1. Chapter 11 The Interstellar Medium

3. Units of Chapter 11 Interstellar Matter Star-Forming Regions
Dark Dust Clouds
The Formation of Stars Like the Sun
Stars of Other Masses
Star Clusters

4. Question 1 Some regions of the Milky Way’s disk appear dark because
a) there are no stars there.
b) stars in that direction are obscured by interstellar gas.
c) stars in that direction are obscured by interstellar dust.
d) numerous black holes capture all the starlight behind them.
Some regions of the Milky Way’s disk appear dark because
Answer: c

5. Question 1 Some regions of the Milky Way’s disk appear dark because
a) there are no stars there.
b) stars in that direction are obscured by interstellar gas.
c) stars in that direction are obscured by interstellar dust.
d) numerous black holes capture all the starlight behind them.
Some regions of the Milky Way’s disk appear dark because
Dust grains are about the same size as visible light, and they can scatter or block the shorter wavelengths.

6. Interstellar Matter The interstellar medium consists of gas and dust.
Gas is atoms and small molecules, mostly hydrogen and helium.
Dust is more like soot or smoke; larger clumps of particles.
Dust absorbs light, and reddens light that gets through.
This image shows distinct reddening of stars near the edge of the dust cloud.

7. Interstellar Matter
Dust clouds absorb blue light preferentially; spectral lines do not shift.

8. Question 2
When a star’s
visible light passes through interstellar dust, the light we see
a) is dimmed and reddened.
b) appears to twinkle.
c) is Doppler shifted.
d) turns bluish in color.
e) ionizes the dust and creates emission lines.
Answer: a

9. Question 2
When a star’s
visible light passes through interstellar dust, the light we see
a) is dimmed and reddened.
b) appears to twinkle.
c) is Doppler shifted.
d) turns bluish in color.
e) ionizes the dust and creates emission lines.
The same process results in wonderful sunsets, as dust in the air scatters the Sun’s blue light, leaving dimmer, redder light.

10. Question 3 Astronomers use the term nebula to refer to
a) outer envelopes of dying stars that drift gently into space.
b) remnants of stars that die by supernova.
c) clouds of gas and dust in interstellar space.
d) distant galaxies seen beyond our Milky Way.
e) All of the above are correct.
Astronomers use the term nebula to refer to
Answer: e

11. Nebula refers to any fuzzy patch – bright or dark – in the sky.
Question 3
a) outer envelopes of dying stars that drift gently into space.
b) remnants of stars that die by supernova.
c) clouds of gas and dust in interstellar space.
d) distant galaxies seen beyond our Milky Way.
e) All of the above are correct.
Astronomers use the term nebula to refer to
Nebula refers to any fuzzy patch – bright or dark – in the sky.

12. Star-Forming Regions
“Nebula” is a general term used for fuzzy objects in the sky.
Dark nebula: dust cloud
Emission nebula: glows, due to hot stars

13. Question 4 Interstellar gas is composed primarily of
a) 90% hydrogen, 9% helium, and 1% heavier elements.
b) molecules including water and CO2.
c) 50% hydrogen, 50% helium.
d) hydrogen, oxygen, and nitrogen.
e) 99% hydrogen, and 1% heavier elements.
Interstellar gas is composed primarily of
Answer: a

14. Question 4 Interstellar gas is composed primarily of
a) 90% hydrogen, 9% helium, and 1% heavier elements.
b) molecules including water and CO2.
c) 50% hydrogen, 50% helium.
d) hydrogen, oxygen, and nitrogen.
e) 99% hydrogen, and 1% heavier elements.
Interstellar gas is composed primarily of
The composition of interstellar gas mirrors that of the Sun, stars, and the jovian planets.

15. Star-Forming Regions
These nebulae are very large and have very low density; their size means that their masses are large despite the low density.

16. Star-Forming Regions
This is the central section of the Milky Way Galaxy, showing several nebulae, areas of star formation.

17. Question 5 The reddish color of emission nebulae indicates that
a) gas and dust is moving away from Earth.
b) hydrogen gas is present.
c) dying stars have recently exploded.
d) cool red stars are hidden inside.
e) dust is present.
The reddish color of emission nebulae indicates that
Answer: b

18. Glowing hydrogen gas emits red light around the Horsehead nebula.
Question 5
a) gas and dust is moving away from Earth.
b) hydrogen gas is present.
c) dying stars have recently exploded.
d) cool red stars are hidden inside.
e) dust is present.
The reddish color of emission nebulae indicates that
Glowing hydrogen gas emits red light around the Horsehead nebula.

