1. Neil F. Comins • William J. Kaufmann III Discovering the Universe
Ninth Edition
CHAPTER 15
The Milky Way Galaxy
The center of the Milky Way Galaxy, as seen by the Chandra X-ray telescope.
The false colors reveal extremely hot gases that surround black holes, neutron
stars, and white dwarves. The image is approximately 300 by 800 ly.
The supermassive black hole at the center of our Galaxy is enshrouded by the hot,
white gas cloud in the center of the image. (NASA/UMass/D. Wang et al.).

2. Understanding the Universe
The Sombrero Galaxy (also designated M104, the 104th galaxy listed in the Messier
catalog) is 29 million ly from us. By combining infrared, optical, and X-ray
observations, we can gain insights into its disk of stars, gas, and dust, along with
its central region, and the hot gas surrounding it. (X-ray: NASA/UMass/Q. D. Wang
et al.; optical: NASA/STScI/AURA/Hubble Heritage; infrared:
NASA/JPL-Caltech/Univ. AZ/R. Kennicutt/SINGS Team)
The Sombrero Galaxy (also designated M104, the 104th galaxy listed in the Messier catalog) is 29 million ly from us. By combining infrared, optical, and X-ray observations, we can gain insights into its disk of stars, gas, and dust, along with its central region, and the hot gas surrounding it.

3. An Astronomer’s Almanac

4. The Universe

5. WHAT DO YOU THINK? What is the shape of the Milky Way Galaxy?
Where is our solar system located in the Milky Way Galaxy?
Is the Sun moving through the Milky Way Galaxy and, if so, about how fast?

6. In this chapter you will discover…
the Milky Way Galaxy—billions of stars along with gas and dust bound together by mutual gravitational attraction
the structure of our Milky Way Galaxy
Earth’s location in the Milky Way
how interstellar gas and dust enable star formation to continue
that observations reveal the presence of significant mass in the Milky Way that astronomers have yet to identify
that there is a massive black hole at the center of our Galaxy

7. Schematic Diagrams of the Milky Way
FIGURE 15-1 Schematic Diagrams of the Milky Way
(a) This edge-on view shows the Milky Way’s disk, containing most of its stars, gas, and dust, and its halo, containing many old stars.
Individual stars in the halo are too dim to show, so the bright regions in the halo represent clusters of stars. (b) Our Galaxy has two major
arms and several shorter arm segments, all spiraling out from the ends of a bar of stars and gas that passes through the Galaxy’s center. The
bar’s existence and the presence of two major arms were confirmed by the Spitzer Space Telescope. (b: NASA/JPL-Caltech/R. Hurt [SSC])
(a) This edge-on view shows the Milky Way’s disk, containing most of its stars, gas, and dust, and its halo, containing many old stars. Individual stars in the halo are too dim to be visible on this scale, so the bright regions in the halo represent clusters of stars.
(b) Our Galaxy has two major arms and several shorter arm segments, all spiraling out from the ends of a bar of stars and gas that passes through the Galaxy’s center. The bar’s existence and the presence of two major arms were confirmed by the Spitzer Space Telescope.

8. High-Tech Telescope of the Mid-Nineteenth Century
FIGURE 15-2 High-Tech Telescope of the Mid-Nineteenth Century
(a) Built in 1845, this structure housed a 1.8-m-diameter telescope, the largest of
its day. The improved resolution it provided over other telescopes
was similar to the improvement that the Hubble Space
Telescope provided over Earthbound optical instruments
when it was launched. The telescope, as shown here, was
restored to its original state in the 3 years fromv1996–1998. (Birr Castle Demesne)
Built in 1845, this structure housed a 1.8-m-diameter telescope, the largest of its day. The improved resolution it provided over other telescopes was similar to the improvement that the Hubble Space Telescope provided over Earthbound optical instruments when it was launched. The telescope, as shown here, was restored to its original state in the 3 years from 1996–1998.

