1. Universe Tenth Edition
Roger Freedman • Robert Geller • William Kaufmann III
Universe Tenth Edition
Chapter 18
The Birth of Stars

2. 18-1: Understanding how stars evolve requires observation as well as ideas from physics
Chapter 18 Opener: A region of star formation about 1400 pc (4000 ly) from Earth in the southern constellation Ara (the Altar). (European Southern Observatory)

3. By reading this chapter, you will learn
18-1 How astronomers have pieced together the story of stellar evolution
18-2 What interstellar nebulae are and what they are made of
18-3 What happens as a star begins to form
18-4 The stages of growth from young protostars to main-sequence stars
18-5 How stars gain and lose mass during their growth
18-6 What insights star clusters add to our understanding of stellar evolution
18-7 Where new stars form within galaxies
18-8 How the death of old stars can trigger the birth of new stars

4. 18-2: Interstellar gas and dust pervade the galaxy
Figure 18-1: (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. Most of the ultraviolet light that makes the nebula glow comes from just five hot massive stars. (a: Australian Astronomical Observatory/David Malin Images; b: NASA,ESA, M. Robberto [Space Telescope Science Institute/ESA] and the Hubble Space Telescope Orion Treasury Project Team)
The Orion Nebula

5. Figure 18-1: (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. Most of the ultraviolet light that makes the nebula glow comes from just five hot massive stars. (a: Australian Astronomical Observatory/David Malin Images; b: NASA,ESA, M. Robberto [Space Telescope Science Institute/ESA] and the Hubble Space Telescope Orion Treasury Project Team)
The Orion Nebula

6. Emission, Reflection, and Dark Nebulae in Orion
Figure 18-2: A variety of different nebulae appear in the sky around Alnitak, the easternmost star in Orion’s belt (see Figure 18-1a). 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. (Stocktrek Images/Roth Ritter/Stocktrek Images/Corbis)
Emission, Reflection, and Dark Nebulae in Orion

7. Ionization and Recombination
Figure 18-3: In an H II region, the characteristic red 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. While many photons are emitted during recombination, only some are visible, and red is the most common.
Ionization and Recombination

8. Ionization and Recombination
Figure 18-3: In an H II region, the characteristic red 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. While many photons are emitted during recombination, only some are visible, and red is the most common.
Ionization and Recombination

9. Figure 18-4: 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. (Australian Astronomical Observatory/David Malin Images)
A Dark Nebula

10. Figure 18-5: 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. (Dr. Stefan Binnewies and Josef Popsel/
Reflection Nebulae

11. Interstellar Reddening
Figure 18-6a: 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.
Interstellar Reddening

12. Interstellar Reddening
Figure 18-6a: 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.
Interstellar Reddening

13. Interstellar Reddening
Figure 18-6b: 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 deeper shade of red. The two nebulae are about 1° apart in the sky. (b: Stocktrek Images/Corbis)
Interstellar Reddening

14. Gas and dust in the Milky Way
Figure 18-7: There are so many stars that they aren’t easy to see individually, resulting in the diffuse whitish band that gives the Milky Way its name. Red glowing gas clouds (emission nebulae or H II regions) can be seen in the foreground, and dark, dusty regions that block starlight are concentrated close to the midplane of the Milky Way Galaxy. This wide-angle photograph also shows the three bright stars that make up the “summer triangle” (see Figure 2-8). (Jerry Lodriguss/Science Source)
Gas and dust in the Milky Way

15. Figure 18-8a: 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. 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. (Australian Astronomical Observatory/David Malin Images)
Spiral Galaxy

16. Figure 18-8b: Spiral galaxies, like our own Milky Way Galaxy, consist of stars, gas, and dust that are largely confined to a flattened, rotating disk. 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. (Instituto de Astrofísica de Canarias/Royal Greenwich Observatory/David Malin)
Spiral Galaxy

17. 18-3: Protostars form in cold, dark nebulae
Figure 18-9: The dark blobs in this photograph of a glowing H II region are clouds of 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 2900 pc (9500 ly) from Earth in the constellation Cassiopeia. The image shows an area about 2.7 pc (8.8 ly) across. (NASA, ESA, and The Hubble Heritage Team [STScI/AURA])
Bok Globules

