There is among our Galaxy's billions of stars a vast assortment in size, mass, temperature, color, and chemical composition. At least nine-tenths of these are main-sequence stars, and most of the rest are white dwarfs. Giants are rare, and supergiants are still more rare. Less than half the Galaxy's stars are single stars; all the other stars are physically related doubles or multiples or are in clusters and groups. Some stars, such as the pulsating and explosive ones, vary in brightness, thereby drawing special attention to themselves. Even at great distances their variability is easy to recognize. But variable stars are actually very rare, probably amounting to less than 0.1 percent of the stars in the Galaxy. Is there a pattern beneath the Galaxy`s rich variety? Very much so, as we shall try to illustrate in this chapter.
Historically, it was not immediately obvious from the sky's appearance that the Sun was immersed in a disk-shaped system of stars, even though the bandlike appearance of the Milky Way is an edgewise view of that disk. The historical path to understanding the structure of our Galaxy was long and arduous, and it also required an understanding of the nature of the faint, smeared-out patches of light called nebulae.
20.1.1 Historical Speculations about the Milky Way
Whether the Universe "have his boundes or bee in deed infinite and without boundes" was the profound question English astronomer Thomas Digges (1546?-1595) asked. This question appeared in Digges's 1576 book Perfit Description of the Celestial Orbes. A few decades later, Galileo also wrote of an infinite and unbounded Universe in his Dialogues, which was probably prompted by the fact that he had seen with his telescope that the Milky Way consists of myriads of stars.
What is the Milky Way? Swedish philosopher Emanuel Swedenborg (1688-1772) speculated in 1734 that the stars formed one vast collection, of which the Solar System was but one constituent. In his 1750 work, An Original Theory of the Universe, the Englishman Thomas Wright (1711-1786) theorized that the Milky Way appears as a bright band of stars because the Sun lies inside a flattened slab of stars. He also suggested that there were other Milky Ways in the Universe. Immanuel Kant (1724-1804), the noted German philosopher, went beyond Wright's idea, suggesting in 1755 that the small, oval, nebulous objects seen with telescopes were other systems of stars or "island universes." This phrase captured popular fancy a century and a half later. A modern photograph of Kant's island universes is shown in Figure 20.1.
Besides these speculations, observational evidence also accumulated that revealed the structure of the Milky Way. In 1785, William Herschel gave the first quantitative proof that the Milky Way was a stellar system shaped like a flat disk or a grindstone. Since Herschel and others did not suspect that starlight might be dimmed by obscuring material between the stars, Herschel deduced that the Sun was near the center of the system.
William Herschel (1738-1822) and his son John (1792-1871), who carried his father's surveys to the southern skies, found over 5000 nebulous objects; these objects were cataloged by John in 1864. Shortly thereafter, the third Earl of Rosse, in Ireland, examined many of these objects with a 1.8-m reflecting telescope, whose nineteenth century appearance is shown in Figure 20.2. Several of the larger nebulosities appeared to consist of gaseous clouds, often with filamentary or ring-shaped forms; still others were resolvable into clusters of stars. However, many were not resolvable into either stars or gas clouds, and some had a spiral-like structures.
If these nebulous objects were outside the Milky Way system, they could logically be expected to be spread more or less uniformly in all directions. But the nebulae were not distributed uniformly over the sky; their number increased in either direction away from the plane of the Milky Way. No one suspected that this effect was illusory: Light was simply dimmed by the cosmic haze in our Galaxy's plane. The nebulae's seeming distribution and the appearance of stars and gas mixed in the same nebula seemed to outweigh the possibility that the spiral nebulae might be unresolved stellar systems, separate from the Milky Way. As more nebulosities were found, the conviction grew that they belonged to one grand system; that is the Milky Way is the full extent of the Universe. The true nature of the Milky Way, the nebulae, and what they had to do with each other and the Milky Way was not to be demonstrated until the late 1920s.
After William Herschel's pioneering work, the next comprehensive study of the distribution of stars was done by Dutch astronomer Jacobus Kapteyn (1851-1922) toward the end of the nineteenth century. In 1922, he published his version of the shape, structure, and dimensions of the stellar system. Known as the Kapteyn universe, his was a Sun-centered system that was 50,000 ly in diameter and 6000 ly thick.
Five years before Kapteyn published his work, however, the American astronomer Harlow Shapley (1885-1972) had arrived at a markedly different conclusion about the Sun's location and the system's dimensions. Shapley found 69 globular clusters that formed a spheroidal configuration centered on the flattened system of stars. He argued that the center of the distribution of globular clusters was the center of the Galaxy, which suggested to him that the Sun was as much as 65,000 ly out from the Galactic center, which is in the direction of Sagittarius. The overall system, he thought, was 300,000 ly across and 30,000 ly thick. Shapley reasoned that the spiral nebulae, such as the nearby spiral galaxy M101 in Ursa Major shown in a modern photograph in Figure 20.3, would be engulfed by our giant Galaxy and therefore were not island universes as some believed.