9. High-Tech Telescope of the Mid-Nineteenth Century
(b) Using his telescope, Lord Rosse made this sketch of the spiral structure of the galaxy M51 and its companion galaxy NGC 5195. (c) A modern photograph of M51 (also called NGC 5194) and NGC 5195. The spiral galaxy M51 in the constellation of Canes Venatici is known as the Whirlpool Galaxy because of its distinctive appearance. The two galaxies are about 20 million ly from Earth.
FIGURE 15-2 High-Tech Telescope of the Mid-Nineteenth Century
(b) Using his telescope, Lord Rosse made this sketch of the spiral structure
of the galaxy M51 and its companion galaxy NGC (c) A modern
photograph of M51 (also called NGC 5194) and NGC The spiral
galaxy M51 in the constellation of Canes Venatici is known as the Whirlpool
Galaxy because of its distinctive appearance. The two galaxies are about 20
million ly from Earth. (b: Lund Humphries; c: NOAO)

10. A Cepheid Variable Star in Galaxy M100
One of the most reliable ways to determine the distance to moderately remote galaxies is to locate Cepheid variable stars in them, as discussed in the text. The distance of 50 million ly (15.2 Mpc) from Earth to the galaxy M100 in the constellation Coma Berenices was determined using Cepheids. (Insets) The Cepheid in this view, one of 20 located to date in M100, is shown at different stages in its brightness cycle, which recurs over several weeks.
FIGURE 15-3 A Cepheid Variable Star in Galaxy M100
One of the most reliable ways to determine the distance to moderately remote
galaxies is to locate Cepheid variable stars in them, as discussed in the text.
The distance of 50 million ly (15.2 Mpc) from Earth to the galaxy M100 in the
constellation Coma Berenices was determined using Cepheids. (Insets) The
Cepheid in this view, one of 20 located to date in M100, is shown at different
stages in its brightness cycle, which recurs over several weeks. (Dr. Wendy L.
Freedman, Observatories of the Carnegie Institution of Washington; NASA)

11. The Period-Luminosity Relation
FIGURE 15-4 The Period-Luminosity Relation
This graph shows the relationship between the periods and average luminosities
of classical (Type I) Cepheid variables and the closely related RR Lyrae stars
(discussed in Chapter 12). Each dot represents a Cepheid or RR Lyrae whose
luminosity and period have been measured.
This graph shows the relationship between the periods and average luminosities of classical (Type I) Cepheid variables and the closely related RR Lyrae stars (discussed in Chapter 12). Each dot represents a Cepheid or RR Lyrae whose luminosity and period have been measured.

12. Our Galaxy
This wide-angle photograph spans half the Milky Way. The Northern Cross is at the left and the Southern Cross is at the right. The center of the Galaxy is in the constellation Sagittarius, in the middle of this photograph. The dark lines and blotches are caused by hundreds of interstellar clouds of gas and dust that obscure the light from background stars, rather than by a lack of stars.
FIGURE 15-5 Our Galaxy
This wide-angle photograph spans half the Milky Way. The center of the
Galaxy is in the constellation Sagittarius, in the middle of this
photograph. The dark lines and blotches are caused by hundreds of
interstellar clouds of gas and dust that obscure the light from
background stars, rather than by a lack of stars. (Dirk Hoppe)

13. A View Toward the Galactic Center
FIGURE 15-6 A View Toward the Galactic Center
More than a million stars in the disk of our Galaxy fill this view, which covers
a relatively clear window just 4° south of the galactic nucleus in Sagittarius.
Beyond the disk stars you can see two prominent globular clusters. Although
most regions of the sky toward Sagittarius are thick with dust, very little
obscuring matter appears in this tiny section of the sky. (Harvard
Observatory)
More than a million stars in the disk of our Galaxy fill this view, which covers a relatively clear window just 4° south of the galactic nucleus in Sagittarius. Beyond the disk stars you can see two prominent globular clusters. Although most regions of the sky toward Sagittarius are thick with dust, very little obscuring matter appears in this tiny section of the sky.

14. Electron Spin and the Hydrogen Atom
FIGURE 15-7 Electron Spin and the Hydrogen Atom
Due to their spin, electrons and protons both act as tiny magnets. When an
electron and the proton it orbits are spinning in the same direction, their
energy is higher than when they are spinning in opposite directions. When
the electron flips from the higher-energy to the lower-energy configuration,
the atom loses a tiny amount of energy that is radiated as a radio photon
with a wavelength of 21 cm.
Due to their spin, electrons and protons both act as tiny magnets. When an electron and the proton it orbits are spinning in the same direction, their energy is higher than when they are spinning in opposite directions. When the electron flips from the higher-energy to the lower-energy configuration, the atom loses a tiny amount of energy that is radiated as a radio photon with a wavelength of 21 cm.

