Universe Tenth Edition Roger Freedman • Robert Geller • William Kaufmann III Universe Tenth Edition Chapter 22 Our Galaxy

Chapter 22 Opener: Two views of the Milky Way: a wide-angle infrared image (upper) and a close-up infrared image (lower). (upper: Ohainaut/ESO/Handout/dpa/Corbis; lower: NASA/JPL-Caltech/S. Stolovy [SSC/Caltech])

By reading this chapter, you will learn 22-1 How astronomers discovered the solar system’s location within the Milky Way Galaxy 22-2 The shape and size of our Galaxy 22-3 How the Milky Way’s spiral structure was discovered 22-4 The evidence for the existence of dark matter in our Galaxy 22-5 What causes the Milky Way’s spiral arms to form and persist 22-6 How astronomers discovered a supermassive black hole at the galactic center

22-1: The Sun is located in the disk of our Galaxy about 8000 parsecs from the galactic center Figure 22-1a: The Milky Way Galaxy is a disk-shaped collection of stars. When we look out at the night sky in the plane of the disk, the stars appear as a band of light that stretches all the way around the sky. When we look perpendicular to the plane of the Galaxy, we see only those relatively few stars that lie between us and the “top” or “bottom” of the disk. Our View of the Milky Way

Our View of the Milky Way Figure 22-1b: This wide-angle photograph shows a 180° view of the Milky Way centered on the constellation Sagittarius (compare with the photograph that opens this chapter). The dark streaks across the Milky Way are due to interstellar dust in the plane of our Galaxy. (Stocktrek Images/Getty Images) Our View of the Milky Way

Herschel’s Map of Our Galaxy Figure 22-2: In a paper published in 1785, the English astronomer William Herschel presented this map of the Milky Way Galaxy. He determined the Galaxy’s shape by counting the numbers of stars in various parts of the sky. Herschel’s conclusions were flawed because interstellar dust blocked his view of distant stars, leading him to the erroneous idea that the Sun is at the center of the Galaxy. (Dr. Jeremy Burgess/Science Source) Herschel’s Map of Our Galaxy

Finding the Center of the Galaxy Figure 22-3: (a) A motorist lost on a foggy night can determine his location by looking for tall buildings that extend above the fog. (b) In the same way, astronomers determine our location in the Galaxy by observing globular clusters that are part of the Galaxy but lie outside the obscuring material in the galactic disk. The globular clusters form a spherical halo centered on the center of the Galaxy. Finding the Center of the Galaxy

Finding the Center of the Galaxy Figure 22-3: (a) A motorist lost on a foggy night can determine his location by looking for tall buildings that extend above the fog. (b) In the same way, astronomers determine our location in the Galaxy by observing globular clusters that are part of the Galaxy but lie outside the obscuring material in the galactic disk. The globular clusters form a spherical halo centered on the center of the Galaxy. Finding the Center of the Galaxy

Finding the Center of the Galaxy Figure 22-3: (a) A motorist lost on a foggy night can determine his location by looking for tall buildings that extend above the fog. (b) In the same way, astronomers determine our location in the Galaxy by observing globular clusters that are part of the Galaxy but lie outside the obscuring material in the galactic disk. The globular clusters form a spherical halo centered on the center of the Galaxy. Finding the Center of the Galaxy

Finding the Center of the Galaxy Figure 22-3: (a) A motorist lost on a foggy night can determine his location by looking for tall buildings that extend above the fog. (b) In the same way, astronomers determine our location in the Galaxy by observing globular clusters that are part of the Galaxy but lie outside the obscuring material in the galactic disk. The globular clusters form a spherical halo centered on the center of the Galaxy. Finding the Center of the Galaxy

Period and Luminosity for Cepheid and RR Lyrae Varaibles Figure 22-4: This graph shows the relationship between period and average luminosity for Cepheid variables and RR Lyrae variables. Cepheids come in a broad range of luminosities: The more luminous the Cepheid, the longer its pulsation period. By contrast, RR Lyrae variables are horizontal branch stars that all have roughly the same average luminosity of about 100 L. Period and Luminosity for Cepheid and RR Lyrae Varaibles