20.1.2 Nebulae, Fuzzy Patches of Light
Astronomers accepted Shapley's finding that the Sun was not the center of the Milky Way, but were disturbed by the enormous scale he claimed for our Galaxy. The first definitive observational evidence that spiral nebulae are stellar systems came in 1912 and the years following, when Vesto Slipher (1875-1969), of the Lowell Observatory, recorded spectra for about 40 spiral nebulae. The spectra consisted of absorption lines on a continuous background, the kind of spectra that would be expected from the composite light of a vast number of stars. However, a peculiar aspect of these spectra was that the majority had absorption lines whose wavelengths were shifted toward the red (or redshifted), with recessional velocities up to 1800 km/s. These large recessional velocities were much greater than those observed for other objects, but there was no ready explanation for the redshifts until years later. On the basis of these results and others, Shapley's one-galaxy concept was challenged by Lick Observatory astronomer Heber Curtis (1872-1942). Curtis insisted that the Milky Way's size had been considerably overestimated. In a great debate before the National Academy of Science on April 26, 1920, Shapley advocated the one-galaxy concept, and Curtis the multigalaxy, or island universe, concept. Their debate was inconclusive. Better observational data were needed, and they were not long in coming.
In 1924, Edwin Hubble (1889-1953), a Mount Wilson Observatory astronomer, began to derive distances to the spiral nebulae by analyzing photographs of NGC 6822 in Capricornus and in the peripheral portions of the two large spirals M31 in Andromeda (Figure 23.1) and M33 in Triangulum (Figure 23.2). Using the new 2.5-m reflector, he was able to obtain photographs showing greater detail: The outer portions of these nebulae could be resolved into swarms of stars including many Cepheid variables, whose distances could be derived from the Cepheid period-luminosity relation. From the Cepheids, Hubble calculated a distance of 900,000 ly for the two spirals (which is actually too small by about a factor of two). However, doubt was at last gone that the spiral nebulae were the island universes Kant had long ago envisioned. Today we know that the Milky Way is a galaxy of stars, just like the billions upon billions of other galaxies spread over vast reaches of space.
Although Western culture had been speculating for over 200 years on the basic structure of the Universe, it has only been within the lifetime of your grandparents that astronomers have known that we live in a Universe of galaxies. This is as important an advance in cosmological thought as the Copernican revolution, and in fact, it may be called a milestone in all intellectual history.
20.2.1 Population Groups
In the early 1940s, Walter Baade (1893-1960) found clues identifying what appeared to be two distinct populations of stars in the Andromeda galaxy. Separating the never-before-resolved central region of that galaxy into its constituent stars, he found multitudes of red giants like those in the globular clusters of our Galaxy. These he named Population II stars to distinguish them from the highly luminous blue supergiants in the spiral arms of the Andromeda galaxy, which he called Population I stars. To Baade it was clear that these population groups were like those in our Galaxy. The massive Population I stars within our Galaxy, which are associated with the bright gaseous nebulae in the spiral arms, are younger than Population II stars, which are found in globular clusters, the Galaxy's halo, and its central bulge.
Today we have found that there are indeed two distinct groups of stars conceptually analogous to Baade`s population groups but with somewhat different characteristics. They are known as the spheroidal population and the disk population. For these two population groups, a definite relationship exists between a star`s physical characteristics and its location in the Galaxy. In many regions, however, the pattern is not pronounced. Astronomers also recognize, because of an accumulation of observations and theories on stellar and galactic evolution, that the two population groups can be subdivided on the basis of their chemical composition, their evolution, their location, and the motions of their stars, as is done in Table 20.1.
For many years, astronomers felt that the way to visualize the substructure of the Galaxy was to see it as made up first of a very thin spiral of the youngest Population I objects, embedded in a succession of thicker lens-shaped groups of older and older stars. The final group of the oldest Population II stars was spherical, as shown in Figure 20.4. This structure was thought to reflect the actual evolution of the Galaxy from a roughly spherical configuration through the succession of lens-shaped groups to the present thin spiral, always leaving stars to occupy the volume defined by the Galaxy at the time of their birth. Views on population groups have changed and continue to change. Today astronomers visualize the Galaxy as divided into a spheroidal population group of old stars, whose characteristics vary roughly in a radial direction out from the Galactic center. The characteristics of the disk population group of younger objects, however, vary roughly with their distance from the center of the Galaxy (Figure 20.4). The older are the disk population objects, the more they have tended to migrate away from the midplane of the Galaxy over time and the less concentrated they are toward the Galactic plane.
20.2.2 Nucleosynthesis, Chemical Evolution
The earlier discussion of stellar evolution sets out a scheme whereby the chemical elements in our Galaxy have evolved. All stars convert some of their hydrogen content to helium, whereas a smaller number of stars will convert helium into carbon. But it is through the lives of massive stars that matter is converted in stages from hydrogen to helium to carbon to oxygen and even to iron in some instances--a process called nucleosynthesis.
Stars will lose part of their mass to the interstellar medium through stellar winds, outbursts like planetary nebulae and novae, and the most colossal outburst of all--supernovae. It is unlikely that stellar winds, planetary nebulae, and possibly novae expel significant amounts of chemically evolved matter back into interstellar space. The real enhancers of the heavy-element composition of the interstellar medium are supernovae (Figure 20.5). During these outbursts, all the elements not already synthesized, primarily the heavy elements beyond iron in the periodic table, are produced very rapidly. Let us digress briefly to see how this synthesis works.