15. A Technique for Mapping the Galaxy
FIGURE 15-8 A Technique for Mapping the Galaxy
Hydrogen clouds at different locations along our line of sight are moving
around the center of the Galaxy at different speeds. The component of
their motion away from us varies with their distance from the solar
system. Radio waves from the various gas clouds, therefore, exhibit
slightly different Doppler shifts, permitting astronomers to sort out the
gas clouds and map the Galaxy. (NASA/JPL-Caltech/R. Hurt [SSC])
Hydrogen clouds at different locations along our line of sight are moving around the center of the Galaxy at different speeds. The component of their motion away from us varies with their distance from the solar system. Radio waves from the various gas clouds, therefore, exhibit slightly different Doppler shifts, permitting astronomers to sort out the gas clouds and map the Galaxy.

16. A Map of the Galaxy
This map, based on radio telescope surveys of 21-cm radiation, shows the distribution of hydrogen gas in a face-on view of the Galaxy. This view just hints at spiral structure. The galactic nucleus is marked with a dot surrounded by a circle. Details in the large, blank, wedge-shaped region toward the upper left of the map are unknown, because gas in this part of the sky is moving perpendicular to our line of sight and thus does not exhibit a detectable Doppler shift. (Inset) This drawing, based on visible-light data, shows that our solar system lies between two arms of the Milky Way Galaxy.
FIGURE 15-9 A Map of the Galaxy
(a) This map, based on radio telescope surveys of 21-cm radiation, shows the distribution of hydrogen gas in a face-on view of the Galaxy. This view
just hints at spiral structure. The galactic nucleus is marked with a dot surrounded by a circle. Details in the large, blank, wedge-shaped region
toward the upper left of the map are unknown, because gas in this part of the sky is moving perpendicular to our line of sight and thus does not
exhibit a detectable Doppler shift. (Inset) This drawing, based on visible-light data, shows that our solar system lies between two arms of
the Milky Way Galaxy. (Courtesy of G. Westerhout; inset: National Geographic)

17. This drawing labels the spiral arms in the Milky Way.
A Map of the Galaxy
FIGURE 15-9 A Map of the Galaxy
(b) This drawing labels the spiral arms in the Milky Way.
(NASA/JPL-Caltech/R. Hart [SSC])
This drawing labels the spiral arms in the Milky Way.

18. Two Views of a Barred Spiral Galaxy
FIGURE Two Views of a Barred Spiral Galaxy
The galaxy M83 is in the southern constellation of Centaurus, about 12 million ly from Earth. (a) At visible wavelengths, spiral
arms are clearly illuminated by young stars and glowing H II regions. (b) A radio view at 21-cm wavelength shows the emission
from neutral hydrogen gas. Note that the spiral arms are more clearly demarcated by visible stars and H II regions than by 21-cm
radio emission. (a: S. Van Dyk/IPAC; b: VLA, NRAO)
The galaxy M83 is in the southern constellation of Centaurus, about 12 million ly from Earth. (a) At visible wavelengths, spiral arms are clearly illuminated by young stars and glowing H II regions. (b) A radio view at 21-cm wavelength shows the emission from neutral hydrogen gas. Note that the spiral arms are more clearly demarcated by visible stars and H II regions than by 21-cm radio emission.

