space was completely unknown, and the clear distinction between "our Galaxy" and "the universe" simply did not exist. Like the Copernican revolution before them, the twin ideas--that the Sun is not at the center of the Galaxy and that the Galaxy is not the center of the universe--required both time and hard observational evidence before they gained widespread acceptance.

Nineteenth-century astronomers were greatly hampered in their efforts to probe and understand the Galaxy by their inability to determine reliable distances to astronomical objects. Prior to the discovery of the main sequence in 1911 and the development of the distance-measurement technique now known as spectroscopic parallax, the locations of objects lying more than a few hundred parsecs from Earth were all but unknown.

As an example of the difficulties that resulted from the lack of a reliable distance scale, let's briefly reconsider the Andromeda Galaxy of Figure 23.1, or the Andromeda spiral nebula, as it was known in the midnineteenth century. With the observational techniques available at the time, it appeared only as a rather fuzzy, indistinct patch of light in the sky, with hints of swirling, spiral structure in it. Astronomers had no means of determining its distance, and they simply assumed that Andromeda and the many other known spiral nebulae were located in our Galaxy and were perhaps somehow similar to emission nebulae. Figure 23.3 shows a face-on view of another such system, in which the spiral structure is more plainly evident.

Figure 23.3 This spiral galaxy, seen nearly face-on, is fairly similar in its overall structure to our own Milky Way Galaxy and Andromeda. It is commonly known as M51--the 51st object in the Messier catalog.

By the end of the nineteenth century, improved telescopes and photographic techniques had allowed astronomers to obtain much better images, showing detail comparable to Figure 23.1. However, the "nebula's" distance was still unknown. In 1888, when such images were first presented publicly, they caused great excitement among astronomers, who thought they were seeing the formation of a star from a swirling gaseous disk! Comparing Figure 23.1 with the figures in Chapter 15 (see especially Figure 15.5), we can perhaps understand how such a mistake could be made--if we believed we were looking at a relatively close, star-sized object. Far from demonstrating that Andromeda was distant and large, the observations seemed to confirm that it was just a small part of our own Galaxy.

Further observations soon made it clear that Andromeda is not a star-forming region. For example, Andromeda's parallax is too small to measure, indicating that it must be at least several hundred parsecs from Earth. Even at 100 pc (which we now know is vastly less than Andromeda's true distance), an object the size of the solar nebula would be impossible to resolve and simply would not look like Figure 23.1.

By the early 1900s, the questions of the size of our Galaxy and the distance to Andromeda and the other spiral nebulae were being hotly debated. One school of thought maintained that they were nebulae much smaller than, and contained within, our own Galaxy. Other astronomers held that they were "island universes" outside the Milky Way Galaxy and comparable to it in size. However, with no firm distance information, both arguments were quite inconclusive. It was not until the late 1920s, with the discovery of the next rung in our cosmic distance "ladder"--the topic of the next section--that the spirals were finally shown to lie well outside our own Galaxy. The issue was finally settled in favor of the island-universe theory.

The "spiral nebulae" are known today as spiral galaxies. The characteristic pinwheel-like structures that give a spiral galaxy its name are called spiral arms. Each arm originates close to the central bulge of the parent galaxy and extends outward throughout much of the galactic disk. Our own Milky Way Galaxy is also of the spiral type, although our location within the Galactic disk makes the spiral structure difficult to discern from the Earth.

Cepheid Star in Distant Galaxy

Studies of nearby stars, gas, and dust, along with comparisons with other galaxies such as Andromeda, can give us a general idea of the overall distribution of matter in our Galaxy. But the story of the spiral nebulae illustrates just how important it is for us to know the distance to an astronomical object before we can determine its nature. The growth in our knowledge of the Galaxy, and the realization that there are many other distant galaxies similar to our own, have gone hand in hand with the development of the cosmic distance scale.

VARIABLE STARS AS DISTANCE INDICATORS

Back One by-product of the laborious effort to catalog stars around the turn of the twentieth century was the systematic study of variable stars. These are stars whose luminosity changes with time--some quite erratically, others more regularly. Only a small fraction of stars fall into this category, but those that do are of great astronomical significance.

We have encountered several examples of variable stars in earlier chapters. Often, the variability is the result of membership in a binary system. Eclipsing binaries, novae, and Type-I supernovae are cases in point. Novae and supernovae collectively are called cataclysmic variables because of their sudden, large changes in brightness. In other instances, however, the variability is a basic trait of the star itself, not dependant on the star being a part of a binary system. We call such a star intrinsically variable.

With regard to Galactic structure, the most important intrinsic variables are the pulsating variable stars (which have nothing to do with pulsars, by the way). The luminosity of a pulsating variable star varies in a smooth and predictable way. Figure 23.4 shows the light curve of one such star, whose brightness rises and falls by about a factor of 2 every 3 days. This type of pulsating star is a Cepheid variable (or simply a "Cepheid"), after the first star of the type to be discovered--Delta Cephei, the fourth brightest star in the constellation Cepheus. Cepheid variables are recognizable by the characteristic shape of their light curves (the rapid rise followed by the slower decline in Figure 23.4b). Different Cepheids have different pulsation periods, ranging from about 1 to 100 days, but the period of any given Cepheid is essentially constant from one cycle to the next.

Figure 23.4 (a) A Cepheid variable star (boxed) on successive nights; two photos, one from each night, have been nearly superimposed. The star is called WW Cygni and is shown at its maximum and minimum brightness. (b) Monitoring the star over the course of a week or so yields a record of the star's changing brightness.

A related class of pulsating variables is the RR Lyrae variable stars (again named after the first known of their kind, in this case the variable star labeled RR in the constellation Lyra). Like the Cepheids, they have a characteristic, easily recognizable light curve (shown in Figure 23.5) that accurately repeats itself. Unlike Cepheids, however, there is little difference in period between one RR Lyrae variable and another.

Figure 23.5 Light curve of the pulsating variable star RR Lyrae. All RR Lyrae-type variables have essentially similar light curves, with a period of less than a day.

Why do Cepheids and RR Lyrae variables pulsate? The basic mechanism was first suggested by the British astrophysicist Sir Arthur Eddington in 1941. The structure of any star is determined in large part by how easily radiation can travel from the core to the photosphere--that is, by the opacity of the interior, the degree to which the gas hinders the passage of light through it. If the opacity rises, the radiation becomes trapped, the internal pressure increases, and the star "puffs up." If the opacity falls, radiation can escape more easily, and the star shrinks. According to theory, under certain circumstances, a star can become unbalanced and enter a state in which the flow of radiation causes the opacity first to rise--making the star expand, cool, and diminish in luminosity--and then to fall, leading to the pulsations we observe.