At various points in the nuclear burning scheme for massive stars, neutrons are released that can be absorbed by nuclei followed by the nucleus' radioactive decay, a process known as the s-process, or the slow neutron capture process. However, during the supernova outburst, nuclei can be bombarded by a flood of neutrons providing very little time for the radioactive decay between absorption of additional neutrons. This process is a rapid neutron capture process and is consequently called the r-process. The consequence of each of these neutron-capture processes is the synthesis of its own vast array of elements, including their isotopes, that constitute the variety of elements found in the period table.
Returning to the supernova outburst, when the expanding shell of ejected matter disperses into space, it soon mixes with and thereby increases the heavy-element composition of the interstellar medium. Successive generations of stars have formed from interstellar clouds enriched with supernova debris. These new stars in turn synthesize in their interiors more heavy elements, some of which are expelled into interstellar space by supernova outbursts from dying massive stars. And eventually, one reaches the current spiral-arm population stars among the gas and dust clouds located in the thin disk portions of the Galaxy; these stars are the youngest and the ones containing the largest abundance of heavy elements.
If this is so, then all the familiar elements of our everyday world that are heavier than hydrogen and to some extent helium, such as carbon in biological life, or oxygen in the air, or aluminum in airplanes, or silicon in glass, or iron in buildings, or gold in rings and bracelets, were synthesized during the lives of massive stars that eventually experienced a supernova outburst. This group of massive stars lived and died before the Solar System formed 5 billion years ago. We and everything about us are the ashes of a long past generation of stars. The matter in our bodies is but one link in our Galaxy's cycle of chemical evolution.
20.2.3 Enriching the Interstellar Medium
All classes of objects moving around the Galactic center continue to occupy to some extent the volume within the Galaxy that they inhabited at the time of their formation. For this reason, astronomers can follow the progress of chemical evolution in the Galaxy, since stars are the furnaces in which heavy elements are synthesized from lighter ones.
Radio astronomy has helped to reveal not a Galaxy of stars, but one of nebulous patches of gas and dust, many of which are birthplaces for new generations of stars. Similarly studies employing ultraviolet, X-ray, or gamma-ray radiation greatly expand astronomy's understanding of stellar evolution. Radiation at these wavelengths is coming from stars in primarily exotic phases of their life, such as birth, during violent outbursts, during mass exchange with companions, or in their death throes. It is also produced by hot gaseous matter around stars that is part of some violent or energetic event connected with the lives of stars. Many of these stars are old, compact stars, such as white dwarfs, hot subdwarfs, neutron stars, and black holes, with some as components in binary systems that had not even been recognized as such. Others are young, high-mass stars that are very hot and very luminous, such as shown in Figure 20.6. In general, what we are learning about the Galaxy is that the birth and evolutionary processes for stars, particularly the massive ones, are much more exotic than once thought, and they involve large volumes of space surrounding stars.
With the various nucleosynthesis processes astronomers can account reasonably well for the relative abundances of the chemical elements observed in successive generations of stars. Early in the Galaxy's life gaseous matter was much denser than now, which favored star formation. The existence of a small amount of heavy elements in the oldest stars suggests that either the Galaxy formed from matter already possessing some heavy elements or an early population of supermassive stars formed and has long since vanished, leaving only their nucleosynthesis efforts for us. The compositions obtained from spectroscopic analysis of the Sun, stars, and nebulae suggest that in the solar neighborhood the abundance of elements heavier than hydrogen and helium has increased over time. There was an increase by a factor of 3 or so between 10 and 15 billion years ago, and the heavy-element abundance has been roughly constant since then. Also, the heavy-element abundance in disk stars appears to decline the more distant they are from the Galactic center. Extensive spectroscopic studies of globular clusters suggest that the clusters near the Galactic center have larger heavy-element abundances than do those far out on the boundary of the Galactic halo.
This pattern of element abundance indicates that the Galaxy started with hydrogen and some helium in a roughly spherical form and in about 5 billion years of star formation produced its spheroidal component. Star formation later became pronounced in a disk-shaped region and continues today in the spiral arms of the disk. The cycle will stop when most of the matter in the Galaxy is tied up in dead stars so that new stars can no longer be formed (Figure 20.5).
20.2.4 General Features of Our Galaxy
From a variety of different measurements astronomers have amassed the information summarized in Table 20.2. From these figures it is evident that our Galaxy is immense in size and similar in many respects to our large neighbor in the constellation Andromeda, some 2 million ly away (Figure 23.1). [For distances measured in millions of light years, we will use the megalight year, abbreviated Mly, where 1 Mly = 1 million ly = 1.0 x 106 ly.]
In general, our Galaxy is a large, flattened, or disk-shaped, system of approximately 400 billion stars. There is a large, roughly spherical nuclear bulge of stars, which occupies about 10 to 15 percent of the inner radius, in the center. Extending out from the nucleus is a thin, grindstone-shaped disk of stars through which thread even thinner spiral arms. If our Galaxy is like many other spiral galaxies, there are two spiral arms (coming out of opposite sides of the nucleus) that wrap around the nucleus in such a fashion that they trail as the Galaxy rotates. Surrounding the disk and nucleus is a spherical halo. About 95 percent of the Galaxy's observable mass is apparently tied up in stars. The remainder is gas and dust grains strewn about the disk with a density greater in the spiral arms than in the regions between the arms. Dark interstellar clouds and bright H II emission nebulae in the arms outline the Galaxy's spiral appearance.