19. Our Galaxy
As seen from the side, three major visible components of our Galaxy are a thin disk, a central bulge, and a two-part halo system. As noted earlier, there is also a central bar. The visible Galaxy’s diameter is about 100,000 ly, and the Sun is about 26,000 ly from the galactic center. The disk contains gas and dust along with Population I (young, metal-rich) stars. The halo is composed almost exclusively of Population II (old, metal-poor) stars. (Inset) The visible matter in our Galaxy fills only a small volume compared to the distribution of dark matter, whose composition is presently unknown. Dark matter’s presence is felt by its gravitational effect on visible matter.
FIGURE Our Galaxy
As seen from the side, three major visible components of our Galaxy are a thin disk, a central bulge, and a two-part halo system. As noted earlier,
there is also a central bar. The visible Galaxy’s diameter is about 100,000 ly, and the Sun is about 26,000 ly from the galactic center. The
disk contains gas and dust along with Population I (young, metal-rich) stars. The halo is composed almost exclusively of Population II (old,
metal-poor) stars. (Inset) The visible matter in our Galaxy fills only a small volume compared to the distribution of dark matter, whose
composition is presently unknown. Dark matter’s presence is felt by its gravitational effect on visible matter.

20. Infrared View of the Milky Way
(a) Taken by the COBE satellite in 1997, this infrared image shows the disk and central bulge of our Galaxy, as they would be seen from outside of the Galaxy. Most of the sources scattered above and below the disk are nearby stars. Stars appear white, whereas interstellar dust appears orange. Note that the dust that obscures light from more distant stars in Figure 15-5 is quite bright in this infrared image. (b) Because we are embedded in it, the
Galaxy appears wrapped around us.
FIGURE Infrared View of the Milky Way
Taken by the COBE satellite in 1997, this infrared image shows the disk and central
bulge of our Galaxy, as they would be seen from outside the Galaxy. Most of the sources
scattered above and below the disk are nearby stars. Stars appear white, whereas
interstellar dust appears orange. Note that the dust that obscures light from more distant
stars in Figure 15-5 is quite bright in this infrared image. (b) Because we are embedded
in it, the Galaxy appears wrapped around us. (The COBE Project, DIRBE, NASA)

21. The Galactic Center
(a) This wide-angle view at infrared wavelengths shows a 50° segment of the Milky Way centered on the nucleus of the Galaxy. Black represents the dimmest regions of infrared emission, with blue the next strongest, followed by yellow and red; white represents the strongest emission. The prominent band diagonally across this photograph is a layer of dust in the plane of the Galaxy. Numerous knots and blobs along the plane of the Galaxy are interstellar clouds of gas and dust heated by nearby stars. (b) This close-up infrared view of the galactic center covers the area outlined by the white rectangle in (a). (c) This infrared image shows about 300 of the brightest stars less than 1 ly from Sagittarius A*, which is at the center of the picture. The distribution of stars and their observed motions around the galactic center imply a very high density (about a million solar masses per cubic light-year) of less luminous stars.
FIGURE The Galactic Center
(a) This wide-angle view at infrared wavelengths shows a 50° segment of the Milky Way centered on the nucleus of the Galaxy. Black represents the
dimmest regions of infrared emission, with blue the next strongest, followed by yellow and red; white represents the strongest emission. The prominent
band diagonally across this photograph is a layer of dust in the plane of the Galaxy. Numerous knots and blobs along the plane of the Galaxy are
interstellar clouds of gas and dust heated by nearby stars. (b) This close-up infrared view of the galactic center covers the area outlined by the white
rectangle in (a). (c) This infrared image shows about 300 of the brightest stars less than 1 ly from Sagittarius A*, which is at the center of the picture.
The distribution of stars and their observed motions around the galactic center imply a very high density (about a million solar masses per cubic
light-year) of less luminous stars. (a, b: NASA; c: R. Schödel et al., MPE/ESO)

22. Two Views of the Galactic Nucleus
(a) A radio image taken at the VLA of the galactic nucleus and environs. This image covers an area of the sky 8 times wider than the Moon. SNR means supernova remnant. The Sgr features are radio-bright objects.
(b) The colored dots show the motion of seven stars in the vicinity of the unseen massive object () at the position of the radio source Sagittarius A*, part of Sgr A. This plot indicates that the stars are held in orbit by a 4 x 106-solar-mass black hole.
FIGURE Two Views of the Galactic Nucleus
(a) A radio image taken at the VLA of the galactic nucleus and environs. This image covers an area of the sky 8 times wider than the Moon. SNR means
supernova remnant. The numbers following each SNR are its right ascension and declination. The Sgr (Sagittarius) features are radio-bright objects.
(b) 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 star) at the position of the radio source Sagittarius A*, part of Sgr A in (a). The orbits were measured over a 14-year period. This plot indicates
that the stars are held in orbit by a 4 × 106-solar-mass black hole. (a: Naval Research Laboratory produced by N. E. Kassim, D. S. Briggs, T. J. W. Lazio,
T. N. LaRosa, J. Imamura, and S. D. Hyman. Originally from the NRAO Very Large Array. Courtesy of A. Pedlar, K. Anantharamiah, M. Gross, and R. Ekers;
b: Keck/UCLA Galactic Center Group)