The conditions necessary to cause pulsations are not found in main-sequence stars. However, they do occur in more evolved stars as they pass through a region of the Hertzsprung-Russell diagram known as the instability strip, shown in Figure 23.6. When a star's temperature and luminosity place it in this strip, the star becomes internally unstable. Both its temperature and its radius pulsate in a regular way, causing the variability we observe. High-mass stars evolve across the upper part of the H-R diagram; when their evolutionary tracks take them into the instability strip, they become Cepheid variables. RR Lyrae variables are low-mass horizontal-branch stars that lie in the lower portion of the instability strip. Similar unstable conditions can occur within stars in other regions of the H-R diagram, particularly on the Hayashi track and the red-giant branch, and other classes of variable stars are indeed associated with such objects. However, we will confine ourselves here to the Cepheids and RR Lyrae variables just discussed. Pulsating variable stars, then, are not a special class of object. They are just normal stars experiencing a brief period of instability as a natural part of stellar evolution.

Figure 23.6 Many pulsating variable stars are found in the so-called instability strip of the H-R diagram. As a high-mass star evolves through the strip, it becomes a Cepheid variable. Low-mass horizontal-branch stars in the instability strip are RR Lyrae variables.

A NEW YARDSTICK

Although Cepheids and other variable stars are interesting objects in their own right, our primary goal here is to use them to obtain information about the large-scale distribution of stars in our Galaxy. The key point about Cepheid variables is this: Their absolute brightnesses (averaged over a complete pulsation cycle) and their pulsation periods are rather tightly connected. Cepheids that vary slowly--that is, that have long periods--have large absolute brightnesses. The converse is also true: Short-period Cepheids have small absolute brightnesses. Figure 23.7 illustrates this relationship for Cepheids found within a thousand parsecs or so of the Earth. Astronomers can plot such a diagram for relatively nearby stars because they can measure their distances using stellar or spectroscopic parallax, as described in Chapter 17. Once the distances are known, the absolute brightnesses (luminosities) of these stars can be determined.

Figure 23.7 A plot of pulsation period versus average absolute brightness (that is, luminosity) for a group of Cepheid variable stars. The two properties are quite tightly linked. The pulsation periods of some RR Lyrae variables are also shown.

This link between period and brightness, shown in Figure 23.7, is known as the period-luminosity relationship. It was discovered in 1908 by Henrietta Leavitt of Harvard University (see Interlude 23-1 below). We know of no exceptions, and this relationship is consistent with theoretical calculations of pulsations in evolved stars. Consequently, we assume that it holds for all Cepheids, near and far. The roughly constant period of the RR Lyrae variables is also marked in Figure 23.7.

The beauty of the period-luminosity relationship is that a simple measurement of a Cepheid variable's pulsation period immediately tells us its luminosity--we just read it off the plot in Figure 23.7. The luminosity of an RR Lyrae star is even easier to determine. All such stars have basically the same absolute brightness--about 100 times that of the Sun--so once a variable star is recognized as being of the RR Lyrae type, its luminosity is known. In either case, comparing absolute and apparent brightnesses gives us an estimate of the star's distance, as described in Chapter 17.

Variable stars make up an important new rung in our cosmic distance ladder. The technique works well provided the star can be clearly identified and its period of variability measured. With Cepheids, this method allows astronomers to estimate distances out to several million parsecs, well beyond the range of either stellar parallax (around 100 pc) or spectroscopic parallax (several thousand pc). In fact, Cepheids can take us all the way to the nearest galaxies. Being less luminous, RR Lyrae stars are not as easily seen as Cepheids, so their useful range is not as great. However, they are much more common, so within their limited range, they are actually more useful than Cepheids.

Figure 23.8 extends our cosmic distance ladder, which we began in Chapter 1, to include Cepheid variables as a fourth method of determining distance. Because the period-luminosity relationship is calibrated using nearby stars, this latest rung inherits any and all uncertainties and errors present in the lower levels. Uncertainties also arise from the "scatter" shown in Figure 23.7. Although the overall connection between period and luminosity is unmistakable, the individual data points do not quite lie on a straight line. Instead, there is a range of possible luminosities corresponding to any measured period.

Figure 23.8 Application of the period-luminosity relationship for Cepheid variable stars allows us to determine distances out to about 15 Mpc with reasonable accuracy.

THE SIZE AND SHAPE OF OUR GALAXY

Many variable stars--specifically, stars of the RR Lyrae type--are found in globular clusters, those tightly bound swarms of old, reddish stars that we first met in Chapter 17. Approximately 140 globular clusters are visible from Earth. Early in the twentieth century, the American astronomer Harlow Shapley used observations of RR Lyrae stars to make two very important discoveries. First, he proved that most globular clusters reside at great distances--many thousands of parsecs--from the Sun. Second, by measuring the directions and distances to each, he was able to determine their three-dimensional distribution in space. In this way, Shapley demonstrated that the globular clusters map out a truly enormous, and roughly spherical, volume of space, about 30 kpc across. But the center of the sphere lies nowhere near our Sun--in fact, it is located nearly 8 kpc away from us in the direction of the constellation Sagittarius.

In a brilliant intellectual leap, Shapley realized that this 30-kpc-wide distribution of globular clusters maps out the true extent of stars in the Milky Way Galaxy--the region that we now call the Galactic halo. The hub of this vast collection of matter, about 8 kpc from the Sun, is the Galactic center. As illustrated in Figure 23.9, we live in the suburbs of this truly huge ensemble of matter, in the Galactic disk (or Galactic plane)--the thin sheet of young stars, gas, and dust that cuts through the center of the halo. Notice, incidentally, the jump in the scale of our units. When talking about stars and "nearby" nebulae, we generally measured distances in parsecs. Now, on a galactic scale, the kiloparsec (kpc) is more appropriate. Soon, as we leave our Galaxy behind, megaparsecs (Mpc) will become the norm.

Figure 23.9 Our Sun does not coincide with the center of the very large collection of globular clusters. Instead, more globular clusters are found in one direction than in any other. The Sun resides closer to the edge of the collection, which measures some 30 kpc across. We now know that the globular clusters outline the true distribution of stars in the Galactic halo.

Shapley's bold interpretation of the globular clusters as defining the overall distribution of stars in our Galaxy was an enormous step forward in human understanding of our place in the universe. Five hundred years ago, Earth was considered the center of all things. Copernicus argued otherwise, demoting our planet to an undistinguished place removed from the center of the solar system. Yet even in the early twentieth century, the prevailing view was that our Sun was the center of not only the Galaxy, but also of the universe. In addition, for reasons that we will discuss in a moment, it was also believed that our Galaxy measured only a few kiloparsecs across. Shapley showed otherwise. With his observations of globular clusters, he simultaneously increased the size of our Galaxy by almost a factor of 10 over earlier estimates and banished our parent Sun to its periphery, virtually overnight!

Curiously, Shapley's dramatic revision of the size of the Milky Way Galaxy and our place in it strengthened his erroneous opinion that the spiral nebulae were part of our Galaxy and that our Galaxy was essentially the entire universe. He regarded as simply beyond belief the idea that there could be other structures as large as our Galaxy. Only in the late 1920s was the Copernican principle extended to the Galaxy itself, when American astronomer Edwin Hubble observed Cepheids in the Andromeda Galaxy and finally succeeded in measuring its distance.