18. 18-4: Protostars evolve into main-sequence stars
Figure 18-10: 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.)
Pre-Main-Sequence Evolutionary Tracks on an H-R Diagram

19. Pre-Main-Sequence Evolutionary Tracks on an H-R Diagram
Figure 18-10: 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.)
Pre-Main-Sequence Evolutionary Tracks on an H-R Diagram

20. Revealing a Hidden Protostar
Figure 18-11: (a) This visible-light view shows a dark nebula (or Bok globule) 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. (NASA/JPL-
Caltech/N. Evans [University of Texas at Austin])
Revealing a Hidden Protostar

21. Main-Sequence Stars of Different Masses
Figure 18-12: 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.
Main-Sequence Stars of Different Masses

22. Main-Sequence Stars of Different Masses
Figure 18-12: 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.
Main-Sequence Stars of Different Masses

23. Main-Sequence Stars of Different Masses
Figure 18-12c: 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.
Main-Sequence Stars of Different Masses

24. Mass Loss from a Supermassive Star
Figure 18-13: The Quintuplet Cluster is 25,000 ly from Earth. Inset: Within the cluster is one of the brightest known stars, called the Pistol. Astronomers calculate that the Pistol formed nearly 3 million years ago and originally had 100–200 M. The structure of the gas cloud suggests the star ejected the gas we see in two episodes, 6000 and 4000 years ago. The gas from any previous ejections is so thinly spread now that we cannot see it. The nebula shown in the inset is more than 1.25 pc (4 ly) across—it would stretch from the Sun nearly to the closest star, Proxima Centauri. (2 MASS/UMass/IPAC-Caltech/NASA/NSF; inset: D. Filger, NASA)
Mass Loss from a Supermassive Star

25. 18-5: During the birth process, stars both gain and lose mass
Figure 18-14: 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 an HH 2 are 0.34 pc (1.1 ly) apart and lie 470 pc (1500 ly) from Earth in the constellation Orion. (J. Hester [Arizona State University], the WFPC-2 Investigation Team, and NASA)
Bipolar Outflow and Herbig-Haro Objects

26. A Circumstellar Accretion Disk and Jets
Figure 18-15: This false-color image shows a T Tauri 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, Herbig-Haro object HH 30, lies 140 pc (460 ly) from Earth. (C. Burrows, the WFPC-2 Investigation Definition Team, and NASA)
A Circumstellar Accretion Disk and Jets

27. A Magnetic Model for Bipolar Outflow
Figure 18-16: (a) Observations suggest that circumstellar accretion disks are threaded by magnetic field lines, as shown here. (b) Magnetic field lines move with the material they thread. (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)
A Magnetic Model for Bipolar Outflow

28. A Magnetic Model for Bipolar Outflow
Figure 18-16: (a) Observations suggest that circumstellar accretion disks are threaded by magnetic field lines, as shown here. (b) Magnetic field lines move with the material they thread. (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)
A Magnetic Model for Bipolar Outflow

29. A Magnetic Model for Bipolar Outflow
Figure 18-16b: Magnetic field lines move with the material they thread. (Adapted from Alfred T. Kamajian/Thomas P. Ray, “Fountain of Youth: Early Days in the Life of a Star,” Scientific American, August 2000)
A Magnetic Model for Bipolar Outflow

30. A Magnetic Model for Bipolar Outflow
Figure 18-16c: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)
A Magnetic Model for Bipolar Outflow

31. Star Formation and Jets
Figure 18-17: While the protostars aren’t visible, some of their jets can be seen emanating from the thick gas and dust from which the stars are forming. As new stars form, their strong winds and intense ultraviolet radiation blow away lower-density material, leaving behind distinct regions with greater density. The denser material makes an ideal star-forming region to produce stars; the first few bright stars are already visible. This is only part of the 30 ly–wide Carina Nebula, which is about 7500 ly away. (NASA, ESA, and M. Livio and the Hubble 20th Anniversary Team [STScI])
Star Formation and Jets