23. Orbits of Stars in Our Galaxy
FIGURE Orbits of Stars in Our Galaxy
This disk galaxy, M58, looks very similar to what the Milky Way Galaxy would
look like from far away. The colored arrows show typical orbits of stars in the
central bulge (blue), disk (red), and halo (yellow). Interstellar clouds,
clusters, and other objects in the various components have similar orbits.
(NOAO/AURA/NSF)
This disk galaxy, M58, looks very similar to what the Milky Way Galaxy would look like from far away. The colored arrows show typical orbits of stars in the central bulge (blue), disk (red), and halo (yellow). Interstellar clouds, clusters, and other objects in the various components have similar orbits.

24. The Nearest Galaxy
(a) The Canis Major Dwarf Galaxy is a dwarf elliptical galaxy that lies some 25,000 ly from the Milky Way. This infrared radiation–based image shows the Milky Way’s spiral arms, as well as the distribution of stars being stripped from the Canis Major Dwarf Galaxy by our Galaxy’s gravitational tidal force. Containing only about 1 billion stars, the Canis Major Dwarf will be completely pulled apart within the next 100 million years or so by the Milky Way. (b) This is a view from Earth of the Canis Major Galaxy and its path of debris.
FIGURE The Nearest Galaxy
(a) The Canis Major Dwarf Galaxy is a dwarf elliptical galaxy that lies some 25,000 ly from the Milky Way. This infrared radiation–based image shows the Milky Way’s
spiral arms, as well as the distribution of stars being stripped from the Canis Major Dwarf Galaxy by our Galaxy’s gravitational tidal force. Containing only about 1
billion stars, the Canis Major Dwarf Galaxy will be completely pulled apart within the next 100 million years or so by the Milky Way. (b) View from Earth of the Canis
Major Dwarf Galaxy and its path of debris. (a: R. Ibata/Strasbourg Observatory, ULP et al., 2MASS, NASA; b: Nicola Martin & Rodrigo Ibata, Observatoire de
Strasbourg, 2003)

25. Differential Rotation of the Galaxy
FIGURE Differential Rotation of the Galaxy
(a) If all stars in the Galaxy had the same angular speed, they would orbit in lockstep. (b) However, stars at different distances from the galactic center
have different angular speeds. Stars and clouds farther from the center take longer to go around the Galaxy than do stars closer to the center. As a result,
stars closer to the Galaxy’s center than the Sun are overtaking the solar system, whereas stars farther from the center are lagging behind us.
(a) If all stars in the Galaxy had the same angular speed, they would orbit in lockstep. (b) However, stars at different distances from the galactic center have different angular speeds. Stars and clouds farther from the center take longer to go around the Galaxy than do stars closer to the center. As a result, stars closer to the Galaxy’s center than the Sun are overtaking the solar system, whereas stars farther from the center are lagging behind us.

26. The Galaxy’s Rotation Curve
FIGURE The Galaxy’s Rotation Curve
The blue curve shows the orbital speeds of stars and gas in the Galaxy, and the dashed red curve shows Keplerian orbits that would be caused by the
gravitational force from all the known objects in the Galaxy. Because the data (blue curve) do not show any such decline, there is, apparently, an
abundance of dark matter that extends to great distances from the galactic center. This additional mass gives the outer stars higher speeds
than they would have otherwise.
The blue curve shows the orbital speeds of stars and gas in the Galaxy, and the dashed red curve shows Keplerian orbits that would be caused by the gravitational force from all the known objects in the Galaxy. Because the data (blue curve) do not show any such decline, there is, apparently, an abundance of dark matter that extends to great distances from the galactic center. This additional mass gives the outer stars higher speeds than they would have otherwise.