Back Since Shapley's time, astronomers have identified many individual stars--that is, stars not belonging to globular clusters--in the Galactic halo. The Galactic disk and halo are now recognized to be quite distinct components of our Galaxy, with very different properties. The spatial distribution of halo stars is unlike that of stars in the disk--material in the disk is confined to a highly flattened region in the Galactic plane that is thin compared with its diameter, whereas the halo is roughly spherical. *But aside from overall shape, the halo has other properties that set it apart from the disk. For one thing, the halo contains essentially no gas or dust--just the opposite of the disk, in which interstellar matter is common. For another, there are clear differences in both appearance and composition between halo and disk stars.

*Actually, there is growing evidence that the halo is somewhat flattened in the direction perpendicular to the disk, but the degree of flattening is quite uncertain. The halo, however, is certainly much less flattened than the disk.

Stars in the Galactic bulge and halo appear distinctly redder than stars found in the disk. We cannot see the big picture of our own Galaxy because of our location within the disk, but careful studies of individual stars near the Sun reveal the trend. Observations of other spiral galaxies show it clearly. In distant galaxies, only the brightest stars can be distinguished as individuals--the remainder merge into a continuous blur of light--but we can say that, on average, their disk stars tend to be bluer in appearance than stars in their bulges (and halos too, although they are much harder to see). This difference in appearance--the bluish tint of the disk and the yellowish or reddish coloration of the bulge--is also evident in Figures 23.1 and 23.3.

Figure 23.10 sketches this twofold distribution for our Galaxy. The local blue stars, observable to distances of a few thousand parsecs, are generally confined to the Galactic plane, as are the young open star clusters and star-forming regions. In contrast, the redder stars--including those found in the old globular clusters--are more uniformly distributed throughout the disk, bulge, and halo. From a distance, our Galactic disk would appear bluish simply because main-sequence O- and B-type blue supergiants are very much brighter than G, K, and M dwarfs, even though the dwarfs dominate the total number.

Figure 23.10 Edge-on view of a spiral galaxy, like our own Milky Way or Andromeda, showing the distributions of young blue stars, open clusters, old red stars, and globular clusters.

The accepted explanation for this marked difference in color between the disk and the halo is this: Whereas the gas-rich Galactic disk is the site of ongoing star formation and so contains stars of all ages, all the stars in the Galactic halo are old. The absence of dust and gas in the halo means that no new stars are forming there, and star formation apparently ceased long ago--at least 10 billion years in the past, judging from the types of halo stars we now observe. (Recall from Chapter 20 that most globular clusters are thought to be between 12 and 15 billion years old.

Support for this scenario comes from studies of the spectra of halo stars, which indicate that these stars are far less abundant in heavy elements (that is, elements heavier than helium) than are stars in the disk. In Chapters 20 and 21 we saw how each successive cycle of star formation and evolution enriches the interstellar medium with the products of stellar nucleosynthesis, leading to a steady increase in heavy elements with time. Thus, the scarcity of these elements in halo stars compared with stars in the disk is consistent with the view that the halo formed long ago. We will discuss the reasons for this disparity between the disk and the halo in a moment, when we study the formation of our Galaxy.

Astronomers often refer to young disk stars as Population I stars and old disk stars as Population II stars. The idea of two stellar "populations" dates back to the 1930s, when the differences between disk and halo stars first became clear. It represents something of an oversimplification, as there is actually a continuous variation in stellar ages throughout the Milky Way Galaxy, not a simple division of stars into two distinct "young" and "old" categories. Nevertheless, the terminology is still widely used.

OPTICAL OBSERVATIONS

In the late eighteenth century, long before the distances to any stars were known, the English astronomer William Herschel tried to estimate the size and shape of our Galaxy simply by counting how many stars he could see in different directions in the sky. Assuming that all stars were of about equal brightness, he concluded that the Galaxy was a roughly lozenge-shaped collection of stars lying in the plane of the Milky Way, with the Sun at the center. Subsequent refinements to this approach led to essentially the same picture and some people went so far as to estimate the dimensions of the "Galaxy" as about 10 kpc in diameter by 2 kpc thick.

As we have just seen, modern astronomers hold a very different view--the Milky Way is now known to be several tens of kiloparsecs across, and the Sun lies nowhere near the center. How could the older estimate have been so flawed? The answer is that the earlier observations were made in the visible part of the electromagnetic spectrum, and astronomers failed to take into account the (then unknown) absorption of visible light by interstellar gas and dust. Only in the 1930s did astronomers begin to realize the true extent and importance of the interstellar medium.

Objects more than a few kiloparsecs away in the plane of the Milky Way are hidden from our view by the effects of interstellar absorption. The central regions of our Galaxy cannot be studied by optical techniques. The apparent fall-off in the density of stars with distance in the plane of the Milky Way is thus not a real thinning of their numbers in space but simply a consequence of the rather murky environment in the Galactic disk. Because the obscuration occurs in all directions in the disk, the fall-off is roughly similar no matter which way we look, and so the Sun appears to be at the center.

In contrast, radiation coming to us from above or below the plane of the Galaxy, where there is less gas and dust along the line of sight, arrives on the Earth relatively unscathed. There is still some patchy obscuration, but the Sun happens to lie in a location where the view out of the disk is largely unimpeded by nearby interstellar clouds. Therefore, optical measurements of the disk's thickness are more accurate than are the measurements of the disk's width. Also, the observed fall-off in the density of stars above and below the Galactic plane is real.

LOCAL STRUCTURE

On the basis of optical, infrared, and radio observations of the many different types of objects--stars, gas, and dust--found within a thousand or so kiloparsecs of the Sun, astronomers have built up a fairly detailed picture of our local neighborhood. At the location of the Sun, some 8 kpc from the center, the Galactic disk is relatively thin--perhaps 300 pc thick, or about 1/100 of its diameter. Don't be fooled, though. Even if you could travel at the speed of light, it would take you 1000 years to traverse the thickness of the Galactic plane. The disk may be thin compared with the diameter of the Galaxy, but it is huge by human standards.

Actually, the thickness of the Galactic disk depends on the kinds of objects you measure. Young stars and interstellar gas are more tightly confined to the plane than are stars such as the Sun, and solar-type stars in turn are more tightly confined than are older K- and M-type dwarfs. Why? Generally, stars form in interstellar clouds close to the disk plane but then tend to drift out of the disk over time, due to their interactions with other stars and molecular clouds. Thus, as stars age, their abundance above and below the disk plane slowly increases. Note that these considerations do not apply to the Galactic halo, whose ancient stars and globular clusters extend far above and below the Galactic plane. The halo is a remnant of an early stage of our Galaxy's evolution and predates the formation of the disk.

Recently, improved observational techniques have revealed an intermediate category of Galactic stars, midway between the old halo and the younger disk, both in age and in spatial distribution. Consisting of stars with estimated ages in the range of 7-10 billion years, this so-called thick disk component of the Milky Way Galaxy measures some 2-3 kpc from top to bottom. Its thickness is too great to be explained by the slow drift just described. Like the halo, it appears to be a vestige of our Galaxy's distant past.