32. 18-6: Young star clusters give insight into star formation and evolution
Figure 18-18: While the protostars aren’t visible, some of their jets can be seen emanating from the thick gas and dust from which the stars are forming. As new stars form, their strong winds and intense ultraviolet radiation blow away lower-density material, leaving behind distinct regions with greater density. The denser material makes an ideal star-forming region to produce stars; the first few bright stars are already visible. This is only part of the 30 ly–wide Carina Nebula, which is about 7500 ly away. (NASA, ESA, and M. Livio and the Hubble 20th Anniversary Team [STScI])
Formation of a Star Cluster

33. Formation of a Star Cluster
Figure 18-18: While the protostars aren’t visible, some of their jets can be seen emanating from the thick gas and dust from which the stars are forming. As new stars form, their strong winds and intense ultraviolet radiation blow away lower-density material, leaving behind distinct regions with greater density. The denser material makes an ideal star-forming region to produce stars; the first few bright stars are already visible. This is only part of the 30 ly–wide Carina Nebula, which is about 7500 ly away. (NASA, ESA, and M. Livio and the Hubble 20th Anniversary Team [STScI])
Formation of a Star Cluster

34. Figure 18-19: NGC 3603 is one the closest star clusters at only 20,000 ly away. Star formation has ended and ultraviolet light from its stars power the largest visible reddish H II region in our Galaxy. (NASA/Hubble Space Telescope Collection)
A Mature Star Cluster

35. A Young Cluster and its H-R Diagram
Figure 18-20: (a) This image shows the young star cluster NGC 2264 in a reddish H II region known as the Fox Fur Nebula. Stars within the cluster provide the ultraviolet light that powers this emission nebula. The cluster 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, and its lower-mass stars have not yet reached the main sequence. (a: © 2004–2013 R. Jay GaBany, Cosmotography.com)
A Young Cluster and its H-R Diagram

37. The Pleiades and its H-R Diagram
Figure 18-21: (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. (Australian Astronomical Observatory/David Malin Images)
The Pleiades and its H-R Diagram

39. 18-7: Star birth can begin in giant molecular clouds
Figure 18-22: 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)
Mapping Molecular Clouds

40. Giant Molecular Clouds in the Milky Way
Figure 18-23: 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)
Giant Molecular Clouds in the Milky Way

41. Figure 18-24: 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 (70 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 (17,200 ly) from Earth in the constellation Centaurus. (NASA; JPL-Caltech; and E. Churchwell, University of Wisconsin-Madison)
A Star-Forming Bubble

42. How O and B Stars Trigger Star Formation
Figure 18-25: 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. This figure only illustrates new star formation in the rightward direction, but it could be spherical in a real cloud. 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)
How O and B Stars Trigger Star Formation

43. How O and B Stars Trigger Star Formation
Figure part 1: 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. This figure only illustrates new star formation in the rightward direction, but it could be spherical in a real cloud. (Adapted from C. Lada, L. Blitz, and B. Elmegreen; inset: R. Thompson, M. Rieke, G. Schneider, and NASA)
How O and B Stars Trigger Star Formation

44. How O and B Stars Trigger Star Formation
Figure 18-25: 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. This figure only illustrates new star formation in the rightward direction, but it could be spherical in a real cloud. 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)
How O and B Stars Trigger Star Formation

45. Figure part 2: 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)

46. Figure part 2: 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)

47. 18-8: Supernovae compress the interstellar medium and can trigger star birth
Figure 18-26: 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)
A Supernova Remnant

48. The Canis Major R1 Association
Figure 18-27: These luminous arcs of gas are studded with numerous young stars. Both the luminous arcs and the young stars within 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. (Davide De Martin)
The Canis Major R1 Association

49. Cosmic Connections 18: 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 main-sequence star through the sequence of events shown here.
How Stars are Born

50. Problem 18-31: 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. (Australian Astronomical Observatory/David Malin Images)
The Trifid Nebula

51. The Trifid Nebula in Infrared and Visible Light
Problem 18-34: The two false-color images opposite 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. (ESA/ISO, ISOCAM, and J. Cernicharo et. al.; IAC, Observatorio del Teide, Tenerife)
The Trifid Nebula in Infrared and Visible Light

52. 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.

53. 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.

54. 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.

55. Key Ideas The more massive the protostar, the more rapidly it evolves.
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.

56. 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.

57. 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.

58. 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.