Stars are most numerous in the nucleus, and their numbers decline outward through the disk and even more rapidly away from the disk into the halo. Within the Galaxy, stars occur singly, in multiple star systems, or in clusters, such as the open clusters of the disk or the globular clusters in the halo. What we see when we scan the Galaxy are the intrinsically bright stars, and not necessarily the most numerous type of star. Thus it is not really possible to do a survey of the Galaxy by type of star. However, using a survey of nearby stars and some educated guesswork, astronomers have devised the estimate for a Galactic census given in Table 20.3. On the assumption that the total number of stars in the Galaxy is about 400 billion, then 90 percent are main-sequence F through M stars, 9 percent are white dwarfs, 0.5 percent are red giants, and 0.5 percent are everything else. All the stars intrinsically brighter than red dwarfs (M V stars) and white dwarfs account for only 24 percent of all stars and only 42 percent of the mass of the stars in the Galaxy. However, they provide 99 percent of the luminosity emitted by the Galaxy. In other words, the bulk (58 percent) of the Galaxy's mass in the form of stars provides only 1 percent of its luminosity!
This is easily grasped when we remember that it would take about 1000 stars intrinsically as bright as Vega (A0 V) to equal the brightness of Rigel (B8 I), about 50,000 stars as bright as the Sun (G2 V), and almost 2.5 billion stars as bright as Wolf 359 (M8 V). Red dwarfs in the solar neighborhood are simply not as numerous as that. As we might expect the same result to hold for other galaxies, we conclude that it is the intrinsically bright stars that are responsible for the light emitted by a galaxy, while much of the mass of a galaxy is tied up in stars that do not contribute appreciably to its brightness. (As we shall see shortly, this imbalance between mass and luminosity is even greater than what we have just indicated.)
The revolution period of the Sun and the solar neighborhood (230 million years) is a reasonable standard for measuring changes in our Galaxy. This time scale is called the Galactic year, and in its units, the Sun and the Solar System are about 20 Galactic years old. The young O and B stars used to trace the spiral structure are less than 0.1 Galactic year in age, whereas the oldest stars in the halo are 40 to 70 Galactic years in age. From the dynamics of the Galaxy, we know that relatively little change can occur in the Galaxy's structure in periods of less than a few Galactic years. And to an external observer, probably very significant changes have occurred in the appearance of our Galaxy in periods of tens of Galactic years.
Mathematics of Finding the Mass of Our Galaxy
Another application of Kepler's modified third law is to use it to obtain an approximate mass for our Galaxy. Assuming that most of the mass lies toward the center in roughly a spherical shape, we can write
PGalaxy2 = 4 p2 RGalaxy3 / G ( MGalaxy + MSun ) ,
where PGalaxy is the period of orbital revolution at the Sun's distance, RGalaxy, MGalaxy is the Galaxy's mass, MSun is the Sun's mass, and G is the gravitational constant. This example is similar to the one on planetary motion in Section 3.5. Because the Sun's circular velocity about the Galactic center is
VSun = 2 p RGalaxy / PGalaxy,
we can restate the preceding equation after some transformation; we also can neglect MSun, which is insignificant compared with MGalaxy, so that
MGalaxy = VSun2 RGalaxy / G.
Substituting in the equation VSun = 2.5 x 107 cm/s, RGalaxy = 2.84 x 1022 cm, and G = 6.67 x 10-8 cm3 / g s2, we find MGalaxy = 2.58 x 1044 g = 1.30 x 1011 MSun This represents the mass inside the Sun's Galactic orbit, which is somewhat less than, but reasonably close to, the value given in Table 20.2.
20.2.6 Galactic Coordinates
To help understand the organization of the Galaxy and its rotation, astronomers have devised a coordinate system (Figure 20.7) based on a perspective appropriate to the Galaxy (Appendix 2). Figure 20.7 shows an idealized sketch of the Galaxy, both in a face-on view of the disk and in a cross sectional view through the disk. On the right-hand side of the figure are shown the Galactic coordinates, which are known as Galactic longitude and latitude. Galactic longitude is the angular distance measured in the central plane of the disk, starting from the Galactic center and measuring along the Milky Way through the constellations shown. The direction 90o away from the direction of the Galactic center (in Sagittarius) is toward the constellations Cepheus and Cygnus; 180o is toward Taurus, Auriga, and Perseus, not far from the Pleiades open cluster; 270o is toward Canis Major and Puppis. Galactic latitude is the angular distance above and below the central plane of the Milky Way. The North Galactic Pole, at 90o N Galactic latitude, lies in the constellation Coma Berenices and in the same hemisphere as the North Celestial Pole. The South Galactic Pole, 90o S, lies in the constellation Sculptor. The Galactic coordinates will help you to visualize the nature of our Galaxy.
20.3.1 Rotation of the Solar Neighborhood
One means of determining the Galaxy's rotation is to use a frame of reference outside the Galaxy, such as distant galaxies. From systematic study of the Doppler shifts of distant galaxies the solar neighborhood is found to be moving toward the galaxies in the direction beyond the stars of Cygnus and away from those beyond the stars of Canis Major. As stated earlier, the velocity of the solar neighborhood, which is about 30,000 ly from the center of the Galaxy, is about 250 km/s (about 0.0008 ly/y) around the Galactic center.