EXPLORING GALACTIC GAS

Back If we want to look beyond our immediate neighborhood and study the full extent of the Galactic disk, we cannot rely on optical observations alone. Interstellar absorption severely limits our vision. In the 1950s, astronomers developed a very important tool needed to explore the true spread of gas in our Galaxy--spectroscopic radio astronomy.

The key to observing Galactic interstellar gas is the 21-cm radio emission line produced by atomic hydrogen. This long-wavelength radiation is largely unaffected by interstellar dust. Thus, it travels more or less unimpeded through the Galactic disk, allowing us to "see" to great distances in this part of the electromagnetic spectrum. In addition, because hydrogen is by far the most abundant element in the interstellar medium, the 21-cm signals are strong enough that a large portion of the disk can be observed in this way. By studying the 21-cm radio lines emitted by atomic hydrogen gas throughout the Galaxy, radio astronomers have mapped out the large-scale distribution and motion of the Galaxy's interstellar clouds.

The 21-cm technique works well for regions abundant in atomic gas. But as noted in Chapter 18, molecular clouds are also widespread throughout the Galaxy. Because these clouds contain little or no atomic hydrogen, they are generally not detectable by their 21-cm emission. The problem of mapping molecular clouds is made all the more difficult because molecular hydrogen--the clouds' main constituent--is itself quite difficult to detect. Its strongest spectral lines just happen to lie in parts of the radio spectrum that are hard to observe with current radio telescopes.

Fortunately, to study the molecular gas in the Galaxy, we can use other molecules that do emit radio radiation. The carbon monoxide molecule is often used for this purpose, mainly because it is abundant in all molecular clouds (although it is still a million times less abundant than molecular hydrogen). Also, one of its characteristic spectral lines happens to occur in a part of the radio domain (at a wavelength of about 3 mm) that is easy to observe. Thus, although carbon monoxide accounts for only a tiny fraction of the matter in molecular clouds, it is a very useful tracer of molecular hydrogen. By measuring atomic hydrogen and carbon monoxide, we can probe most of the disk of the Milky Way.

RADIO MAPS OF THE MILKY WAY

The interstellar clouds in the Galactic disk exhibit an organized pattern on a grand scale. According to radio studies, the center of the gas distribution coincides roughly with that of the globular-cluster system, about 8 kpc from the Sun. (In fact, this figure of 8 kpc is derived most accurately from radio observations of the Galactic gas, whose center is normally taken to define the center of our Galaxy.) Radio-emitting gas has been observed out to at least 40-50 kpc from the Galactic center. Over much of the inner 20 kpc or so of the disk, the gas is confined quite closely to the Galactic plane. Beyond that, the gas distribution spreads out somewhat, to a thickness of several kiloparsecs, and shows definite signs of being "warped," possibly because of the gravitational influence of a pair of nearby galaxies (to be discussed in Chapter 24; see also Figure 23.12).

Near the center, the gas in the disk fattens markedly in the Galactic bulge. The high gas density in the inner part of the bulge makes it the site of vigorous ongoing star formation, and both very old and very young stars mingle there. The bulge is flattened in shape, measuring some 6 kpc across in the plane of the disk, but only about 4 kpc from top to bottom. Recent detailed studies of the motion of gas and stars in and near the bulge suggest that it may really be football-shaped, with the long axis of the football lying in the Galactic plane (see Figure 23.10).

Figure 23.11 shows two real images--(a) an infrared view of our own Galaxy and (b) an optical view of a distant galaxy like our own--that reinforce our belief in the correctness of this Galactic model.

Figure 23.11 (a) A wide-angle infrared image of the plane and bulge of our Milky Way, as observed by the Cosmic Background Explorer (COBE) satellite. (b) An image of the galaxy NGC 891, whose shape is believed to be quite similar to the Milky Way's.

Radio studies provide perhaps the best direct evidence that we really do live in a spiral galaxy. As shown in Figure 23.12, they show clear evidence that our Galaxy has spiral arms. One of these arms, as best we can tell, wraps around a large part of the entire disk and contains our Sun. The 30-kpc diameter marked on Figure 23.10 indicates the approximate extent of both the luminous "stellar" component of our Galaxy and of the known spiral structure--about the same as the diameter of the Galactic globular-cluster distribution.

Figure 23.12 An artist's conception of our Milky Way Galaxy seen face-on. This illustration is based on data accumulated by legions of astronomers during the past few decades, including radio maps of gas density in the Galactic disk. Painted from the perspective of an observer 100 kpc above the Galactic plane, the spiral arms are at their best-determined positions. All the features are drawn to scale (except for the oversized yellow dot near the top, which represents our Sun). The two small blotches to the left are dwarf galaxies, called the Magellanic Clouds. We will study them in Chapter 24.

Now let's turn our attention to the dynamics of the Milky Way Galaxy--that is, to the motion of the stars and gas it contains. Are the internal motions of our Galaxy's members chaotic and random, or are they part of some gigantic "traffic pattern"? The answer depends on our perspective. The motion of stars and clouds we see on small scales (within a few tens of parsecs of the Sun) seems random, but on larger scales (hundreds or thousands of parsecs) the motion appears much more orderly.

DISK AND HALO ORBITS

Back We see the gas and stars in the Galactic disk only from our vantage point on Earth as we orbit the Sun. Yet, as we look around the Galactic disk in different directions, a clear pattern of motion emerges, as summarized in Figure 23.13. The spectral lines emitted from Galactic material--both nearby stars (on average) and the more widely distributed interstellar gas--in the upper right quadrant and the lower left quadrant of Figure 23.13 are blueshifted. At the same time, the interstellar regions sampled in the upper left quadrant and the lower right quadrant are redshifted. In short, some regions (the blueshifted directions) in the Galaxy are approaching the Sun, whereas others (the redshifted ones) are receding from us. The important point is that they are moving in a systematic fashion.

Figure 23.13 Diagram of the four Galactic quadrants in which stars and interstellar clouds show systematic Doppler motions. This information tells us that the disk of the Galaxy is spinning in a well-ordered way. These quadrants are drawn (as dashed lines) to intersect at the Sun, not at the Galactic center, because it is from the viewpoint of our own planetary system that the observations are made. The longer the arrow, the greater the angular speed of the disk material.

Careful study of this fourfold pattern of Doppler-shifted stars and gas leads to the following important conclusion: The entire Galactic disk is rotating about the Galactic center. In the vicinity of the Sun, the orbital speed is about 220 km/s. At the Sun's distance of 8 kpc from the Galactic center, material takes about 225 million years--an interval of time sometimes known as 1 Galactic year--to complete one circuit.