19. Star-Forming Regions
Emission nebulae generally glow red – this is the Hα line of hydrogen.
The dust lanes visible in the previous image are part of the nebula, and are not due to intervening clouds.

20. Star-Forming Regions
How nebulae work

21. Star-Forming Regions
There is a strong interaction between the nebula and the stars within it; the fuzzy areas near the pillars are due to photoevaporation.

22. Star-Forming Regions
Emission nebulae are made of hot, thin gas, which exhibits distinct emission lines.

23. Tarantula Nebula

24. Dark Dust Clouds
Average temperature of dark dust clouds is a few tens of kelvins.
These clouds absorb visible light (left), and emit radio wavelengths (right).

25. Dark Dust Clouds
This cloud is very dark, and can be seen only because of the background stars.

26. Dark Dust Clouds
The Horsehead Nebula is a particularly distinctive dark dust cloud.

27. Dark Dust Clouds
Interstellar gas emits low-energy radiation, due to a transition in the hydrogen atom.

28. Question 6 21-centimeter radiation is important because
a) its radio waves pass unaffected through clouds of interstellar dust.
b) it arises from cool helium gas present throughout space.
c) it can be detected with optical telescopes.
d) it is produced by protostars.
e) it reveals the structure of new stars.
21-centimeter radiation is important because
Answer: a

29. Question 6 21-centimeter radiation is important because
a) its radio waves pass unaffected through clouds of interstellar dust.
b) it arises from cool helium gas present throughout space.
c) it can be detected with optical telescopes.
d) it is produced by protostars.
e) it reveals the structure of new stars.
21-centimeter radiation is important because
Cool atomic hydrogen gas produces 21-cm radio radiation as its electron “flips” its direction of spin.

30. Dark Dust Clouds
This is a contour map of H2CO near the M20 Nebula. Other molecules that can be useful for mapping out these clouds are carbon dioxide and water.
Here, the red and green lines correspond to different rotational transitions. (frequencies)

31. Dark Dust Clouds
These are CO (carbon monoxide) emitting clouds in the outer Milky Way, probably corresponding to regions of star formation.

32. Question 7 Complex molecules in space are found
a) in the photospheres of red giant stars.
b) primarily inside dense dust clouds.
c) in the coronas of stars like our Sun.
d) scattered evenly throughout interstellar space.
e) surrounding energetic young stars.
Complex molecules in space are found
Answer: b

33. Question 7 Complex molecules in space are found
a) in the photospheres of red giant stars.
b) primarily inside dense dust clouds.
c) in the coronas of stars like our Sun.
d) scattered evenly throughout interstellar space.
e) surrounding energetic young stars.
Complex molecules in space are found
A radio telescope image of the outer portion of the Milky Way, revealing molecular cloud complexes.

34. The Formation of Stars Like the Sun
Star formation happens when part of a dust cloud begins to contract under its own gravitational force; as it collapses, the center becomes
hotter and
hotter until
nuclear fusion
begins in the
core.

35. The Formation of Stars Like the Sun
When looking at just a few atoms, the gravitational force is nowhere near strong enough to overcome the random thermal motion.
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36. The Formation of Stars Like the Sun
Stars go through a number of stages in the process of forming from an interstellar cloud.

37. Question 8
a) Clouds fragment into smaller objects, forming many stars at one time.
b) One star forms; other matter goes into planets, moons, asteroids, & comets.
c) Clouds rotate & throw off mass until only enough is left to form one star.
How do single stars form within huge clouds of interstellar gas and dust?
Answer: a

38. Question 8
a) Clouds fragment into smaller objects, forming many stars at one time.
b) One star forms; other matter goes into planets, moons, asteroids, & comets.
c) Clouds rotate & throw off mass until only enough is left to form one star.
How do single stars form within huge clouds of interstellar gas and dust?
The theory of star formation predicts stars in a cluster would form about the same time.

39. The Formation of Stars Like the Sun
Stage 1:
Interstellar cloud starts to contract, probably triggered by shock or pressure wave from nearby star. As it contracts, the cloud fragments into smaller pieces.

40. The Formation of Stars Like the Sun
Stage 2:
Individual cloud fragments begin to collapse. Once the density is high enough, there is no further fragmentation.
Stage 3:
The interior of the fragment has begun heating, and is about 10,000 K.

41. The Formation of Stars Like the Sun
The Orion Nebula is thought to contain interstellar clouds in the process of condensing, as well as protostars.
Orion Nebula Mosaic

42. The Formation of Stars Like the Sun
Stage 4:
The core of the cloud is now a protostar, and makes its first appearance on the H–R diagram.

