1. Roger A. Freedman • William J. Kaufmann III
Universe
Eighth Edition
CHAPTER 18
The Birth of Stars

2. A region of star formation about 1400 pc (4000 ly) from Earth in the southern constellation Ara (the Altar).
(European Southern Observatory)

3. HW
Chapter 18 online quiz due Thursday night 11/11

4. Figure 18-1 The Orion Nebula
(a) The middle “star” of the three that make up Orion’s sword is actually an interstellar cloud called
the Orion Nebula.
(b) The nebula is about 450 pc (1500 ly) from Earth and
contains about 300 solar masses of material. Within the area shown by
the box are four hot, massive stars called the Trapezium. They produce
the ultraviolet light that makes the nebula glow. (a: R. C. Mitchell, Central
Washington University; b: Anglo-Australian Observatory)

5. Figure 18-1 The Orion Nebula
(a) The middle “star” of the three that make up Orion’s sword is actually an interstellar cloud called
the Orion Nebula.
(a: R. C. Mitchell, Central Washington University)

6. Figure 18-1 The Orion Nebula
(b) The nebula is about 450 pc (1500 ly) from Earth and
contains about 300 solar masses of material. Within the area shown by
the box are four hot, massive stars called the Trapezium. They produce
the ultraviolet light that makes the nebula glow.
(b: Anglo-Australian Observatory)

7. Figure 18-2 Emission, Reflection, and Dark Nebulae in Orion
A variety of different nebulae appear in the sky around Alnitak,
the easternmost star in Orion’s belt (see Figure 20-1a). All the nebulae lie
approximately 500 pc (1600 ly) from Earth. They are actually nowhere
near Alnitak, which is only 250 pc (820 ly) distant. This photograph shows
an area of the sky about 1.5° across. (Royal Observatory, Edinburgh)

8. Figure 18-3 Ionization and Recombination
The characteristic glow of emission nebulae (like those shown in
Figure 18-1 and Figure 18-2) comes from gas atoms that are excited by ultraviolet radiation from
nearby hot stars.

9. Figure 18-4 A Dark Nebula
When first discovered in the late 1700s, dark nebulae
were thought to be “holes in the heavens” where very few stars are
present. In fact, they are opaque regions that block out light from the
stars beyond them. The few stars that appear to be within Barnard 86 lie
between us and the nebula. Barnard 86 is in the constellation Sagittarius
and has an angular diameter of 4 arcminutes, about 1/7 the angular
diameter of the full moon. (Anglo-Australian Observatory)

10. Figure 18-5 Reflection Nebulae
Wispy reflection nebulae called NGC
surround several stars in the constellation Corona Australis (the Southern
Crown). Unlike emission nebulae, reflection nebulae do not emit their own
light, but scatter and reflect light from the stars that they surround. This
scattered starlight is quite blue in color. The region shown here is about
23 arcminutes across. (David Malin/Anglo-Australian Observatory)

11. Figure 18-6 Interstellar Reddening
(a) Dust grains in interstellar space scatter or
absorb blue light more than red light. Thus light from a distant object
appears redder than it really is.

12. Figure 18-6 Interstellar Reddening
(b) The emission nebulae NGC 3603 and
NGC 3576 are different distances from Earth. Light from the more distant
nebula must pass through more interstellar dust to reach us, so more
interstellar reddening occurs and NGC 3603 is a ruddier shade of red. The
two nebulae are about 1° apart in the sky. (Anglo-Australian Observatory)

13. Figure 18-7 Gas and Dust in the Milky Way
Glowing gas clouds (emission nebulae
or H II regions) and dark, dusty regions are concentrated close to the
midplane of the Milky Way Galaxy, of which our Sun is part. This
wide-angle photograph also shows the three bright stars that make up
the “summer triangle” (see Figure 2-8). (© Jerry Lodriguss/Photo Researchers).

