Fig. 12.1 . A panoramic telescopic view of the Milky Way, the luminous concentration of bright stars and dark intervening dust clouds that extends in a band across the celestial sphere. We live in this disk, and look out through it. Our view is eventually blocked by the build-up of interstellar dust, and the light from more distant regions of the disk cannot get through. The center of the Milky Way is located at the center of the image, in the direction of the constellation Sagittarius. Although the disk appears wider in that direction, the center is not visible through the dust. The Large and Small Magellanic Clouds can be seen as bright swirls of light below the plane to the right of center. (This map of the Milky Way was hand-drawn from many photographs by Martin and Tatjana Keskula under the direction of Knut Lundmark; courtesy of the Lund Observatory, Sweden.)
Fig. 12.2 . A million stars are crowded together in globular star clusters, like the two shown here. Many are located in a great spherical halo that encloses our Milky Way. A relatively few, like these, are concentrated toward the central nucleus. These globular clusters are designated NGC 6522 and NGC 6528. (Courtesy of KPNO/AURA.)
Fig. 12.3 . As shown in 1918 by the American astronomer Harlow Shapley (1885-1972), the globular star clusters are distributed in a roughly spherical system whose center coincides with the core of our Milky Way. The Sun is located in the disk, about 27,700 light-years away from the center. The disk and central bulge are shown edge-on in a negative print of an infrared image taken from the InfraRed Astronomical Satellite. The infrared observations can penetrate the obscuring veil of interstellar dust that hides the distant Milky Way from observation at optically visible wavelengths. It is this dust that limited astronomer’s view of stars to a much smaller Kapteyn Universe, centered on the Sun.
Fig. 12.4 . Luminous emission nebulae, known as ionized hydrogen, or H II, regions, act like beacons that mark out the inner spiral structure of our Milky Way. The Sun and solar system are located at the upper center, in a local spur from the nearest arm. The figure is centered at the center of our Milky Way. The scale, shown in the lower right, extends across 16,300 light-years.
Fig. 12.5 . This drawing depicts our Milky Way as viewed from above its plane. The stars and interstellar material are concentrated within spiral arms. The Sun lies within one of these spiral arms at a distance of 27,700 light-years, from the center, designated here as 8,500 parsecs or 8.5 kpc. This distance is 1.75 billion times the distance between the Earth and the Sun.
Fig. 12.6 . Stars slowly revolve about an unseen center, whose gravity controls their motions. This diagram portrays the orbits of infrared stars near the center of our Milky Way. The annual positions of seven stars have been determined over a fifteen-year period (colored dots), determining their curved trajectories and inferring the mass of the invisible center. These orbits indicate that a super-massive black hole is located at the center of the Milky Way; it has a mass of 4.1 million times the mass of the Sun. The display covers the central 1.0 x 1.0 seconds of arc, which at a distance of about 27,700 light-years corresponds to a width of about 0.13 light-years or 1,140 light-hours. (Courtesy of Andrea M. Ghez/UCLA galactic center group.)
Fig. 12.7 . Lord Rosse (1800-1867) discovered the spiral structure of Messier 51, abbreviated M 51, using his 1.8-m (72-inch) telescope in the spring of 1845, and he subsequently found at least a dozen other nebulae with a spiral shape. In his description of this drawing of M 51, published in 1850, Rosse attributed the spiral pattern to rotation of the nebula. Camille Flammarion (1842-1925) included this sketch in his popular books about astronomy, leading to a growing awareness of spiral nebulae, and it might have inspired the swirls of starlight found in Vincent Van Gogh’s (1853-1890) painting of the Starry Night. We now know that M 51, also designated NGC 5194 and called the Whirlpool Galaxy, is a magnificent, rotating spiral galaxy located 35 million light-years away, with a small, irregular companion NGC 5195 separated from the center of M 51 by about 10 million light-years. [Reproduced from The Earl of Rosse, Observations of the Nebulae, Philosophical Transactions of the Royal Society, pages 110-124, plate 35 (1850).]