Period and Luminosity for Cepheid and RR Lyrae Varaibles Figure 22-4: This graph shows the relationship between period and average luminosity for Cepheid variables and RR Lyrae variables. Cepheids come in a broad range of luminosities: The more luminous the Cepheid, the longer its pulsation period. By contrast, RR Lyrae variables are horizontal branch stars that all have roughly the same average luminosity of about 100 L. Period and Luminosity for Cepheid and RR Lyrae Varaibles

RR Lyrae Varaibles in a Globular Cluster Figure 22-5: The arrows point to three RR Lyrae variables in the globular cluster M55, located in the constellation Sagittarius. From the average apparent brightness (as seen in this photograph) and average luminosity (known to be roughly 100 L) of these variable stars, astronomers have deduced that the distance to M55 is 6500 pc (20,000 ly). (Harvard-Smithsonian Center for Astrophysics) RR Lyrae Varaibles in a Globular Cluster

22-2: Observations at nonvisible wavelengths reveal the shape of the Galaxy Figure 22-6: (a) This view was constructed from observations made at far-infrared wavelengths by the IRAS spacecraft. Interstellar dust, which is mostly confined to the plane of the Galaxy, is the principal source of radiation in this wavelength range. (b) Observing at near-infrared wavelengths, as in this composite of COBE data, allows us to see much farther through interstellar dust than we can at visible wavelengths. Light in this wavelength range comes mostly from stars in the plane of the Galaxy and in the bulge at the Galaxy’s center. (NASA) The Infrared Milky Way

Our Galaxy (Schematic Edge-on View) Figure 22-7: There are three major components of our Galaxy: a disk, a central bulge, and a halo. The disk contains gas and dust along with metal-rich (Population I) stars. The halo is composed almost exclusively of old, metal-poor (Population II) stars. The central bulge is a mixture of Population I and Population II stars. These images of NGC 7331, which is about 15 million pc (50 million ly) from Earth, were made with the Spitzer Space Telescope (see Section 6-7, especially Figure 6-26). Our Galaxy (Schematic Edge-on View)

NGC 7331: A Near-Twin of the Milky Way Figure 22-8a: If we could view our Galaxy from a great distance, it would probably look like this galaxy in the constellation Pegasus. As in Figure 22-6, the far-infrared image reveals the presence of dust in the galaxy’s plane. (NASA; JPL-Caltech; M. Regan [STScI]; and the SINGS Team) NGC 7331: A Near-Twin of the Milky Way

NGC 7331: A Near-Twin of the Milky Way Figure 22-8b: If we could view our Galaxy from a great distance, it would probably look like this galaxy in the constellation Pegasus. The near-infrared image shows the distribution of stars. These images of NGC 7331, which is about 15 million pc (50 million ly) from Earth, were made with the Spitzer Space Telescope (see Section 6-7, especially Figure 6-26). (NASA; JPL-Caltech; M. Regan [STScI]; and the SINGS Team) NGC 7331: A Near-Twin of the Milky Way

Star Orbits in the Milky Way Figure 22-9: The different populations of stars in our Galaxy travel along different sorts of orbits. The galaxy in this visible-light image is the Milky Way’s near-twin NGC 7331, the same galaxy shown at infrared wavelengths in Figure 22-8. (Russell Croman/Science Source) Star Orbits in the Milky Way

Star Orbits in the Milky Way Figure 22-9: The different populations of stars in our Galaxy travel along different sorts of orbits. The galaxy in this visible-light image is the Milky Way’s near-twin NGC 7331, the same galaxy shown at infrared wavelengths in Figure 22-8. (Russell Croman/Science Source) Star Orbits in the Milky Way

Stellar Populations: Disk Versus Central Bulge Figure 22-10: The disk and central bulge of the the Milky Way contain rather different populations of stars. The same is true for the galaxy NGC 1309, which has a similar structure to the Milky Way Galaxy and happens to be oriented face-on to us. NGC 1309 is about 30 million pc (100 million ly) from us in the constellation Eridanus. (NASA, ESA, The Hubble Heritage Team [STScI/AURA] and A. Riess [STScI]) Stellar Populations: Disk Versus Central Bulge

Magnetic Interactions in the Hydrogen Atom Figure 22-11: (a) The energy of a pair of magnets is high when their north poles or their south poles are near each other, and low when they have opposite poles near each other. (b) Thanks to their spin, electrons and protons are both tiny magnets. When the electron flips from the higher-energy configuration (with its spin in the same direction as the proton’s spin) to the lower-energy configuration (with its spin opposite to the proton’s spin), the atom loses a tiny amount of energy and emits a radio photon with a wavelength of 21 cm. Magnetic Interactions in the Hydrogen Atom

Figure 22-11a: The energy of a pair of magnets is high when their north poles or their south poles are near each other, and low when they have opposite poles near each other.