14. Figure 18-8 Spiral Galaxies
Spiral galaxies, like our own Milky Way
Galaxy, consist of stars, gas, and dust that are largely
confined to a flattened, rotating disk.
(a) This face-on view of M83 shows
luminous stars and H II regions along the spiral arms.
(a: David Malin/Anglo-Australian Observatory)

15. Figure 18-8 Spiral Galaxies
Spiral galaxies, like our own Milky Way
Galaxy, consist of stars, gas, and dust that are largely
confined to a flattened, rotating disk.
(b) This edge-on view of NGC 891 shows a dark band caused by dust in this galaxy’s
interstellar medium. Although in different parts of the sky, both galaxies
are about 7 million pc (23 million ly) from Earth and have angular
diameters of about 13 arcminutes.
(b: Instituto de Astrofísica de Canarias/Royal Greenwich Observatory/David Malin)

16. Figure 18-9 Bok Globules
The dark blobs in this photograph of a glowing
H II region are clouds of dust and gas called Bok globules. A
typical Bok globule is a parsec or less in size and contains from one to a
thousand solar masses of material. The Bok globules and H II region in
this image are part of a much larger star-forming region called NGC 281,
which lies about 9500 ly (2900 pc) from Earth in the constellation
Cassiopeia. The image shows an area about 8.8 ly (2.7 pc) across. (NASA,
ESA, and The Hubble Heritage Team (STScI/AURA))

17. Main-sequence stars have masses greater than about 0. 08 solar masses
Main-sequence stars have masses greater than about 0.08 solar masses. The reason for this is that gas clouds smaller than 0.08 solar masses
do not develop the necessary high temperature and pressure required for nuclear fusion when they collapse.
are too small to begin to collapse.
generate enough energy that they fragment into smaller mass objects when they collapse.
become white dwarf stars.
do not exist as far as we know.
Q18.9

18. Main-sequence stars have masses greater than about 0. 08 solar masses
Main-sequence stars have masses greater than about 0.08 solar masses. The reason for this is that gas clouds smaller than 0.08 solar masses
do not develop the necessary high temperature and pressure required for nuclear fusion when they collapse.
are too small to begin to collapse.
generate enough energy that they fragment into smaller mass objects when they collapse.
become white dwarf stars.
do not exist as far as we know.
A18.9

19. Figure 18-10 Pre–Main-Sequence Evolutionary Tracks
As a protostar evolves, its
luminosity and surface temperature both change. The tracks shown here
depict these changes for protostars of seven different masses. Each
dashed red line shows the age of a protostar when its evolutionary track
crosses that line. (We will see in Section 18-5 that protostars lose quite a
bit of mass as they evolve: The mass shown for each track is the value
when the protostar finally settles down as a main-sequence star.)

20. Figure 18-11 Revealing a Hidden Protostar
(a) This visible-light view shows a dark
nebula called L1014 in the constellation Cygnus (the Swan). No stars are
visible within the nebula.
(b) The Spitzer Space Telescope was used to
make this false-color infrared image of the outlined area in (a). The bright
red-yellow spot is a protostar within the dark nebula.
(a: Deep Sky Survey; b: NASA/JPL-Caltech/N. Evans, Univ. of Texas at Austin)

21. Figure 18-12 Main-Sequence Stars of Different Masses
Stellar models show that when a protostar evolves into a main-sequence
star, its internal structure depends on its mass. Note: The three stars
shown here are not drawn to scale. Compared with a 1-M main-sequence
star like that shown in (b), a 6-M main-sequence star like that in (a) has
more than 4 times the radius, and a 0.2-M main-sequence star like that
in (c) has only one-third the radius.

22. Figure 18-12 Main-Sequence Stars of Different Masses
Stellar models show that when a protostar evolves into a main-sequence
star, its internal structure depends on its mass. Note: The three stars
shown here are not drawn to scale. Compared with a 1-M main-sequence
star like that shown in (b), a 6-M main-sequence star like that in (a) has
more than 4 times the radius, and a 0.2-M main-sequence star like that
in (c) has only one-third the radius.