Fig. 12.8 . The nearest spiral galaxy, the Andromeda Nebula, also known as M 31 and NGC 224, is located at a distance of about 2.6 million light-years, so its light takes about 2.6 million years to reach us. Both the Andromeda Nebula and our Galaxy are spiral galaxies with total masses of about a million million, or 1012, solar masses, and roughly a hundred billion, or 1011, optically visible stars. The several distinct stars surrounding the diffuse light from Andromeda are stars within our own Galaxy; these stars lie well in front of Andromeda. Two smaller galaxies are also shown in this image. They are M 32, also designated as NGC 221, shown at the edge of the Andromeda Nebula, and NGC 205, that is located somewhat further away. These are elliptical systems at about the same distance as M 31, but with only about one-hundredth of its mass. (Courtesy of Karl-Schwarzschild Observatorium, Tautenburg.)
Fig. 12.9 . This natural-color Hubble Space Telescope image shows the spiral galaxy NGC 4911 in the Coma cluster of galaxies, which lies about 320 million light-years away from the Earth. Central clouds of interstellar gas and dust are silhouetted against glowing, young, star clusters and clouds of hydrogen gas. (Courtesy of NASA/ESA/Hubble Heritage Team/STScI/AURA.)
Fig. 12.10 . A plot of the distance of extragalactic nebulae, or galaxies, versus the radial velocity at which each galaxy is receding from Earth, published in 1929 by the American astronomer Edwin Hubble (1889-1953). The linear relationship between the distance and radial velocity indicates that the Universe is expanding. Vesto M. Slipher (1875-1969) determined most of these velocities more than a decade before this diagram was drawn. Here the velocity is in units of kilometers per second, abbreviated km s-1, and the distance is in units of millions of parsecs, or Mpc, where 1 Mpc is equivalent to 3.26 million light-years. Hubble underestimated the distances of the spiral nebulae, so the distance scale for modern versions of this diagram is about seven times larger. The filled circles and solid line represent the solution for individual nebulae; the open circles and dashed line are for groups of them, and the cross represents the mean distance of 22 nebulae whose distances could not be determined individually.
Fig. 12.11 . This plot of galaxy distance versus recession velocity is analogous to that obtained by Edwin Hubble (1889-1953), in his 1929 discovery of the expansion of the Universe (Fig. 12.10). The slope of the linear fit (solid line) to the data (dots) measures the expansion rate of the universe, a quantity called the Hubble constant, designated H0. The data shown here summarize eleven years of efforts to measure this constant by using the Hubble Space Telescope to determine the distances and velocities of Cepheid variable stars in nearby galaxies. The distance is in units of a million parsecs, or Mpc, where 1 Mpc is equivalent to 3.26 million light-years, and the radial velocity is given in units of kilometers per second, denoted as km s-1. The fit to these data indicate that H0 = 75 ± 10 km s-1 Mpc-1, and that this constant lies well within the limits of 50 and 100 in the same units (dashed lines). [Adapted from Wendy L. Freedman et al., Final results from the Hubble Space Telescope Key Project to Measure the Hubble Constant, Astrophysical Journal 553, 47-72 (2001).]
Fig. 12.12 . More than 1,000 identified galaxies are located within the Coma cluster, also designated Abell 1656, which has a mean distance of 321 million light-years and is more than 20 million light-years in diameter. Each of these galaxies contains hundreds of billions of stars. Most of the galaxies that inhabit the central portion of this cluster are giant elliptical galaxies. Several spiral galaxies are found farther out from the center, such as the one shown in the upper left of this mosaic of images taken from the Hubble Space Telescope. Nearly every object in this picture is a galaxy. It is a section of the cluster that is several million light-years across, and is located about one-third of the way out from the center. (Courtesy of NASA/ESA/Hubble Heritage/STScI/AURA.)