43. The Formation of Stars Like the Sun
These jets are being emitted as material condenses onto a protostar.

44. The Formation of Stars Like the Sun
These protostars are in Orion.

45. The Formation of Stars Like the Sun
Planetary formation has begun, but the protostar is still not in equilibrium – all heating comes from the gravitational collapse.

46. The Formation of Stars Like the Sun
The last stages can be followed on the H–R diagram:
The protostar’s luminosity decreases even as its temperature rises because it is becoming more compact.

47. The Formation of Stars Like the Sun
At stage 6, the core reaches 10 million K, and nuclear fusion begins. The protostar has become a star.
The star continues to contract and increase in temperature, until it is in equilibrium. This is
stage 7: the star has reached the main sequence and will remain there as long as it has hydrogen to fuse in its core.

48. Stars of Other Masses
This H–R diagram shows the evolution of stars somewhat more and somewhat less massive than the Sun. The shape of the paths is similar, but they wind up in different places on the main sequence.

49. Stars of Other Masses
If the mass of the original nebular fragment is too small, nuclear fusion will never begin. These “failed stars” are called brown dwarfs.

50. Star Clusters
Because a single interstellar cloud can produce many stars of the same age and composition, star clusters are an excellent way to study the effect of mass on stellar evolution.

51. Star Clusters
This is a young star cluster called the Pleiades. The H–R diagram of its stars is on the right. This is an example of an open cluster.

52. Star Clusters
This is a globular cluster – note the absence of massive main-sequence stars, and the heavily populated red giant region.

53. Cluster Location

54. Question 9
a) OB associations.
b) molecular cloud complexes.
c) aggregates.
d) globular clusters.
e) hives.
Very young stars in small clusters of members are known as
Answer: a

55. Question 9
a) OB associations.
b) molecular cloud complexes.
c) aggregates.
d) globular clusters.
e) hives.
Very young stars in small clusters of members are known as
NGC 3603 is a newborn cluster of hot young blue Type O and B stars – a perfect OB association.

56. Star Clusters
These images are believed to show a star cluster in the process of formation within the Orion Nebula.

57. Question 10 All stars in a stellar cluster have roughly the same
a) temperature.
b) color.
c) distance.
d) mass.
e) luminosity.
All stars in a stellar cluster have roughly the same
Answer: c

58. Question 10 All stars in a stellar cluster have roughly the same
a) temperature.
b) color.
c) distance.
d) mass.
e) luminosity.
All stars in a stellar cluster have roughly the same
Stars in the Pleiades cluster vary in temperature, color, mass, and luminosity, but all lie about 440 light-years away.

59. Star Clusters
The presence of massive, short-lived O and B stars can profoundly affect their star cluster, as they can blow away dust and gas before it has time to collapse.
This is a simulation of such a cluster.
Carina Nebula

60. Question 11 Stars are often born within groups known as a) clans.
b) spiral waves.
c) aggregates.
d) clusters.
e) swarms.
Stars are often born within groups known as
Answer: d

61. Question 11 Stars are often born within groups known as a) clans.
b) spiral waves.
c) aggregates.
d) clusters.
e) swarms.
Stars are often born within groups known as
The Pleiades – a nearby open cluster – is a group of relatively young stars about 400 light-years from the Sun.

62. Question 12 Globular clusters are typically observed
a) in the plane of our Galaxy.
b) above or below the plane of our Galaxy.
c) near to our Sun.
d) in the hearts of other galaxies.
Globular clusters are typically observed
Answer: b

63. Question 12 Globular clusters are typically observed
a) in the plane of our Galaxy.
b) above or below the plane of our Galaxy.
c) near to our Sun.
d) in the hearts of other galaxies.
Globular clusters are typically observed
Globular clusters orbit the center of the Milky Way, and are usually seen above or below the galactic plane far from our Sun.

64. Summary of Chapter 11 Interstellar medium is made of gas and dust.
Emission nebulae are hot, glowing gas associated with the formation of large stars.
Dark dust clouds, especially molecular clouds, are very cold. They may seed the beginnings of star formation.
Dark clouds can be studied using the 21-cm emission line of molecular hydrogen.
Star formation begins with fragmenting, collapsing cloud of dust and gas.

65. Summary of Chapter 11, cont.
The cloud fragment collapses due to its own gravity, and its temperature and luminosity increase. When the core is sufficiently hot, fusion begins.
Collapsing cloud fragments and protostars have been observed.
Mass determines where a star falls on the main sequence.
One cloud typically forms many stars, as a star cluster.