At present, astronomers estimate that within about 3000 ly of the Galactic center, the central regions rotate somewhat like a solid wheel, so that the orbital velocity increases from the center outward. From about 4000 ly through the position of the Sun at 30,000 ly and on to about 60,000 ly from the center, the rotational velocity ranges from 200 to 300 km/s. Beyond about 60,000 ly, if indeed the Galaxy should extend beyond that distance, the velocity is probably nearer to 200 km/s.
20.3.2 Do All Parts of the Galaxy Rotate?
Astronomers have compared the rotation of the halo of our Galaxy, whose most visible occupants are the hundred or so globular clusters, with that of the disk. From such a comparison, it seems that the rotation of the halo, if any, cannot be much more than about 50 km/s. In summary, the spheroidal population group in the Galactic halo rotates the slowest, if at all, the older stars of the disk population group rotate a little faster, and the youngest stars of the disk population group appear to rotate the fastest. Thus not only different rates of rotation are measured, but these different rates can be associated with distinctive parts of the Galaxy, as defined by the ages and characteristics of its component stars.
20.3.3 Galactic Orbits for Various Population Groups
Galactic rotation is the composite of the orbits of individual stars like the Sun. The Sun's orbit about the Galactic center is very nearly a circle (an ellipse of small eccentricity). In addition, two or three times per orbit the Sun oscillates up and down perpendicular to the Galactic plane, sometimes above the midplane (0o Galactic latitude) and sometimes below. The variation is probably not more than 1000 ly, or one-third the thickness of the disk of the Galaxy, as shown in Figure 20.8.
The youngest stars, such as the O and B stars, H II emission nebulae, some open clusters, and the interstellar medium--all of which are disk population objects--move in nearly circular orbits about the Galactic center. These objects are also strongly confined to the midplane of the Galactic disk. Somewhat older stars (those up to about 8 billion years in age) are also orbiting in small-eccentricity elliptical orbits like that of the Sun. Moreover, they oscillate farther above and below the midplane of the Galactic disk than do younger stars. Indeed, the stars and interstellar matter of the disk population group that truly define the rotation of our Galaxy. The stars of the spheroidal population group, however, move in large-eccentricity elliptical orbits that carry them to higher Galactic latitudes than is the case for the disk population stars, as shown in Figure 20.8.
20.3.4 The Missing Mass Problem, First Piece of Evidence
In the early 1930s, studies were made of the amount of mass that must lie in the Galactic disk in order to account for the positions and motions of stars just above or below the disk of our Galaxy. In Newtonian gravitational theory, the amount of matter per unit volume, the matter density, should through its gravitational effects influence the motions and consequently the positions of these stars. For example, the greater the matter density in the Galactic disk, the greater the gravitational attraction of the disk, and consequently the more difficult it is for stars to pull away and rise far above or below the disk. Conversely, the smaller the matter density, the less the gravitational attraction, and consequently the easier it is for stars to pull away from their neighbors in the Galactic disk. The amount of matter found in these early studies and most subsequent studies addressing the same question is around 0.01 Msun/ly3. But the amount of luminous matter found from studies of the distribution of stars and interstellar matter is 0.005 Msun/ly3, as shown in Table 20.2, or about one-half the amount required to produce the observed motions outside the Galactic disk. Efforts to show that the additional mass in the disk is faint stars and interstellar matter have all failed, and so the mystery has remained for over fifty years.
This mystery is not the only evidence for unseen or nonluminous or dark matter either in our Galaxy or in other galactic systems. Therefore we will come back to this problem later.
20.4. Spiral Structure of Our Galaxy
20.4.1 Tracing the Spiral Arms of Our Galaxy
As was discussed in Chapter 16, interstellar dust, concentrated in interstellar clouds in the plane of our Galaxy, obstructs our view of the Galaxy depending on the direction in which we look; the greatest obstruction is in general along the plane of the Galaxy and the least perpendicular to the plane. Despite the presence of obscuring dust, optical astronomers in the early 1950s identified segments of spiral arms that weave through the Galactic disk by tracing the locations where O and B associations and their H II emission nebulosities are most prominent. One of them, the Orion arm, shown in Figure 20.9, contains the Sun on the inside portion that lies toward the Galactic center. The segment we see arches over an angle covering some 10,000 ly. Near the Sun the arm has a short extension, called the Orion spur, that extends outward. A segment of a second arm, known as the Perseus arm, arcs in the same general direction as the Orion arm and lies nearby 6000 ly beyond the Orion arm, closer to the edge of the Galaxy. A segment of a third arm, the Sagittarius-Carina arm, lying inside the Orion arm, has been observed toward the Galactic center about 6000 ly from the Sun. There is also some evidence for a segment of an arm even farther out, beyond the Perseus arm, and another segment of an arm (maybe two) inside the Sagittarius-Carina arm, closer to the Galactic center.
One of the obstacles to the use of O and B stars as tracers of spiral arms is that they are all too far away to determine their distances by trigonometric parallax. Hence the less reliable inverse-square law of light must be used, and consequently, the distances to the arms are known only to about +10 percent, that is, to about 1000 ly out of every 10,000 ly. Since the spiral arm is only a few thousand light years across, spiral structure becomes impossible to distinguish from the locations of O and B stars that are more distant than 15,000 to 25,000 ly from the Sun.