23. Figure 18-12 Main-Sequence Stars of Different Masses
Stellar models show that when a protostar evolves into a main-sequence
star, its internal structure depends on its mass. Note: The three stars
shown here are not drawn to scale. Compared with a 1-M main-sequence
star like that shown in (b), a 6-M main-sequence star like that in (a) has
more than 4 times the radius, and a 0.2-M main-sequence star like that
in (c) has only one-third the radius.

24. Figure 18-13 Mass Loss from Young, Massive Stars
(a) The Omega Nebula, also known as M17, is a region of star formation in
the constellation Sagittarius about 1700 pc (5500 ly) from Earth.
(a: Palomar Observatory DSS)

25. Figure 18-13 Mass Loss from Young, Massive Stars
(b) This infrared image allows us to see through dust, revealing recently formed
stars that cannot be seen in (a).
(b: 2MASS/UMass/ IPAC-Caltech/NASA/NSF)

26. Figure 18-13 Mass Loss from Young, Massive Stars
(c) The most massive young stars eject
copious amounts of hot gas. Red indicates X-ray emission from gas at a
temperature of K; blue indicates even hotter gas at a
temperature of K. Astronomers do not see such X-ray emission
from the Orion Nebula (Figure 18-1), which has many young stars but
very few massive ones.
(c: NASA/CXC/PSU/L. Townsley et al.)

27. Figure 18-14 Bipolar Outflow and Herbig-Haro Objects
The two bright knots of glowing, ionized gas called HH 1 and HH 2 are
Herbig-Haro objects. They are created when fast-moving gas ejected from
a protostar slams into the surrounding interstellar medium, heating the
gas to high temperature. HH 1 and HH 2 are 0.34 parsec (1.1 light-year)
apart and lie 470 pc (1500 ly) from Earth in the constellation Orion.
(J. Hester, the WFPC-2 Investigation Definition Team, and NASA)

28. Figure 18-15 A Circumstellar Accretion Disk and Jets
This false-color image shows a star surrounded by an accretion disk, which
we see nearly edge-on. Red denotes emission from ionized gas, while
green denotes starlight scattered from dust particles in the disk. The
midplane of the accretion disk is so dusty and opaque that it appears
dark. Two oppositely directed jets flow away from the star, perpendicular
to the disk and along the disk’s rotation axis. This star lies 140 pc (460 ly)
from Earth. (C. Burrows, the WFPC-2 Investigation Definition Team, and NASA)

29. Figure 18-16 A Magnetic Model for Bipolar Outflow
(a) Observations suggest that
circumstellar accretion disks are threaded by magnetic field lines, as
shown here.
(b), (c) The contraction and rotation of the disk make the
magnetic field lines distort and twist into helices. These helices steer
some of the disk material into jets that stream perpendicular to the plane
of the disk, as in Figure
(Adapted from Alfred T. Kamajian/Thomas P. Ray, “Fountain of Youth: Early Days in the Life of a Star,” Scientific American, August 2000)

30. Figure 18-16 A Magnetic Model for Bipolar Outflow
(a) Observations suggest that
circumstellar accretion disks are threaded by magnetic field lines, as
shown here.
(b), (c) The contraction and rotation of the disk make the
magnetic field lines distort and twist into helices. These helices steer
some of the disk material into jets that stream perpendicular to the plane
of the disk, as in Figure
(Adapted from Alfred T. Kamajian/Thomas P. Ray, “Fountain of Youth: Early Days in the Life of a Star,” Scientific American, August 2000)

31. Figure 18-16 A Magnetic Model for Bipolar Outflow
(a) Observations suggest that
circumstellar accretion disks are threaded by magnetic field lines, as
shown here.
(b), (c) The contraction and rotation of the disk make the
magnetic field lines distort and twist into helices. These helices steer
some of the disk material into jets that stream perpendicular to the plane
of the disk, as in Figure
(Adapted from Alfred T. Kamajian/Thomas P. Ray, “Fountain of Youth: Early Days in the Life of a Star,” Scientific American, August 2000)