Magnetic Interactions in the Hydrogen Atom Figure 22-11b: Thanks to their spin, electrons and protons are both tiny magnets. When the electron flips from the higher-energy configuration (with its spin in the same direction as the proton’s spin) to the lower-energy configuration (with its spin opposite to the proton’s spin), the atom loses a tiny amount of energy and emits a radio photon with a wavelength of 21 cm. Magnetic Interactions in the Hydrogen Atom

22-3: Observations of cold hydrogen clouds and star forming regions reveal that our galaxy has spiral arms Figure 22-12: This image was made by mapping the sky with radio telescopes tuned to the 21-cm wavelength emitted by neutral interstellar hydrogen (H I). The entire sky has been mapped onto an oval, and the plane of the Galaxy extends horizontally across the image as in Figure 22-6. Black and blue represent the weakest emission, and red and white the strongest. (Courtesy of C. Jones and W. Forman, Harvard-Smithsonian Center for Astrophysics) The Sky at 21 cm

Spin Flip Transitions in Medicine Box 22-1: In magnetic resonance imaging, a magnetic field is used whose strength varies from place to place. The difference in energy between the opposed and aligned orientations of a proton depends on the strength of the magnetic field, so radio waves will only be absorbed at places where this energy difference is equal to the energy of a radio photon. (This equality is called resonance, which is how magnetic resonance imaging gets its name.) By varying the magnetic field strength over the body and the wavelength of the radio waves, and by measuring how much of the radio wave is absorbed by different parts of the body, it is possible to map out the body’s tissues. The accompanying false- color image shows such a map of a patient’s head. Unlike X-ray images, which show only the densest parts of the body, such as bones and teeth, magnetic resonance imaging can be used to view less dense (but water-containing) soft tis- sue. Just as the 21-cm radio emission has given astronomers a clear view of what were hidden regions of our Galaxy, magnetic resonance imaging allows modern medicine to see otherwise invisible parts of the human body. Spin Flip Transitions in Medicine

A Technique for Mapping Our Galaxy Figure 22-13: If we look within the plane of our Galaxy from our position at S, hydrogen clouds at different locations (shown as 1, 2, 3, and 4) along our line of sight are moving at slightly different speeds relative to us. As a result, radio waves from these various gas clouds are subjected to slightly different Doppler shifts. This permits radio astronomers to sort out the gas clouds and thus map the Galaxy. A Technique for Mapping Our Galaxy

A Map of Neutral Hydrogen in Our Galaxy Figure 22-14: This map, constructed from radio-telescope surveys of 21-cm radiation, shows the distribution of hydrogen gas in a reconstructed (or hypothetical) face-on view of our Galaxy. The map suggests a spiral structure. Details in the blank, wedge-shaped region at the bottom of the map are unknown. Gas in this part of the Galaxy is moving perpendicular to our line of sight and thus does not exhibit a detectable Doppler shift. (Image courtesy of Leo Blitz, Ph.D.) A Map of Neutral Hydrogen in Our Galaxy

Figure 22-15: The galaxy M83 lies in the southern constellation Hydra about 5 million pc (15 million ly) from Earth. (a) This visible-light image clearly shows the spiral arms. The presence of young stars and H II regions indicates that star formation takes place in spiral arms. (b) This radio view at a wavelength of 21 cm shows the emission from neutral interstellar hydrogen gas (H I). Note that essentially the same pattern of spiral arms is traced out in this image as in the visible-light photograph. (c) M83 has a different appearance in this near-infrared view. The starlight has been removed in this image to reveal the infrared emission of dust throughout the galaxy. (a: ©Australian Astronomical Observatory/David Malin Images; b: VLA, NRAO; c: NASA/JPL-Caltech) A Spiral Galaxy

Our Galaxy Seen Face-on: Artist’s Impressions Figure 22-16: (a) The Galaxy’s diameter is about 50,000 pc (160,000 ly), and our solar system is about 8000 pc (26,000 ly) from the galactic center. The elongated central bulge is about 8300 pc (27,000 ly) long and is oriented at approximately 45° to a line running from the solar system to the galactic center. (b) Our solar system is located between the Sagittarius and Perseus arms, two of the major spiral arms in the Milky Way. (a: NASA/JPL-Caltech/R. Hurt, SSC; b: NG Maps/National Geographic Creative) Our Galaxy Seen Face-on: Artist’s Impressions