Are there other tracers of spiral structure? Yes, several are possible. The Cepheid variable stars are very luminous supergiant stars with readily recognizable brightness variations (Section 13.5). They are also stars that are somewhat older than the O and B stars, but they are nevertheless relatively young and still near their birthplaces. Thus the Cepheids ought to be good candidates for spiral arm tracers. The results for Cepheids in our Galaxy, however, are not convincing. Other spiral arm tracers on which astronomers are working are the dark interstellar clouds, red supergiants, supernovae remnants, pulsars, and a number of X-ray sources. For example, it appears that the inner edges of most spiral arms are marked by the presence of dark interstellar clouds. However, it is probably the radio wavelength view of the Galaxy that provides astronomers with the second-best evidence for spiral structure after the O and B associations and their H II emission nebulae.
20.4.2 Radio Mapping of Spiral Structure
Radio astronomers analyzing Doppler shifts in 21-cm radiation emitted by neutral hydrogen in many directions through the Milky Way have mapped the distribution of atomic hydrogen in detail (Figure 20.10), and more segments of arms have been discovered in 21-cm surveys of the Galaxy than can be recognized in visible light.
By way of illustrating radio mapping techniques, let us consider a small segment of the arm structure as revealed in Figure 20.11 by the 21-cm line profiles along Galactic longitude 85o. What are the distances from the Sun to the three observed maxima (labeled A, B, and C) in Figure 20.11b, where the intensity of the Doppler-shifted 21-cm radiation is greatest? To determine this, we can use the Galactic rotation data derived by optical astronomers in Figure 20.11c to establish a model of the arm structure with an approximate scale of distances. From this model we can calculate what the radial velocities should be where segments of the spiral arms cross our line of sight at Galactic longitude 85o, assuming that hydrogen gas moves in nearly circular orbits around the center of the Galaxy as do O and B stars.
The arm region at A in Figure 20.11a is orbiting at approximately the same distance from the Galactic center as the Sun. The difference in the motions between the Sun and region A projected along our line of sight produces a slight negative Doppler shift of the 21-cm line corresponding to a distance of about 1600 ly. Because the arm region at B is farther from the Galactic center than the Sun, its orbital motion projected along our line of sight produces a large negative Doppler shift corresponding to a distance of about 12,000 ly. Finally, in region C, which is farthest from the Galactic center, the orbital motion is the slowest. Its projection along our line of sight produces the largest negative Doppler shift, corresponding to a distance of about 25,000 ly.
As was discussed in Chapter 16, radiation from carbon monoxide is also used as a probe of the Galaxy's spiral structure (Figure 20.10) because regions suitable for the formation of carbon monoxide molecules are also reasonable locations for molecular hydrogen, which is a very poor emitter in radio wavelengths. Approximately one-half of all the hydrogen in the interstellar medium is in molecular form, and it is not heavily concentrated where there are heavy concentrations of atomic hydrogen, so that its location must be inferred from its association with carbon monoxide. The molecular clouds containing carbon monoxide identify the location of the three spiral arms evident in visible light in Figure 20.9. However, just as one sees, from Figures 20.9 and 20.10, that the spiral features evident in atomic hydrogen are not identical with the spiral arms seen in visible light, so also the carbon monoxide spiral features are not precisely those seen in either atomic hydrogen or visible light.
Regardless of how well young stars, atomic hydrogen, or molecular hydrogen outline the same spiral features, astronomers have no doubt that the Galaxy has a spiral structure (Figure 20.12). In summary, the spiral structure begins at 10,000 to 15,000 ly out from the Galactic center and extends through the disk to distances of at least 50,000 ly. The arms are a few thousand light years across and are separated by distances of about 10,000 ly. If our Galaxy is similar to nearby spiral galaxies, such as M31 (Figure 23.1), M33 (Figure 23.2), M51 (Chapter 21 opener), and M81 (Figure 1.6), then it probably has two arms alternately weaving their way out through the disk. Astronomers are, however, still a long way from fitting the segments of arms we observe into a coherent pattern.
20.4.3 Maintaining Spiral Structure
Gravity is the dominant force in shaping and preserving the structure of spiral arms seen in normal spiral galaxies. Because the arms trail behind in rotation, the more rapid motion of the spiral arms close to the nucleus should wind them so tightly after several Galactic years that the spiral pattern of the Galaxy would soon disappear. From the large number of spiral galaxies that exist it is evident that spiral structure is fairly stable and is probably not a transient feature. There are two promising theories to explain why spiral arms persist.
One theory says that spiral density waves, or waves of compression, move through the gaseous and stellar matter of the disk as a result of gravitational variations in the disk. These waves spiral outward from the center at a constant rotational rate somewhat slower than the rotation of stars and clouds of interstellar matter. The waves do not consist of moving matter, but are a moving pattern of compression that is overtaken by the faster moving interstellar matter. As a result, when interstellar clouds move through the compressional waves, gas and dust pile up in a spiral pattern that is dense enough to initiate star formation (Figure 20.13). However, interstellar matter is not dense enough between the spiral arms to contract under its own gravity to form stars. To help visualize a spiral density wave, think of the following analogy: imagine a slowly moving road crew painting white divider lines on a freeway where traffic moves in only one direction. The traffic piles up wherever the crew is working before it can proceed normally. From an airplane the congestion would seem to be moving slowly forward as the crew plods along.