Furthermore, the disk does not rotate at a uniform rate. Rather, it spins differentially--that is, stars and gas at different distances from the Galactic center take different lengths of time to complete one orbit, as we would expect from Kepler's laws. (Contrast this with solid-body rotation, in which every piece of the disk would move with the same angular speed and so would have the same orbital period, like a record spinning on a turntable.) Radio observations demonstrate that the inner regions of the Galactic disk take much less time to orbit the Galactic center than do the outer parts. Similar differential rotations are observed in Andromeda (and, in fact, in all other spiral galaxies). Thus stars in the Galactic disk do not move smoothly together but ceaselessly change their positions relative to one another as they orbit the Galactic center.

This picture of orderly circular orbital motion about the Galactic center applies only to the Galactic disk. Stars in the Galactic halo (as well as stars in the thick disk and Galactic bulge) are not so well behaved. The old globular clusters and the faint, reddish individual stars that make up the halo do not share the well-defined rotation of the disk. Instead, their orbits are largely random. *Although they do orbit the Galactic center, halo objects move in all directions, their paths filling the halo's entire three-dimensional volume. At any given radius, halo stars move at speeds comparable to the disk's rotation speed, but in every direction, not just one--their orbits carry them repeatedly through the disk plane and out the other side. Figure 23.14 illustrates this motion and contrasts it with the much more regular orbits in the disk. Some well-known stars in the vicinity of the Sun--the bright giant Arcturus, for example--are actually halo stars that are "just passing through" the disk on orbits that take them far above and below the Galactic plane.

*In fact, the stars that comprise the halo do have some net rotation about the Galactic center, but it is overwhelmed by the larger random component of their motion.

Figure 23.14 Stars in the Galactic disk move in orderly, circular orbits about the Galactic center. In contrast, halo stars have orbits with largely random orientations and eccentricities. The orbit of a typical halo star takes it high above the Galactic plane, through the disk, then far below the plane on the other side of the Galaxy.

THE FORMATION OF OUR GALAXY

/There are many differences between the Galactic disk and the Galactic halo; a few of them are listed in Table 23-1. Is there some evolutionary scenario that can naturally account for the present-day disk-halo structure we see? The answer is that there is, and it takes us all the way back to the birth of our Galaxy, some 10-15 billion years ago. Not all the details are agreed upon by all astronomers, but the overall picture is now fairly widely accepted.

When the first stars and globular clusters formed, the gas in our Galaxy had not yet accumulated into a thin disk. Instead, it was spread out over a rather irregular, and quite extended, region of space, spanning many tens of kiloparsecs in all directions. When the first stars formed, they were distributed throughout this volume. Their distribution today (in the halo) reflects that fact--it is an imprint of their birth. Many astronomers believe that the very first stars formed even earlier, in smaller systems that later merged to create our Galaxy. The present-day halo would look the same in either case. Figure 23.15 illustrates this view of our Galaxy's evolution.

Figure 23.15 Astronomers reason that, early on, our Galaxy was rather irregularly shaped, with gas distributed throughout its volume. Possibly it formed via the merger of several smaller systems, as depicted in (a) and (b). When stars formed during these stages, there was no preferred direction in which they moved and no preferred location in which they were found. In time, rotation caused the gas and dust to fall to the Galactic plane and form a spinning disk, as in (c). The older stars were left behind, forming the halo. (d) New stars forming in the disk inherit its overall rotation and so orbit the Galactic center on ordered, circular orbits.

During the past 10-15 billion years, rotation has flattened the gas in our Galaxy into a relatively thin disk, which contains virtually all the gas, dust, and young stars in the Milky Way. Physically, this process is rather similar to the flattening of the solar nebula to a disk during the formation of the solar system, as described in Chapter 15, except on a vastly larger scale. Star formation in the halo ceased billions of years ago when the raw materials fell to the Galactic plane. Ongoing star formation in the disk gives the plane its bluish tint, but the halo's short-lived blue stars have long since burned out, leaving only the long-lived red stars that give it its characteristic pinkish glow. The Galactic halo is ancient, whereas the disk is full of youthful activity. The thick disk, with its intermediate-age stars, may represent an intermediate stage of star formation that occurred while the gas was still flattening into the plane.

The chaotic orbits of the halo stars are also explained by this theory. When the halo developed, the irregularly shaped Galaxy was rotating only very slowly, so there was no strongly preferred direction in which matter tended to move. As a result, halo stars were free to travel along nearly any path once they formed (or when their parent systems merged). As the Galactic disk formed, however, conservation of angular momentum caused it to spin more rapidly. Stars forming from the gas and dust of the disk inherit its rotational motion and so move on well-defined, circular orbits. Again, the thick disk's properties suggest that it formed while gas was still sinking to the Galaxy's midplane and had not yet reached its final (present-day) rotation rate.

In principle, the structure of our Galaxy bears witness to the conditions that created it. In practice, however, the interpretation of the observations is made difficult by the sheer complexity of the system we inhabit and by the many competing physical processes that have modified its appearance since it formed. As a result, the early stages of the Milky Way are still very poorly understood. We will return to the subject of galaxy formation in Chapter 24.

Back Studies of spiral structure in our own Galaxy and in others indicate that the spiral arms are made up of much more than just interstellar clouds. Young objects--such as O- and B-type stars, open clusters, and emission nebulae--all reside in the arms, too. In fact, these objects are generally not found outside the spiral arms. The obvious conclusion is that the spiral arms are the part of the Galactic disk where star formation actually takes place. The brightness of these young stellar objects is the main reason that the spiral arms of other galaxies are easily seen from afar (see Figure 23.3).

A central problem facing astronomers trying to understand spiral structure is how that structure persists over long periods of time. The basic issue is really very simple: We know that the inner parts of the Galactic disk rotate more rapidly than the outer regions. This differential rotation makes it impossible for any large-scale structure "tied" to the disk material to survive.

To understand why differential rotation poses a problem, consider that our Sun, at roughly 8 kpc from the Galactic center, takes about 225 million years to complete one circuit of the Milky Way. Thus our 4.5 billion-year-old solar system has cycled around the Galactic center some 20 times since being formed. In the same time, however, stars lying closer to the Galactic center have completed many more revolutions, while those farther out toward the edge of the disk have made far fewer. The result of this differential motion, illustrated in Figure 23.16, is that a spiral pattern consisting always of the same group of stars and gas clouds would necessarily "wind up" and disappear within a few hundred million years. Yet spiral arms clearly do exist in our own galaxy, and their prevalence in other disk galaxies suggests that they last for considerably longer than this. Thus, whatever the spiral arms are, they cannot simply be dense star-forming regions orbiting along with the rest of the Galactic disk.

Figure 23.16 This painting illustrates the fact that the disk of our Galaxy rotates differentially-- stars close to the center take less time to orbit the Galactic center than those at greater distances. If spiral arms were somehow tied to the material of the Galactic disk, this differential rotation would cause the spiral pattern to wind up and disappear in a few hundred million years. Spiral arms would be too short-lived to be consistent with the numbers of spirals actually observed.

How then do the Galaxy's spiral arms retain their structure over long periods of time in spite of differential rotation? A leading explanation for the existence of spiral arms holds that they are spiral density waves--coiled waves of gas compression that move through the Galactic disk, squeezing clouds of interstellar gas and triggering the process of star formation as they go. The spiral arms we observe are outlined by the denser-than-normal clouds of gas they create, and by the new stars formed as a result of the spiral wave's passage.