Fig. 12.13 . Very distant and faint galaxies can be investigated by observing them through a cluster of galaxies. The powerful gravitation of the cluster acts like a lens, bending, focusing and magnifying the light of more distant galaxies that lie behind it (see Fig. 12.14). The gravitational lens action can distort the light from the background galaxies into faint arcs, or produce magnified images of individual galaxies that would otherwise remain invisible. (Courtesy of NASA/JPL-Caltech.)
Fig. 12.14 . A Hubble Space Telescope image of a rich cluster of galaxies designated Abell 2218. It is about three billion light-years away from Earth. A typical rich cluster contains hundreds and even thousands of galaxies, each composed of hundreds of billions of stars and possibly up to ten times more mass within invisible dark matter. The galaxy cluster Abell 2218 is so massive and so compact that its gravity bends and focuses the light from galaxies that lie behind it. Multiple images of these background galaxies are distorted into long faint arcs. Magnified or ring images of individual background galaxies can also be observed. [Courtesy of NASA/STScI/Andrew Fruchter/the ERO team (Sylvia Baggett/STScI, Richard Hook/ST-ECF/Zoltan Levay, STScI).]
Fig. 12.15 . When a background and foreground galaxy are perfectly aligned, the closer galaxy acts as a gravitational lens, bending and magnifying the light of the more distant galaxy and forming a glowing “Einstein” ring. A double ring is captured in this Hubble Space Telescope image, indicating an exceptionally rare alignment of a massive foreground galaxy with two background ones; the distances of the three galaxies are estimated at 3 billion, 6 billion and 11 billion light-years from Earth. (Courtesy of NASA/ESA/Raphael Gavazzi and Tommaso Trea, U. C. Santa Barbara and the SLACS team.)
Fig. 12.16 . Gravitational interaction of the antennae galaxies, catalogued as NGC 4038 and NGC 4039, produces long arms of young stars in their wakes. The colliding galaxies are located about 62 million light-years from Earth, and have been merging together for the last 800 million years. As the two galaxies continue to churn together, clouds of interstellar gas and dust are shocked and compressed, triggering the birth of new stars. This composite image is from the Chandra X-ray Observatory (blue), the Hubble Space Telescope (gold and brown) and the Spitzer Space Telescope (red). The blue x-rays show huge clouds of hot, interstellar gas, the red data show infrared radiation from warm dust clouds that have been heated by newborn stars, and the gold and brown data reveal both star-forming regions and older stars. (Courtesy of NASA/ESA/SAO/CXC/JPL-Caltech/STScI.)
Fig. 12.17 . By measuring the recession velocity, or redshift, of galaxies, astronomers have determined their distance and combined it with their location in the sky to obtain the three-dimensional distribution of galaxies. The map shown here is for galaxies within one billion light-years (far left or far right) from the Earth (center). Since galaxies started to form about 12 billion years ago this is a relatively nearby part of the universe. It includes recession velocities of up to 30,0000 km s-1, at a redshift of 0.1. The galaxies are concentrated in long, narrow sheet-like walls encircling large empty places known as voids, about 100 million light-years across. The Sloan Great Wall (left) spans about 1.4 billion light-years. It may be gravitationally unbound, perhaps beginning to fall apart, but this great wall includes superclusters of galaxies that may stay bound together by their mutual gravitational pull. The Sloan Great Wall was discovered using data from the Sloan Digital Sky Survey in 2004. Other superclusters, or clusters of galaxy clusters, are labeled in the diagram, which is from the Two Degree Field galaxy survey. (Courtesy of Willem Schaap, Kapteyn Institute, U. Groningen et al., 2dF Galaxy Redshift Survey).
Fig. 12.18 . One moment in the ever-changing distribution of galaxies studied using a supercomputer to trace out their formation, evolution and clustering. The width of this image is about 10 million light-years. (Courtesy of Volker Springel, the Millennium Simulation Project/Max Planck Institute for Astrophysics, Garching, Germany.)