32. Figure 18-17 A Star Cluster with an H II Region
The star cluster M16 is thought to be no more than
800,000 years old, and star formation is still taking place within adjacent
dark, dusty globules. The inset shows three dense, cold pillars of gas and
dust silhouetted against the glowing background of the red emission
nebula (called the Eagle Nebula for its shape). The pillar at the upper left
extends about 0.3 parsec (1 light-year) from base to tip, and each of
its “fingers” is somewhat broader than our entire solar system.
(Anglo-Australian Observatory; J. Hester and P. Scowen, Arizona State
University; NASA)

33. Figure 18-17 A Star Cluster with an H II Region
The star cluster M16 is thought to be no more than
800,000 years old, and star formation is still taking place within adjacent
dark, dusty globules. The inset shows three dense, cold pillars of gas and
dust silhouetted against the glowing background of the red emission
nebula (called the Eagle Nebula for its shape). The pillar at the upper left
extends about 0.3 parsec (1 light-year) from base to tip, and each of
its “fingers” is somewhat broader than our entire solar system.
(Anglo-Australian Observatory; J. Hester and P. Scowen, Arizona State
University; NASA)

34. Figure 18-18 A Young Star Cluster and Its H-R Diagram
(a) This photograph shows an H II region and the young star cluster NGC 2264 in the constellation
Monoceros (the Unicorn). It lies about 800 pc (2600 ly) from Earth.
(b) Each dot plotted on this H-R diagram represents a star in NGC 2264
whose luminosity and surface temperature have been determined. This
star cluster probably started forming only 2 million years ago.
(Anglo-Australian Observatory)

35. Figure 18-19 The Pleiades and Its H-R Diagram
(a) The Pleiades star cluster is
117 pc (380 ly) from Earth in the constellation Taurus, and can be seen
with the naked eye.
(b) Each dot plotted on this H-R diagram represents a
star in the Pleiades whose luminosity and surface temperature have been
measured. (Note: The scales on this H-R diagram are different from those
in Figure 18-18b.) The Pleiades is about 50 million (5  107) years old.
(Anglo-Australian Observatory)

36. Figure 18-20 Mapping Molecular Clouds
A radio telescope was tuned to a
wavelength of 2.6 mm to detect emissions from carbon monoxide (CO)
molecules in the constellations Orion and Monoceros. The result was this
false-color map, which shows a 35°  40° section of the sky. The Orion
and Horsehead star-forming nebulae are located at sites of intense
CO emission (shown in red and yellow), indicating the presence of a
particularly dense molecular cloud at these sites of star formation. The
molecular cloud is much thinner at the positions of the Cone and Rosette
nebulae, where star formation is less intense. (Courtesy of R. Maddalena,
M. Morris, J. Moscowitz, and P. Thaddeus)

37. Figure 18-21 Giant Molecular Clouds in the Milky Way
This perspective drawing shows the locations of giant molecular clouds in an inner part of our
Galaxy as seen from a vantage point above the Sun. These clouds lie
primarily along the Galaxy’s spiral arms, shown by red arcs. The distance
from the Sun to the galactic center is about 8000 pc (26,000 ly). (Adapted
from T. M. Dame and colleagues)

38. Figure 18-22 A Star-Forming Bubble
Radiation and winds from the hot, young O and
B stars at the center of this Spitzer Space Telescope image have carved
out a bubble about 20 pc (65 ly) in diameter in the surrounding gas and
dust. The material around the surface of the bubble has been compressed
and heated, making the dust glow at the infrared wavelengths used to
record this image. The compressed material is so dense that new stars
have formed within that material. This glowing cloud, called RCW 79, lies
about 5300 pc (17,300 ly) from Earth in the constellation Centaurus.
(NASA; JPL-Caltech; and E. Churchwell, University of Wisconsin-Madison)