The Rotation of Our Galaxy Figure 22-17: Each schematic diagram follows three stars (the Sun and two others) orbiting the center of the Galaxy at different distances from the galactic center. (a) This case of stars with similar, nearly uniform, orbital speeds is what we have in our Galaxy. Although they start off lined up in this illustration, the stars become increasingly separated as they move along their orbits. With a uniform speed for all stars, stars inside the Sun’s orbit overtake and move ahead of the Sun, while stars far from the galactic center lag behind the Sun. (b) The stars would remain lined up if the Galaxy rotated like a solid disk. This orientation is not what is observed. (c) If stars orbited the galactic center in the same way that planets orbit the Sun, stars inside the Sun’s orbit would overtake us faster than they are observed to do. The Rotation of Our Galaxy

The Rotation of Our Galaxy Figure 22-17: Each schematic diagram follows three stars (the Sun and two others) orbiting the center of the Galaxy at different distances from the galactic center. (a) This case of stars with similar, nearly uniform, orbital speeds is what we have in our Galaxy. Although they start off lined up in this illustration, the stars become increasingly separated as they move along their orbits. With a uniform speed for all stars, stars inside the Sun’s orbit overtake and move ahead of the Sun, while stars far from the galactic center lag behind the Sun. (b) The stars would remain lined up if the Galaxy rotated like a solid disk. This orientation is not what is observed. (c) If stars orbited the galactic center in the same way that planets orbit the Sun, stars inside the Sun’s orbit would overtake us faster than they are observed to do. The Rotation of Our Galaxy

The Rotation of Our Galaxy Figure 22-17: Each schematic diagram follows three stars (the Sun and two others) orbiting the center of the Galaxy at different distances from the galactic center. (a) This case of stars with similar, nearly uniform, orbital speeds is what we have in our Galaxy. Although they start off lined up in this illustration, the stars become increasingly separated as they move along their orbits. With a uniform speed for all stars, stars inside the Sun’s orbit overtake and move ahead of the Sun, while stars far from the galactic center lag behind the Sun. (b) The stars would remain lined up if the Galaxy rotated like a solid disk. This orientation is not what is observed. (c) If stars orbited the galactic center in the same way that planets orbit the Sun, stars inside the Sun’s orbit would overtake us faster than they are observed to do. The Rotation of Our Galaxy

The Rotation of Our Galaxy Figure 22-17: Each schematic diagram follows three stars (the Sun and two others) orbiting the center of the Galaxy at different distances from the galactic center. (a) This case of stars with similar, nearly uniform, orbital speeds is what we have in our Galaxy. Although they start off lined up in this illustration, the stars become increasingly separated as they move along their orbits. With a uniform speed for all stars, stars inside the Sun’s orbit overtake and move ahead of the Sun, while stars far from the galactic center lag behind the Sun. (b) The stars would remain lined up if the Galaxy rotated like a solid disk. This orientation is not what is observed. (c) If stars orbited the galactic center in the same way that planets orbit the Sun, stars inside the Sun’s orbit would overtake us faster than they are observed to do. The Rotation of Our Galaxy

22-4: The rotation of our Galaxy reveals the presence of dark matter Figure 22-18: The blue curve shows the orbital speeds of stars and gas in the disk of the Galaxy out to a distance of 18,000 parsecs from the galactic center. (Very few stars are found beyond this distance.) The dashed red curve indicates how this orbital speed should decline beyond the confines of most of the Galaxy’s visible mass. Because there is no such decline, there must be an abundance of invisible dark matter that extends to great distances from the galactic center. The Galaxy’s Rotation Curve

The Galaxy’s Rotation Curve Figure 22-18: The blue curve shows the orbital speeds of stars and gas in the disk of the Galaxy out to a distance of 18,000 parsecs from the galactic center. (Very few stars are found beyond this distance.) The dashed red curve indicates how this orbital speed should decline beyond the confines of most of the Galaxy’s visible mass. Because there is no such decline, there must be an abundance of invisible dark matter that extends to great distances from the galactic center. The Galaxy’s Rotation Curve