This explanation of spiral structure avoids the problem of differential rotation because the wave pattern is not tied to any particular piece of the Galactic disk. The spirals we see are merely patterns moving through the disk, not great masses of matter being transported from place to place. The density wave moves through the collection of stars and gas comprising the disk just as a sound wave moves through air or an ocean wave passes through water, compressing different parts of the disk at different times. Even though the rotation rate of the disk material varies with its distance from the Galactic center, the wave itself remains intact, defining the Galaxy's spiral arms.

In fact, over much of the inner part of the Galactic disk (within about 15 kpc of the center), the spiral wave pattern is predicted to rotate more slowly than the stars and gas. Thus, Galactic material actually catches up with the wave, is temporarily slowed down and compressed as it passes through, then continues on its way. (For a more down-to-earth example of an analogous process, see Interlude 23-2 below.) As shown in Figure 23.17, the slowly moving spiral density wave is outrun by the faster rotation of the disk. As gas enters the arm from behind, it is compressed and forms stars. Dust lanes mark the regions of highest-density gas. The most prominent stars--the bright O and B blue giants--live for only a short time, so OB associations, young star clusters, and emission nebulae are found only within the arms, near their birthsites. Their brightness emphasizes the spiral structure. Further downstream, ahead of the spiral arms, we see mostly older stars and star clusters. These have had enough time since their formation to outdistance the wave and pull away from it. Over millions of years, their random individual motions distort and eventually destroy their original spiral configuration, and they become part of the general disk population.

Figure 23.17 Density-wave theory holds that the spiral arms seen in our own and many other galaxies are actually waves of gas compression and star formation moving through the material of the galactic disk. Gas (red arrows) enters the arm (white arrows) from behind, is compressed, and forms stars. The spiral pattern we see in this painting at right is delineated by dust lanes, regions of high gas density, and newly formed O and B stars. The inset photograph at left shows the spiral galaxy NGC 1566, which displays many of the features described in the text. Note, incidentally, that although the figure shows a "two-armed" spiral, astronomers are not completely certain how many arms make up the spiral structure in our own Galaxy (see Figures 23.10 and 23.12). The theory makes no strong predictions on this point.

An alternative possibility is that the formation of stars drives the waves, instead of the other way around. Imagine a row of newly formed massive stars somewhere in the disk. As these stars form, the emission nebulae that appear around them send shock waves through the surrounding gas, possibly triggering new star formation. Similarly, when the stars explode in supernovae, more shocks are formed. As illustrated in Figure 23.18(a), the formation of one group of stars thus provides the mechanism for the creation of more stars. Computer simulations suggest that it is possible for the "wave" of star formation thus created to take on the form of a partial spiral and for this pattern to persist for some time. However, this process, sometimes known as self-propagating star formation, can produce only pieces of spirals, as are seen in some galaxies (see Figure 23.18b). It apparently cannot produce the galaxy-wide spiral arms seen in other galaxies and present in our own. It may well be that there is more than one process at work in the spectacular spirals we see.

Figure 23.18 (a) Self-propagating star formation. In this view of the formation of spiral arms, the shock waves produced by the formation and later evolution of a group of stars provides the trigger for new rounds of star formation. We have used supernova explosions to illustrate the point here, but the formation of emission nebulae and planetary nebulae are also important. (b) This process may well be responsible for the "partial" spiral arms seen in some galaxies, such as NGC 300, shown here in true color. The distinct blue appearance derives from the vast numbers of young stars that pepper its ill-defined spiral arms.

An important question (one that unfortunately is not answered by either of the theories just described) is: Where do these spirals come from? What was responsible for generating the density wave in the first place, or for creating the line of newborn stars whose evolution drives the advancing spiral arm? Scientists speculate that (1) instabilities in the gas near the Galactic bulge, (2) the gravitational effects of our satellite galaxies (the Magellanic Clouds, to be discussed in Chapter 24), or (3) the possible asymmetry within the bulge itself may have had a big enough influence on the disk to get the process going. The first possibility is supported by growing evidence that many other spiral galaxies seem to have experienced gravitational interactions with neighboring systems in the relatively recent past (see Chapter 24). However, many astronomers still regard the other two possibilities as equally likely. For example, they point to isolated spirals, whose structure clearly cannot be the result of an external interaction. The truth of the matter is that we still don't know for sure how galaxies--including our own--acquire such beautiful spiral arms.

The Ongoing Search for Dark Matter

GALACTIC ROTATION

Back What can we learn by studying the motions of the clouds and stars that make up our Galaxy? One very important quantity that we can measure is the mass of the Milky Way. To see how this is accomplished, let us apply the same principles used in studies of planetary motions to observations of Galactic dynamics. Recall from Chapter 2 that Kepler's third law (as modified by Newton) connects the orbital period, orbit size, and masses of any two objects in orbit around one another. We expressed this law as follows:

In the solar system, the total mass includes both the mass of the Sun and the mass of an orbiting planet. Because the mass of any planet is small compared with that of the Sun, we can safely ignore the planet. The mass we compute is the mass of the Sun, to good accuracy. Similarly, in applying Kepler's law to weigh the Galaxy, we can neglect the Sun's mass, which is extremely small compared to the mass of the Galaxy. Once we know the orbit of the Sun around the Galactic center, we have all the quantities we need to apply the equation. We can consider the result to be a measure of the Galaxy's mass.

We must make one important clarification at this point. In the case of a planet orbiting the Sun, there is no ambiguity about what mass is being measured--it is the Sun's. However, the case of the Sun orbiting the center of the Galaxy is more complicated. The Galaxy's matter is distributed over a large volume of space and is not concentrated at the Galactic center (as the Sun's mass is concentrated at the center of the solar system). Some of the Galaxy's mass lies inside the Sun's orbit (that is, within 8 kpc of the Galactic center), and some lies outside, at large distances from both the Sun and the Galactic center. The question naturally arises: Which portion of the Galaxy's mass controls the Sun's orbit? Isaac Newton answered this question three centuries ago: The Sun's orbital period is determined by the portion of the Galaxy that lies within the orbit of the Sun. This is the mass computed from the foregoing equation.

The distance from the Sun to the Galactic center is about 8 kpc, and the Sun's orbital period, as we have seen, is 225 million years. Substituting these numbers into the above equation, we find that the mass of the Milky Way Galaxy within the Sun's orbit is almost 1011 solar masses--100 billion times the mass of our Sun. The Milky Way Galaxy is truly vast, both in size and in mass.

DARK MATTER

Back The mass we have just computed for the Galaxy is the mass of stars, gas, dust, and everything else residing inside the Sun's orbit around the Galactic center. As we have just seen (recall Figure 23.12), a lot more matter lies outside that radius, in the form of stars and interstellar gas. Yet the luminous portion of the Milky Way Galaxy--the region outlined by the globular clusters and by the spiral arms--is merely the "tip of the Galactic iceberg." There is strong evidence that our Galaxy is in reality very much larger.