The extensive dust lanes frequently observed along inner edges of spiral arms in other galaxies are evidence that these compressional waves are real. The newly formed blue supergiants and H II regions, like brilliant beacons, illuminate and indicate the present locations of the spiral arms. In time, the wave lags behind the star-formation region, and the H II regions and the short-lived massive blue stars soon disappear. Long-lived stars of small mass, such as the Sun, are left to mix with and become part of the disk population of stars. Thus there is a continual re-forming of the arms.
A second theory for persistent spiral arm structure is that of a repeating process in which clusters of stars are created from shock waves generated by supernova outbursts within the differentially rotating Galactic disk. Star formation is triggered by the rapidly expanding shock-wave fronts from the supernova explosions that compress interstellar clouds (Section 17.5). Among the stars formed there are additional massive ones that soon live out their lives and explode as supernovae, generating shock waves that trigger the next round of star formation. Thus the process is perpetuated from one generation to the next, while the differential rotation of the Galaxy stretches new star groups into the recognized spiral features.
It may be that within our Galaxy both theories are operating simultaneously. One possible combination is that the density waves initiate the positions of the spiral arms and supernovae outbursts intensify arm formation. Galactic studies with Space Telescope, when it is in orbit, may one day settle the question of the formation and maintenance of spiral structure.
As stated earlier, the characteristic properties of the spheroidal component of the Galaxy vary in a radial direction from the Galactic center. This component contains those regions of the Galaxy identified as the halo, the bulge, and the nucleus, through which the density of stars decreases outward very significantly. Globular clusters and RR Lyrae variables are typical of the spheroidal population group and can be found throughout its volume. In the solar neighborhood, subdwarfs and high-velocity stars are part of this population group, and although they are only detected near the Sun, such stars presumably exist everywhere within the spheroidal population. All spheroidal population stars are old, from 10 to 15 billion years in age, and have typical masses of 0.8MSun, since all its more massive stars have already evolved to become white dwarfs. The spheroidal component contains little, if any, gas and dust except in the nucleus.
20.5.1 The Nucleus
The nucleus of our Galaxy lies far beyond the stars of Sagittarius, where it is obscured from our view by immense quantities of interstellar dust that lie within 10,000 to 15,000 ly of the Sun. Thus the great star cloud in Sagittarius (Figure 14.2) probably represents only the outer edge of the true nucleus. What would the nucleus look like if we could see it unobscured? Estimates are that it would be about the brightness and size of the full Moon, fading away rapidly into the halo and merging gradually with the disk stars along the Galactic plane.
Little is known about the stellar composition of the nucleus because its stars emit most of their radiant energy in the visible part of the spectrum, which is prevented from reaching us by interstellar dust. Old spheroidal population stars, such as globular clusters and RR Lyrae variables, increase in number toward the Galactic center. Thus it is probable that the nucleus is a dense concentration of old stars. Its brightest stars should be red giants of relatively low mass.
To support this contention, astronomers can appeal to the reasonably unobscured view they have of the nuclei of some nearby spiral galaxies. An H-R diagram for the nucleus of our Galaxy should resemble that for the old open cluster NGC 188 in Figure 18.4 and not that for the globular cluster M92 in Figure 18.5. The density of nucleus stars is likely to be up to a few million times that of solar neighborhood stars; that is to say, nucleus stars are spread a few thousand astronomical units apart, whereas the spread for stars in the solar neighborhood is a few hundred thousand astronomical units. The overall shape of the nucleus is that of a slightly flattened sphere.
A much larger fraction of photons in the infrared region than in the visible region of the spectrum can complete the journey from the Galaxy's nucleus to the Sun. Moreover, in the radio, X-ray, and gamma-ray regions, very few photons are lost during the 30,000-year trip through the plane of the Galaxy. However, X-ray and gamma-ray photons have trouble penetrating the Earth's atmosphere, so most of what astronomers know about the central regions of our Galaxy has been obtained by radio and infrared observations.
20.5.2 The Nucleus as Seen in Various Wavelength Regions
As was discussed in Section 5.4, data received by a radio telescope are processed by a computer to produce contour maps that show the location of the most intense regions of radio emission (Figure 20.14). A similar technique is used for infrared radiation (Figure 20.15). Results of such studies indicate that on the outer boundary of the Galaxy's nucleus, 15,000 ly from the Galactic center, there is a ring of giant H II emission nebulae, like the Orion Nebula, along with a number of giant molecular clouds. Inside this ring, about 10,000 ly on this side and 8000 ly on the other side of the center, are rotating and expanding arms of neutral hydrogen. Figure 20.18 attempts to picture these arms and the region in the center as one might see them from above the Galactic disk north of the Sun.
Farther in toward the Galactic center lies a disk of gas that extends about 4000 to 5000 ly from the center. Another disk of gas that has a higher temperature lies farther inside the first one; it extends out about 1000 ly from the center. Embedded within this second disk are giant complexes of dust and molecules, in which are apparently located very hot young stars and their associated H II emission nebulae. Estimates of the mass inside a radius of about 1000 ly indicate that it amounts to perhaps 1 to 2 billion solar masses, with a couple of percent in gaseous form. That is, 1 percent or so of the observable Galaxy's total mass is confined to 0.01 percent of the volume of the disk.