The Galaxy and its Dark Matter Halo Figure 22-19: The dark matter in our Galaxy forms a spherical halo whose center is at the center of the visible Galaxy. The extent of the dark matter halo is unknown, but its diameter is at least 100 kiloparsecs. The total mass of the dark matter halo is at least 10 times the combined mass of all of the stars, dust, gas, and planets in the Milky Way. The Galaxy and its Dark Matter Halo

Microlensing by Dark Matter in the Galactic Halo Figure 22-20a: If a dense object such as a brown dwarf or black hole passes between Earth and a distant star, the gravitational curvature of space around the dense object deflects the starlight and focuses it in our direction. This effect is called microlensing. Microlensing by Dark Matter in the Galactic Halo

Microlensing by Dark Matter in the Galactic Halo Figure 22-20a: If a dense object such as a brown dwarf or black hole passes between Earth and a distant star, the gravitational curvature of space around the dense object deflects the starlight and focuses it in our direction. This effect is called microlensing. Microlensing by Dark Matter in the Galactic Halo

Microlensing by Dark Matter in the Galactic Halo Figure 22-20: This light curve shows the gravitational microlensing of light from a star in the Galaxy’s central bulge. Astronomers do not know the nature of the object that passed between Earth and this star to cause the microlensing. (Courtesy of the MACHO and GMAN Collaborations) Microlensing by Dark Matter in the Galactic Halo

Microlensing by Dark Matter in the Galactic Halo Figure 22-20: This light curve shows the gravitational microlensing of light from a star in the Galaxy’s central bulge. Astronomers do not know the nature of the object that passed between Earth and this star to cause the microlensing. (Courtesy of the MACHO and GMAN Collaborations) Microlensing by Dark Matter in the Galactic Halo

Detecting Dark Matter WIMPS Figure 22-21: This detector sits near the bottom of an old gold mine 4850 feet underground. If a WIMP collides with a xenon particle, the so-called weak interaction ends up producing light that can be detected by photomultipliers. Electrons are also produced in collisions to varying degrees, which helps to determine if the collisions were actually due to WIMPs, and not some other particle. While Earth shields out cosmic rays that would otherwise overwhelm the many fewer WIMP detections, the tank shown here is also surrounded by 72,000 gallons of water that helps to shield neutrons (also a source of noise) that are emitted by natural radioactivity from the surrounding rock. Detecting Dark Matter WIMPS

22-5: Spiral arms are caused by density waves that sweep around the Galaxy Figure 22-22: This series of drawings shows that spiral arms in galaxies like the Milky Way cannot simply be assemblages of stars. If they were, the spiral arms would “wind up” and disappear in just a few hundred million years. The Winding Dilemma

Figure 22-22: This series of drawings shows that spiral arms in galaxies like the Milky Way cannot simply be assemblages of stars. If they were, the spiral arms would “wind up” and disappear in just a few hundred million years. The Winding Dilemma

A Density Wave on the Highway Figure 22-23: A density wave in a spiral galaxy is analogous to a crew of painters moving slowly along the highway, creating a moving traffic jam. Like such a traffic jam, a density wave in a spiral galaxy is a slow-moving region where stars, gas, and dust are more densely packed than in the rest of the galaxy. As the material of the galaxy passes through the density wave, it is compressed. This triggers star formation, as Figure 22-24 shows. A Density Wave on the Highway

A Density Wave on the Highway Figure 22-23: A density wave in a spiral galaxy is analogous to a crew of painters moving slowly along the highway, creating a moving traffic jam. Like such a traffic jam, a density wave in a spiral galaxy is a slow-moving region where stars, gas, and dust are more densely packed than in the rest of the galaxy. As the material of the galaxy passes through the density wave, it is compressed. This triggers star formation, as Figure 22-24 shows. A Density Wave on the Highway

Star Formation in the Density-Wave Model Figure 22-24: A spiral arm is a region where the density of material is higher than in the surrounding parts of a galaxy. Interstellar matter moves around the galactic center rapidly (shown by the red arrows) and is compressed as it passes through the slow-moving spiral arms (whose motion is shown by the blue arrows). This compression triggers star formation in the interstellar matter, so that new stars appear on the “downstream” side of the densest part of the spiral arms. Star Formation in the Density-Wave Model