39. Figure 18-23 How O and B Stars Trigger Star Formation
Stellar winds and ultraviolet radiation
from young O and B stars produce a shock wave that compresses gas farther into the
giant molecular cloud. This stimulates star formation, producing more O and B stars,
which stimulate still more star formation, and so on. Meanwhile, older stars are left
behind. The inset shows a massive star that has spawned other, smaller stars in
this way. These stars are about 770 pc (2500 ly) from Earth in the Cone Nebula, a
star-forming region in the constellation Monoceros. The younger stars are just 0.04 to
0.08 ly (2500 to 5000 AU) from the central star. (Adapted from C. Lada, L. Blitz, and
B. Elmegreen; inset: R. Thompson, M. Rieke, G, Schneider, and NASA)

40. Figure 18-23 How O and B Stars Trigger Star Formation
Stellar winds and ultraviolet radiation
from young O and B stars produce a shock wave that compresses gas farther into the
giant molecular cloud. This stimulates star formation, producing more O and B stars,
which stimulate still more star formation, and so on. Meanwhile, older stars are left
behind. The inset shows a massive star that has spawned other, smaller stars in
this way. These stars are about 770 pc (2500 ly) from Earth in the Cone Nebula, a
star-forming region in the constellation Monoceros. The younger stars are just 0.04 to
0.08 ly (2500 to 5000 AU) from the central star. (Adapted from C. Lada, L. Blitz, and
B. Elmegreen; inset: R. Thompson, M. Rieke, G, Schneider, and NASA)

41. Figure 18-23 How O and B Stars Trigger Star Formation
Stellar winds and ultraviolet radiation
from young O and B stars produce a shock wave that compresses gas farther into the
giant molecular cloud. This stimulates star formation, producing more O and B stars,
which stimulate still more star formation, and so on. Meanwhile, older stars are left
behind. The inset shows a massive star that has spawned other, smaller stars in
this way. These stars are about 770 pc (2500 ly) from Earth in the Cone Nebula, a
star-forming region in the constellation Monoceros. The younger stars are just 0.04 to
0.08 ly (2500 to 5000 AU) from the central star. (Adapted from C. Lada, L. Blitz, and
B. Elmegreen; inset: R. Thompson, M. Rieke, G, Schneider, and NASA)

42. Figure 18-24 A Supernova Remnant
This composite image shows
Cassiopeia A, the remnant of a supernova that occurred
about 3000 pc (10,000 ly) from Earth. In the roughly 300 years since the
supernova explosion, a shock wave has expanded about 3 pc (10 ly)
outward in all directions from the explosion site. The shock wave has
warmed interstellar dust to a temperature of about 300 K (Spitzer Space
Telescope infrared image in red), and has heated interstellar gases to
temperatures that range from 104 K (Hubble Space Telescope visible-light
image in yellow) to 107 K (Chandra X-ray Observatory X-ray image in
green and blue). (NASA; JPL-Caltech; and O. Krause, Steward Observatory)

43. Figure 18-25 The Canis Major R1 Association
This luminous arc of gas,
about 30 pc (100 ly) long, is studded with numerous young
stars. Both the luminous arc and the young stars can be traced to the
same source, a supernova explosion. The shock wave from the supernova
explosion is exciting the gas and making it glow; the same shock wave
also compresses the interstellar medium through which it passes,
triggering star formation. (Courtesy of H. Vehrenberg)

44. If a clump of interstellar matter is cold and dense enough, it will
begin to collapse thanks to the mutual gravitational attraction of
its parts. If the clump is massive enough, it will evolve into a mainsequence
star through the sequence of events shown here.

45. If a clump of interstellar matter is cold and dense enough, it will
begin to collapse thanks to the mutual gravitational attraction of
its parts. If the clump is massive enough, it will evolve into a mainsequence
star through the sequence of events shown here.