To determine the mass of the Galaxy on larger scales--that is, to find how much matter is contained within spheres of progressively larger radii--we must measure the orbital motion of stars and gas farther from the Galactic center than is the Sun. Astronomers have found that the most effective way of doing this is by making radio observations of gas in the Galactic disk, because radio waves are relatively unaffected by interstellar absorption and allow us to probe to great distances, far outside the Sun's orbit. On the basis of these studies, radio astronomers have determined our Galaxy's rotation rate at various distances from the Galactic center. The resultant plot of rotation speed versus distance from the center is the called the Galactic rotation curve. It is shown in Figure 23.19.

Figure 23.19 The rotation curve for the Milky Way Galaxy plots rotation speed against distance from the Galactic center. We can use this curve to compute the mass of the Galaxy that lies within any given radius. The dashed curve is the rotation curve that would be expected if the Galaxy "ended" abruptly at a radius of 15 kpc, the limit of most of the known spiral structure and the globular cluster distribution. The fact that the curve does not fall off, but in fact rises beyond that point, indicates that there must be additional matter beyond that radius.

Knowing the Galactic rotation curve, we can repeat our earlier calculation to compute the total mass that lies within any given distance from the Galactic center. We find, for example, that the mass within about 15 kpc from the center--the volume defined by the globular clusters and the known spiral structure--is some 2 × 1011 solar masses, about twice the mass contained within the Sun's orbit. Does most of the matter in the Galaxy "cut off" at this point, where the luminosity drops off sharply? Surprisingly, the answer is "No."

If most of the matter in the Galaxy ended at the edge of the visible structure, Newton's laws of motion would predict that the orbital speed of stars and gas beyond 15 kpc would decrease outward, just as the orbital speeds of the planets diminish with increasing distance from the Sun. The dashed line in Figure 23.19 indicates what the rotation curve would look like in that case. However, the true rotation curve is quite different. Far from falling off at larger distances, it actually rises slightly out to the limits of our measurement capabilities. This implies that the amount of mass contained within successively larger radii continues to grow beyond the orbit of the Sun, apparently out to a distance of at least 40 or 50 kpc. According to our earlier formula, the amount of mass within 40 kpc is approximately 6 × 1011 solar masses. In other words, roughly twice as much mass lies outside the luminous part of our galaxy--the part made up of stars, star clusters, and spiral arms--as lies inside!

Astronomers now believe that our Galaxy is surrounded by an extensive, invisible dark halo, which dwarfs the inner halo of stars and globular clusters and extends well beyond the 15-kpc radius once thought to be the limit of our Galaxy. As we will see, researchers have found evidence for dark halos in many other galaxies, leading them to suspect that most galaxies are actually much larger and more massive than their optical images suggest. But what is this mass? We do not detect enough stars or interstellar matter to account for the mass that our computations tell us must be there. We are inescapably drawn to the conclusion that most of the mass in our own and in other galaxies exists in the form of invisible dark matter, which we presently simply do not understand.

The term dark here does not refer just to matter undetectable in visible light. The material has (so far) escaped detection at all wavelengths, from radio to gamma rays. Only by its gravitational pull do we know of its existence. Dark matter is not hydrogen gas (atomic or molecular), nor is it made up of stars. Given the amount of matter that must be accounted for, we would have been able to detect it by now with present-day equipment if it were in either of those forms. Its nature and its consequences for the evolution of galaxies and the universe are among the most important questions in astronomy today.

Many candidates have been suggested for this dark matter, although none is proven. Among the strongest contenders are the brown dwarfs discussed in Chapter 19--low-mass prestellar objects that never reached the point of nuclear burning in their cores. These objects could exist in great numbers throughout the Galaxy, yet could be exceedingly hard to see. The fact that we observe so many low-mass stars strongly suggests that the even lower-mass brown dwarfs may be a very common "by-product" of star formation. Black dwarfs, the end result of low-mass stellar evolution (Chapter 20) are another possibility. Again, they would be very hard to detect, although it seems unlikely that many stars have actually had time to reach this advanced evolutionary stage. Black holes also might supply the unseen mass, although their very existence is still debated, and very few candidates exist. However, given that they are the evolutionary product of (relatively rare) massive stars, it is hard to believe that there could be enough of them to hide large amounts of Galactic matter.

All these candidates are stellar in nature--they are associated with star formation or the late stages of stellar evolution. A radically different alternative is that the dark matter is made up of exotic subatomic particles that pervade the entire universe. Although there is (as yet) no hard experimental evidence for them, many theoretical astrophysicists believe that these particles could have been produced in abundance during the very earliest moments of our universe. If the particles have survived to the present day, there might be enough of them to account for all the dark matter we believe must be out there. This idea is hard to test, however, because any particles of this nature that might exist would be very hard to detect. Several detection experiments have been attempted--so far, without success.

THE SEARCH FOR STELLAR DARK MATTER

As noted earlier, it is unlikely that appreciable amounts of dark matter are hidden in any kind of yet unseen stars. Small, dim red-dwarf stars were once considered ideal candidates for dark matter. The reasoning was that, just as pebbles are more plentiful on a beach than rocks, then perhaps low-mass stars are very widespread, but not yet detected. However, recent observations of ancient globular clusters--where such red dwarfs might be especially abundant--have shown that faint red dwarfs are actually sparse in the Milky Way. Analysis of imagery like that shown in Figure 23.20 suggests that there is a cut-off point at about 0.2 M , below which nature rarely makes such dim, low-mass stars.

Figure 23.20 Very sensitive visible observations with the Hubble Space Telescope have apparently ruled out faint red-dwarf stars as candidates for dark matter. The object shown here, the globular cluster NGC 6397, is one of many regions searched in the Milky Way. The inset, 0.4 pc on a side, shows a high-resolution Hubble view. The scores of diamonds have been overlaid at positions where red dwarfs might (statistically) have been expected if they did indeed make up the dark matter, but were not found.

Recently, one very promising means of detecting stellar dark matter has begun to show results. It involves a key element of Albert Einstein's theory of general relativity (see Chapter 22, especially Interlude 22-2)--the prediction that a beam of light can be deflected by a gravitational field, which has already been verified in the case of starlight passing close to the Sun. Although this effect is small, it has the potential for making otherwise invisible stellar objects observable from the Earth. Here's how.

Imagine looking at a distant star as a faint, compact object such as a brown dwarf, or a distant white dwarf, happens to cross your line of sight. As illustrated in Figure 23.21, the intervening dwarf deflects a little more starlight than usual toward the observer, resulting in a temporary, but quite substantial, brightening of the background star. In some ways, the effect is like the focusing of light by a lens, and the process is known as gravitational lensing. The foreground dwarf is referred to as a gravitational lens. The amount of brightening and the duration of the effect depend on the mass, distance, and velocity of the lensing object. Typically, apparent brightness increases from 2 to 5 times its usual amount for a period of several weeks. Thus, even though the dwarf cannot be seen directly, its effect on the light of the background star makes it detectable. (In Chapter 25 we will encounter other instances of gravitational lensing in the universe, but on very much larger scales.)