Star Formation in the Density-Wave Model Figure 22-24: A spiral arm is a region where the density of material is higher than in the surrounding parts of a galaxy. Interstellar matter moves around the galactic center rapidly (shown by the red arrows) and is compressed as it passes through the slow-moving spiral arms (whose motion is shown by the blue arrows). This compression triggers star formation in the interstellar matter, so that new stars appear on the “downstream” side of the densest part of the spiral arms. Star Formation in the Density-Wave Model

Star Formation in the Whirlpool Galaxy Figure 22-25: The spiral galaxy M51 (called the Whirlpool) is a real-life example of the density-wave model illustrated in Figure 22-24. (a) This infrared image shows where dust has piled up as the material within M51 passes through its spiral arms. Radio images of M51 show that hydrogen gas also piles up in the same locations, thus beginning the formation of new stars. (b) By the time stars complete their formation process, their motion around the galaxy has swept them “downstream” of the positions of greatest dust density, just as depicted in Figure 22-24. (a: NASA, JPL-Caltech, and R. Kennicutt [Univ. of Arizona]; b: DSS) Star Formation in the Whirlpool Galaxy

Cosmic Connections 22: Different populations of stars are found in different neighborhoods of our home galaxy. (The galaxy shown here is another spiral galaxy similar to our own.) The variations from one galactic region to another are due to the presence or absence of ongoing star formation. Stars in the Milky Way

Figure 22-26a: The differences from one spiral galaxy to another suggest that more than one process can create spiral arms. NGC 628 is a grand-design spiral galaxy with thin, well-defined spiral arms, (Gemini Observatory—GMOS Team) Variety in Spiral Arms

Figure 22-26b: The differences from one spiral galaxy to another suggest that more than one process can create spiral arms. NGC 4414 is a flocculent spiral galaxy with fuzzy, broken, and poorly defined spiral arms. (Olivier Vallejo [Observatoire de Bordeaux], HST, ESA, NASA) Variety in Spiral Arms

22-6: Infrared, radio, X-ray and gamma-ray observations are used to probe the galactic center Figure 22-27: (a) In this false-color infrared image, the reddish band is dust in the plane of the Galaxy and the fainter bluish blobs are interstellar clouds heated by young O and B stars. (b) This close-up infrared view covers the area outlined by the white rectangle in (a). (c) Adaptive optics reveals stars densely packed around the galactic center. (a, b: NASA; c: European Southern Observatory) The Galactic Center

Stars Orbiting Sagittarius A* Figure 22-28: The colored dots superimposed on this infrared image show the motion of seven stars in the vicinity of the unseen massive object (denoted by the yellow five-pointed star) at the position of the radio source Sagittarius A*. The orbits were measured over an 15-year period. Analysis of the orbits indicates that the stars are held in orbit by a black hole of 4.1 million solar masses. The blue dots for S0-16 show this star reached 4% the speed of light at its closest approach to the black hole. (Keck/UCLA Galactic Center Group) Stars Orbiting Sagittarius A*

The Energetic Center of the Galaxy Figure 22-29: (a) The area shown in this radio image has the same angular size as the full moon. Sagittarius A*, at the very center of the Galaxy, is one of the brightest radio sources in the sky. Magnetic fields shape nearby interstellar gas into immense, graceful arches. (b) This composite of images at X-ray wavelengths from 0.16 to 0.62 nm shows lobes of gas on either side of Sagittarius A*. The character of the X-ray emission shows that the gas temperature is as high as 2 × 107 K. ( VLA, F. Sadeh et al./ NRAO; b: F. K. Baganoff et al./CXC/MIT/NASA) The Energetic Center of the Galaxy

The Energetic Center of the Galaxy Figure 22-29: (a) The area shown in this radio image has the same angular size as the full moon. Sagittarius A*, at the very center of the Galaxy, is one of the brightest radio sources in the sky. Magnetic fields shape nearby interstellar gas into immense, graceful arches. (b) This composite of images at X-ray wavelengths from 0.16 to 0.62 nm shows lobes of gas on either side of Sagittarius A*. The character of the X-ray emission shows that the gas temperature is as high as 2 × 107 K. ( VLA, F. Sadeh et al./ NRAO; b: F. K. Baganoff et al./CXC/MIT/NASA) The Energetic Center of the Galaxy