46. Q31. The visible-light photograph below shows the Trifid Nebula
in the constellation Sagittarius. Label the following features
on this photograph: (a) reflection nebulae (and the star
or stars whose light is being reflected); (b) dark nebulae;
(c) H II regions; (d) regions where star formation may be occurring.
Explain how you identified each feature.
A31. (a) The reflection nebula is blue.
(b) The star that causes the reflection nebula is bright and close to the nebula.
(c) H II regions are luminous and reddish.
(d) Star formation is occurring in the dark nebulae.

47. Q34. The two false-color images below show a portion of the
Trifid Nebula (see Question 31). The reddish-orange view is
a false-color infrared image, while the bluish picture (shown
to the same scale) was made with visible light. Explain why
the dark streaks in the visible-light image appear bright in the
infrared image.
A34. The dark streaks appear dark in the visible-light image because the dust absorbs visible light. The same streaks appear bright in the infrared image because the dust grains, having been warmed by the absorption to the visible light, emit the energy in the infrared.

48. Key Ideas
Stellar Evolution: Because stars shine by thermonuclear reactions, they have a finite life span. The theory of stellar evolution describes how stars form and change during that life span.
The Interstellar Medium: Interstellar gas and dust, which make up the interstellar medium, are concentrated in the disk of the Galaxy. Clouds within the interstellar medium are called nebulae.

49. Key Ideas
Dark nebulae are so dense that they are opaque. They appear as dark blots against a background of distant stars.
Emission nebulae, or H II regions, are glowing, ionized clouds of gas. Emission nebulae are powered by ultraviolet light that they absorb from nearby hot stars.
Reflection nebulae are produced when starlight is reflected from dust grains in the interstellar medium, producing a characteristic bluish glow.

50. Key Ideas
Protostars: Star formation begins in dense, cold nebulae, where gravitational attraction causes a clump of material to condense into a protostar.
As a protostar grows by the gravitational accretion of gases, Kelvin-Helmholtz contraction causes it to heat and begin glowing. Its relatively low temperature and high luminosity place it in the upper-right region on an H-R diagram.
Further evolution of a protostar causes it to move toward the main sequence on the H-R diagram. When its core temperatures become high enough to ignite steady hydrogen burning, it becomes a main-sequence star.
The more massive the protostar, the more rapidly it evolves.

51. Key Ideas
Mass Loss by Protostars: In the final stages of pre–main-sequence contraction, when thermonuclear reactions are about to begin in its core, a protostar may eject large amounts of gas into space.
Low-mass stars that vigorously eject gas are called T Tauri stars.
A circumstellar accretion disk provides material that a young star ejects as jets. Clumps of glowing gas called Herbig-Haro objects are sometimes found along these jets and at their ends.

52. Key Ideas
Star Clusters: Newborn stars may form an open or galactic cluster. Stars are held together in such a cluster by gravity. Occasionally a star moving more rapidly than average will escape, or “evaporate,” from such a cluster.
A stellar association is a group of newborn stars that are moving apart so rapidly that their gravitational attraction for one another cannot pull them into orbit about one another.

53. Key Ideas
O and B Stars and Their Relation to H II Regions: The most massive protostars to form out of a dark nebula rapidly become main sequence O and B stars. They emit strong ultraviolet radiation that ionizes hydrogen in the surrounding cloud, thus creating the reddish emission nebulae called H II regions.
Ultraviolet radiation and stellar winds from the O and B stars at the core of an H II region create shock waves that move outward through the gas cloud, compressing the gas and triggering the formation of more protostars.

54. Key Ideas
Giant Molecular Clouds: The spiral arms of our Galaxy are laced with giant molecular clouds, immense nebulae so cold that their constituent atoms can form into molecules.
Star-forming regions appear when a giant molecular cloud is compressed. This can be caused by the cloud’s passage through one of the spiral arms of our Galaxy, by a supernova explosion, or by other mechanisms.