27. Microlensing by Dark Matter in the Galactic Halo
FIGURE Microlensing by Dark Matter in the Galactic Halo
(a) Gravitational fields cause light to change direction. A white dwarf, brown dwarf,
or black hole in the Galaxy’s halo passing between Earth and a more distant star
will focus the starlight in our direction, making distant objects appear brighter than
they are normally. (b) The light curve of the gravitational microlensing of light from
a star in the Galaxy’s nuclear bulge by an intervening object.
(a) Gravitational fields cause light to change direction. A white dwarf, brown dwarf, or black hole in the Galaxy’s halo passing between Earth and a more distant star will focus the starlight in our direction, making distant objects appear brighter than they are normally. (b) The light curve of the gravitational microlensing of light from a star in the Galaxy’s central bulge by an intervening object.

28. Summary of Key Ideas

29. Discovering the Milky Way
A century ago, astronomers were divided on whether or not the Milky Way Galaxy and the universe were the same thing.
The Shapley–Curtis debate was the first major public discussion between astronomers as to whether the Milky Way contains all the stars in the universe.
Cepheid variable stars are important in determining the distance to other galaxies.
Edwin Hubble proved that there are other galaxies far outside of the Milky Way.

30. The Structure of Our Galaxy
Our Galaxy has a disk about 100,000 ly in diameter and about 2000 ly thick, with a high concentration of interstellar dust and gas. It contains around 200 billion stars.
Interstellar dust obscures our view into the plane of the galactic disk at visual wavelengths. However, hydrogen clouds can be detected beyond this dust by the 21-cm radio waves emitted by changes in the relative spins of electrons and protons in the clouds, as well as by other nonvisible emissions.

31. The Structure of Our Galaxy
The center, or galactic nucleus, has been studied at gamma-ray, X-ray, infrared, and radio wavelengths, which pass readily through intervening interstellar dust and H II regions that illuminate the spiral arms. These observations have revealed the dynamic nature of the galactic nucleus, but much about it remains unexplained.
A supermassive black hole of about 4.3 x 106 solar masses exists in the galactic nucleus.
The galactic nucleus of the Milky Way is surrounded by a flattened sphere of stars, called the central bulge, through which a bar of stars and gas extends.

32. The Structure of Our Galaxy
A disk with at least four bright arms of stars, gas, and dust spirals out from the ends of the bar in the galactic central bulge.
Young OB associations, H II regions, and molecular clouds in the galactic disk outline huge spiral arms where stars are forming.
The Sun is located about 26,000 ly from the galactic nucleus, between the spiral arms. The Sun moves in its orbit at a speed of about 878,000 km/h and takes about 230 million years to complete one orbit around the center of the Galaxy.

33. The Structure of Our Galaxy
The entire Galaxy is surrounded by two halos of matter. The inner halo includes a spherical distribution of globular clusters and field stars, as well as large amounts of dark matter. It orbits in the same general direction as the disk. The outer halo is composed of dark matter and very old stars, which have retrograde orbits.

34. Key Terms central bulge dark matter (missing mass) disk (of a galaxy)
distance modulus
galactic cannibalism
galactic nucleus
galaxy
halo (of a galaxy)
microlensing
Milky Way Galaxy
missing mass
nebula (plural nebulae)
nuclear bulge
rotation curve (of a galaxy)
Sagittarius A
Shapley–Curtis debate
spin (of an electron or proton)
spiral arm
synchrotron radiation
21-cm radio radiation

35. WHAT DID YOU THINK? What is the shape of the Milky Way Galaxy?
The Milky Way is a barred spiral galaxy. A bar of stars, gas, and dust runs through its central region. It has two major spiral arms, several minor arms, and is surrounded by a complex spherical halo system of stars and dark matter.

36. WHAT DID YOU THINK?
Where is our solar system located in the Milky Way Galaxy?
The solar system is between the Sagittarius and Perseus spiral arms, about 26,000 ly from the center of the Galaxy (about halfway out to the visible edge of the galactic disk).

37. WHAT DID YOU THINK?
Is the Sun moving through the Milky Way Galaxy and, if so, about how fast?
Yes. The Sun orbits the center of the Milky Way Galaxy at a speed of 878,000 km/h.