Galactic Gamma-Ray Bubbles Figure 22-30: This is an artist’s illustration of giant gamma-ray “bubbles” seen above and below our galactic plane, possibly due to the black hole at the galactic center. Each of the bubbles extends about 25,000 light-years away from the plane. The bubbles are estimated to be a few million years old. The gamma ray data also contain hints of linear jet-like features, which are sometimes found around black holes, but the existence of the jets is far from clear. If the jets are real, producing them might have required the black hole to consume a giant hydrogen cloud of about 10,000 solar masses, with only a small portion escaping into the jets. (David A. Aguilar [CfA]) Galactic Gamma-Ray Bubbles

M74 in Visible and Ultraviolet Wavelengths Problem 22-41: The accompanying image shows the spiral galaxy M74, located about 55 million light-years from Earth in the constellation Pisces (the Fish). It is actually a superposition of two false- color images. The red portion is an optical image taken at visible wavelengths, while the blue portion is an ultraviolet image made by NASA’s Ultraviolet Imaging Telescope, which was carried into orbit by the space shuttle Columbia during the Astro-1 mission in 1990. Compare the visible and ultraviolet images and, from what you know about stellar evolution and spiral structure, explain the differences you see. (NASA, UIT) M74 in Visible and Ultraviolet Wavelengths

Infrared Images of Two Spiral Galaxies Problem 22-42: The accompanying figure shows infrared images of two spiral galaxies. Explain which of these is a grand-design spiral galaxy and which is a flocculent spiral galaxy. Explain your reasoning. (NASA; JPL-Caltech; R. Kennicutt [University of Arizona]; and the SINGS Team) Infrared Images of Two Spiral Galaxies

Key Ideas The Shape and Size of the Galaxy: Our Galaxy has a disk about 50 kpc (160,000 ly) in diameter and about 600 pc (2000 ly) thick, with a high concentration of interstellar dust and gas in the disk. The galactic center is surrounded by a large distribution of stars called the central bulge. This bulge is not perfectly symmetrical, but may have a bar or peanut shape. The disk of the Galaxy is surrounded by a spherical distribution of globular clusters and old stars, called the galactic halo. There are about 200 billion (2  1011) stars in the Galaxy’s disk, central bulge, and halo.

Key Ideas The Sun’s Location in the Galaxy: Our Sun lies within the galactic disk, some 8000 pc (26,000 ly) from the center of the Galaxy. Interstellar dust obscures our view at visible wavelengths along lines of sight that lie in the plane of the galactic disk. As a result, the Sun’s location in the Galaxy was unknown for many years. This dilemma was resolved by observing parts of the Galaxy outside the disk. The Sun orbits around the center of the Galaxy at a speed of about 790,000 km/h. It takes about 220 million years to complete one orbit.

Key Ideas The Rotation of the Galaxy and Dark Matter: From studies of the rotation of the Galaxy, astronomers estimate that the total mass of the Galaxy is about 1012 M. Only about 10% of this mass is in the form of visible stars, gas, and dust. The remaining 90% is in some nonvisible form, called dark matter, that extends beyond the edge of the luminous material in the Galaxy. Our Galaxy’s dark matter may be a combination of MACHOs (dim, star-sized objects), and is hypothesized to consist mostly of WIMPs (relatively massive subatomic particles).

Key Ideas The Galaxy’s Spiral Structure: OB associations, H II regions, and molecular clouds in the galactic disk outline huge spiral arms. Spiral arms can be traced from the positions of clouds of atomic hydrogen. These can be detected throughout the galactic disk by the 21-cm radio waves emitted by the spin-flip transition in hydrogen. These emissions easily penetrate the intervening interstellar dust.

Key Ideas Theories of Spiral Structure: There are two leading theories of spiral structure in galaxies. According to the density-wave theory, spiral arms are created by density waves that sweep around the Galaxy. The gravitational field of this spiral pattern compresses the interstellar clouds through which it passes, thereby triggering the formation of the OB associations and H II regions that illuminate the spiral arms. According to the theory of self-propagating star formation, spiral arms are caused by the birth of stars over an extended region in a galaxy. Differential rotation of the galaxy stretches the star-forming region into an elongated arch of stars and nebulae.

Key Ideas The Galactic Nucleus: The innermost part of the Galaxy, or galactic nucleus, has been studied through its radio, infrared, and X-ray emissions (which are able to pass through interstellar dust). A strong radio source called Sagittarius A* is located at the galactic center. This marks the position of a supermassive black hole with a mass of about 4.1 106 M.