Figure 23.21 Gravitational lensing by a foreground dark object such as a brown dwarf can temporarily cause a background star to brighten significantly, providing a possible means of detecting stellar dark matter.

Of course, the probability of one star passing almost directly in front of another, as seen from Earth, is extremely small. But by observing millions of stars every few days over a period of years (using automated telescopes and high-speed computers to reduce the burden of coping with so much data), astronomers hope to see enough of these events to let them measure the mass of stellar dark matter in the Galactic halo. The technique presents an exciting, and potentially very important, new means of probing the structure of our Galaxy. In late 1993, three groups of researchers--in the United States, Europe, and Australia--each announced the detection of a lensing event. Subsequent reports from these groups seem to suggest that they are seeing too few lensing events to support the theory that brown dwarfs and faint, low-mass stars account for all of the dark matter inferred from dynamical studies. However, it is still too early to draw any definite conclusions.

Bear in mind, though, that the identity of the dark matter is not necessarily an "all-or-nothing" proposition. It is perfectly conceivable that more than one type of dark matter exists. For example, it is quite possible that most of the dark matter in the inner (visible) parts of galaxies could be in the form of brown dwarfs and very low-mass stars, while the dark matter on larger scales might be primarily in the form of exotic particles. We will return to this perplexing problem in later chapters.

Sgr A* at the Galactic Center

Back Theory predicts that the Galactic bulge, and especially the region close to the Galactic center, should be densely populated with billions of stars. However, we are unable to see this region of our Galaxy--the interstellar medium in the Galactic disk shrouds what otherwise would be a stunning view. Figure 23.22 shows the (optical) view we do have of the region of the Milky Way toward the Galactic center, in the general direction of the constellation Sagittarius.

Figure 23.22 A photograph of stellar and interstellar matter in the direction of the Galactic center. Because of heavy obscuration, even the largest optical telescopes can see no farther than 1/10 the distance to the center. The M8 nebula can be seen at the extreme top center. The field is roughly 20°, top to bottom, and is a continuation of the bottom part of Figure 18.6. The circle indicates the location of the core of our Galaxy.

With the help of infrared and radio techniques, we can peer more deeply into the central regions of our Galaxy than we can with optical techniques. Infrared observations (see Figure 23.23a) indicate that the Galaxy's core harbors roughly 50,000 stars per cubic parsec. That's a stellar density about a million times greater than in our solar neighborhood. Had any planets formed along with these Galactic-center stars, they would probably have been ripped from their orbits and obliterated, as these stars must experience frequent close encounters and even collisions. Infrared radiation has also been detected from what appear to be huge clouds rich in dust. In addition, radio observations indicate that a ring of molecular gas 500 pc across, containing some 30,000 solar masses of material, surrounds the central source.

Figure 23.23 (a) An infrared image of the region around the center of our Galaxy shows many bright stars packed into a relatively small volume. The average density of matter in this region is estimated to be about a million times that in the solar neighborhood. The boxed region near the top is about 200 parsecs across. (b) The central portion of our Galaxy, seen in the radio part of the spectrum. This VLA image shows a region about 200 pc across surrounding the Galactic center (which lies within the bright "blob" at bottom right). The long-wavelength radio emission cuts through the Galaxy's dust, providing an image of matter in the immediate vicinity of the Galaxy's center. (c) The spiral-like pattern of radio emission arising from Sagittarius A, the very center of the Galaxy. This image's scale is about 10 pc and suggests a rotating ring of matter only 5 pc across.

High-resolution radio observations show more structure on small scales. Figure 23.23(b) shows the bright radio source Sagittarius A, which lies at the center of the circle in Figure 23.22 and within the boxed region in Figure 23.23(a)--and, we think, at the center of our Galaxy. On a scale of about 100 pc, extended filaments can be seen. Their presence suggests to many astronomers that strong magnetic fields operate in the vicinity of the center, creating structures similar in appearance (but much larger in size) to those observed on the active Sun. On even smaller scales, the observations suggest a rotating ring or disk of matter only a few parsecs across.

What could be the cause of all this activity? An important clue comes from the Doppler broadening of infrared spectral lines from the central swirling whirlpool of gas (Figure 23.23b). The extent of the broadening indicates that the gas is moving very rapidly. In order to keep this gas in orbit, whatever is at the center must be extremely massive--a million solar masses or more. Given the twin requirements of large mass and small size, a leading contender is a black hole. The hole itself is not the source of the energy, of course. Instead, the vast accretion disk of matter being drawn toward the hole by its enormous gravity emits the energy as it falls in, just as we saw (on a much smaller scale) in Chapter 22 when we discussed X-ray emission from neutron stars and stellar-mass black holes. The strong magnetic fields are thought to be generated within the accretion disk itself as matter spirals inward. We will see later that astronomers have reason to suspect that similar events are occurring at the centers of many other galaxies.

Figure 23.24 places these findings into a simplified perspective. In a series of six images, an artist has captured the important results of long-wavelength radio and infrared studies of the Milky Way's heart. Each painting is centered on the Galaxy's core, and each increases in resolution by a factor of 10.

Figure 23.24 Six artist's conceptions, each centered on the Galactic core and each increasing in resolution by a factor of 10. Frame (a) shows the same scene as Figure 23.12. Frame (f) is an artist's rendition of a vast whirlpool within the innermost parsec of our Galaxy.

Frame (a) renders the Galaxy's overall shape, as painted in Figure 23.12. The scale of this frame measures about 100 kpc from top to bottom. Frame (b) spans a distance of 10 kpc from top to bottom and is nearly filled by the great circular sweep of the innermost spiral arm. Moving in to a 1-kpc span, frame (c) depicts a ring of matter made mostly of giant molecular clouds and gaseous nebulae. This entire flattened, circular feature is rotating at about 100 km/s. The origin of this ring of gas is still unclear. In frame (d), at 100 pc, a pinkish region of ionized gas surrounds the reddish heart of the Galaxy. (Of course, we cannot see the colors of these regions--astronomers infer them on the basis of the gases' temperatures and densities.) The source of energy producing this vast ionized cloud is unknown, although it is presumed to be related to activity in the Galactic center. Frame (e), spanning 10 pc, depicts the tilted, spinning whirlpool of hot (104 K) gas that marks the core of our Galaxy. The innermost sanctum of this gigantic whirlpool is painted in frame (f), in which a swiftly spinning, white-hot disk of gas at millions of kelvins nearly engulfs an enormously massive object too small in size to be pictured (even as a minute dot) on this scale.

If our knowledge of the Galaxy's center seems sketchy, that's because it is. Astronomers are still deciphering the clues hidden within its invisible radiation. We are only beginning to appreciate the full magnitude of this strange new realm deep in the heart of the Milky Way. In some respects, our research is not yet mature science. Rather, it's exploration--but absolutely fascinating exploration, enabling us to return from our telescopes with tales of new wonders at the core of our Galaxy.