Even closer to the Galactic center, radio contour maps show an elongated distribution of radio emissions centered on the Galactic equator. These sources of intense radio emissions are clouds of hot interstellar gas like the H II emission nebulae in the spiral arms. Somewhat different from the others, the strongest source is known to radio astronomers as Sagittarius A. It is a small, very bright source of radio emissions that is not visible optically. Sagittarius A is the most powerful source of nonthermal radiation in the Galaxy, and it can be seen on all contour maps of radio emissions covering the Galactic center. Most astronomers now think that Sagittarius A is the actual center of our Galaxy.
What else do we know that would confirm this supposition? Approximately coinciding with the location of Sagittarius A is a source of infrared, ultraviolet, and X-ray radiation. The infrared observations suggest that the source is at most a few light years in size with an estimated mass of several million solar masses. To some astronomers, since the central region of our Galaxy is the seat of violent, energetic events, one interpretation of the compact massive object at the center is that it is one or more black holes. Considerable amounts of matter in the form of stars and interstellar matter should then be swallowed by this black hole from surrounding regions.
20.5.3 The Halo
Surrounding the disk of the Galaxy is the halo, whose shape is that of a slightly flattened sphere. It is known to contain a small number of individual stars, 100 or so globular clusters that are situated far above the disk, and possibly another 100 or so globular clusters in or near the disk and nucleus.
The halo is apparently a remnant of the Galaxy's very early history. The stars and globular clusters in it are some of the oldest stars in the Galaxy (Chapter 18) and show how conditions in the Galaxy were very different at the time the globular clusters formed. The halo stars are found to be very deficient in elements heavier than helium compared with the youngest stars in the spiral arms of the Galactic plane. The stars of the spheroidal component appear to have a smaller heavy-element abundance the farther they are from the Galactic center. Stars in the nucleus, once also thought to be deficient in heavy elements, apparently are not, even though most are quite old. Stars elsewhere, then, formed under conditions that were different from those in the distant parts of the halo.
It was thought for many years that the halo's mass was only a small percentage of the disk's mass. But today there is evidence suggesting that the mass of the halo must be at least comparable with the disk's in order for the Galaxy's disk to maintain stability over its life span. If that were true, then the halo might add about 100 billion solar masses to the total mass of the Galaxy. At one extreme this mass could be tied up in stars of very low luminosity and small mass, or at the other extreme it could be hot gas. This is not the end of the surprises about the halo that appear to be unfolding.
20.5.4 Missing Mass Problem, Second Piece of Evidence
As early as 1974, some astronomers were suggesting that the observable Galaxy--disk, nucleus, and halo stars--is embedded in a still larger and more massive, but unseen, spheroidal halo of nonluminous material, called the halo's dark component (Figure 20.16). Estimates of the size of the halo`s dark component include values for its radius of up to 350,000 ly (almost 6 times that of the visible halo) and a mass of up to 2 x 1012 MSun (or about 7 to 10 times the mass of the observable Galaxy). If the observable Galaxy constitutes only 10 to 15 percent of the total mass, what are the arguments astronomers have for the existence of all that dark matter? Moreover, what is the form of this matter that it cannot be observed directly?
The basic argument for the existence of a massive halo that engulfs our Galaxy is one of stability and permanency. First, the Galaxy needs to have a great deal of mass to stabilize gravitationally, for the billions of years of the Galaxy's existence, the relatively thin and delicate disk with its spiral structure. Second, if the observable Galaxy provided all the gravitational attraction in the system, then the orbital velocity of stars in the disk should decrease beyond 20,000 ly from the Galactic center, as do the Keplerian orbits of distant planets in our Solar System. However, as pointed out earlier, orbital velocities do not decline; they are approximately constant or even slightly increasing well beyond the distance of the Sun from the Galactic center. Third, the motions of the nearest galaxies to our own, which are small galaxies, suggest that our Galaxy is much more massive than what we observe in the radiation all across the electromagnetic spectrum from luminous matter. And finally, the rotational velocities of other spiral galaxies suggest that there is dark matter far from the centers of those visible galaxies as well. If these arguments are not invalidated by further study and observation, then our Galaxy is a much larger and more massive system than we once thought.
What is the form of this dark matter (some refer to it as the "dark population")? It does not appear to be cool gaseous matter containing neutral hydrogen and carbon monoxide because it does not emit 21-cm radio radiation. If some of it were hot gaseous matter, then it should emit short-wavelength radiation in the ultraviolet or X-ray regions. Indeed ultraviolet studies suggest that the halo does contain some hot gas with a temperature of about 100,000 K, that extends 25,000 to 35,000 ly on either side of the Galactic disk and cycles in and out of the disk like giant fountains. However, this gas does not even closely account for the amount of mass needed. The best guess at this point is that if a massive halo of dark matter exists, most of it is in the form of old stars of very low luminosity, such as red dwarfs, white dwarfs, brown dwarfs, black dwarfs or black holes. Whatever form the dark matter is in, it has presented astronomers with a fascinating problem.
Earlier we had discussed the problem of discrepancies in the matter density in the Galactic disk between results derived from stellar motion studies and that from luminous matter. Is that problem part of the dark matter in the halo or is it something entirely different? The amount of matter in the halo will also effect the motions of stars trying to pull away from the Galactic disk, so that the two problems are parts of the same mystery. We will encounter this problem again but on a larger scale when we discuss clusters of galaxies.
Having considered our own Galaxy, let us move to the broader topic of galaxies and their distribution in the Universe in the next three chapters.