• Home
  • Textbooks
  • Universe
  • The Birth of Stars

Universe

Roger Freedman, William J. Kaufmann

Chapter 18

The Birth of Stars - all with Video Answers

Educators


Chapter Questions

01:38

Problem 1

If no one has ever seen a star go through the complete formation process, how are we able to understand how stars form?

Farhanul Hasan
Farhanul Hasan
Numerade Educator
01:29

Problem 2

Why is it more difficult to observe the life cycles of stars than the life cycles of planets or animals?

Charles Machakwa
Charles Machakwa
Numerade Educator
01:58

Problem 3

If an interstellar medium fills the space between the stars, how is that we are able to see the stars at all?

Averell Hause
Averell Hause
Carnegie Mellon University
01:12

Problem 4

Summarize the evidence that interstellar space contains (a) gas and (b) dust.

Matthew Miranda
Matthew Miranda
Numerade Educator
00:30

Problem 5

What are H II regions? Near what kinds of stars are they found? Why do only these stars give rise to H II regions?

Donald Albin
Donald Albin
Numerade Educator
04:41

Problem 6

What are stationary absorption lines? In what sort of spectra are they seen? How do they give evidence for the existence of the interstellar medium?

Daniel Azubuike
Daniel Azubuike
Numerade Educator
00:39

Problem 7

In Figure 18-2, what makes the Horsehead Nebula dark? What makes IC 434 glow?

Zachary Warner
Zachary Warner
Numerade Educator
02:07

Problem 8

Why is the daytime sky blue? Why are distant mountains purple? Why is the Sun red when seen near the horizon at sunrise or sunset? In what ways are your answers analogous to the explanations for the bluish color of reflection nebulae and the process of interstellar reddening?

kj
Karl Jacob
Numerade Educator
00:42

Problem 9

To see the constellation Coma Berenices (Berenice's Hair) you must look perpendicular to the plane of the Milky Way. By contrast, the Milky Way passes through the constellation Cassiopeia (named for a mythical queen). Would you expect H II regions to be more abundant in Coma Berenices or in Cassiopeia? Explain your reasoning.

Zachary Warner
Zachary Warner
Numerade Educator
01:31

Problem 10

The interior of a dark nebula is billions of times less dense than the air that you breathe. How, then, are dark nebulae able to block out starlight?

Sarah Mccrumb
Sarah Mccrumb
Numerade Educator
00:13

Problem 11

Why are low temperatures necessary in order for protostars to form inside dark nebulae?

Dading Chen
Dading Chen
Numerade Educator
08:01

Problem 12

Compare and contrast Barnard objects and Bok globules. How many Sun-sized stars could you make out of a Barnard object? Out of a Bok globule?

Sarah Mccrumb
Sarah Mccrumb
Numerade Educator
00:43

Problem 13

Describe the energy source that causes a protostar to shine. How does this source differ from the energy source inside a main-sequence star?

Zachary Warner
Zachary Warner
Numerade Educator
01:42

Problem 14

What is an evolutionary track? How can evolutionary tracks help us interpret the H-R diagram?

Parvati Devi
Parvati Devi
Numerade Educator
05:07

Problem 15

What happens inside a protostar to slow and eventually halt its gravitational contraction?

Sarah Mccrumb
Sarah Mccrumb
Numerade Educator
02:14

Problem 16

Why are the evolutionary tracks of high-mass stars different from those of low-mass stars? For which kind of star is the evolution more rapid? Why?

Sarah Mccrumb
Sarah Mccrumb
Numerade Educator
04:41

Problem 17

Why are protostars more easily seen with an infrared telescope than with a visible-light telescope?

Sarah Mccrumb
Sarah Mccrumb
Numerade Educator
05:44

Problem 18

In what ways is the internal structure of a $1-\mathrm{M}_{\odot}$ mainsequence star different from that of a $5-\mathrm{M}_{\odot}$ main-sequence star? From that of a $0.5-\mathrm{M}_{\odot}$ main-sequence star? What features are common to all these stars?

Sarah Mccrumb
Sarah Mccrumb
Numerade Educator
02:08

Problem 19

What sets the limits on the maximum and minimum masses of a main-sequence star?

Rodger Claar
Rodger Claar
Numerade Educator
02:36

Problem 20

What are T Tauri stars? How do we know that they eject matter at high speed? How does their rate of mass loss compare to that of the Sun?

Sarah Mccrumb
Sarah Mccrumb
Numerade Educator
00:28

Problem 21

What are Herbig-Haro objects? Why are they often found in pairs?

Zachary Warner
Zachary Warner
Numerade Educator
03:09

Problem 22

Why do disks form around contracting protostars? What is the connection between disks and bipolar outflows?

Sarah Mccrumb
Sarah Mccrumb
Numerade Educator
02:14

Problem 23

Young open clusters like those shown in Figures 18-18 and 18-19 are found only in the plane of the Galaxy. Explain why this should be.

Sarah Mccrumb
Sarah Mccrumb
Numerade Educator
00:58

Problem 24

Why are observations at millimeter wavelengths so much more useful in exploring interstellar clouds than observations at visible wavelengths?

Zachary Warner
Zachary Warner
Numerade Educator
03:29

Problem 25

What are giant molecular clouds? What role do these clouds play in the birth of stars?

Sarah Mccrumb
Sarah Mccrumb
Numerade Educator
00:21

Problem 26

Giant molecular clouds are among the largest objects in our Galaxy. Why, then, were they discovered only relatively recently?

Dading Chen
Dading Chen
Numerade Educator
01:34

Problem 27

Consider the following stages in the evolution of a young star cluster: (i) $\mathrm{H}$ II region; (ii) dark nebula; (iii) formation of $\mathrm{O}$ and B stars; (iv) giant molecular cloud. Put these stages in the correct chronological order and discuss how they are related.

Zachary Warner
Zachary Warner
Numerade Educator
01:40

Problem 28

Briefly describe four mechanisms that compress the interstellar medium and trigger star formation.

Farhanul Hasan
Farhanul Hasan
Numerade Educator
02:00

Problem 29

If you looked at the spectrum of a reflection nebula, would you see absorption lines, emission lines, or no lines? Explain your answer. As part of your explanation, describe how the spectrum demonstrates that the light was reflected from nearby stars.

Joseph Fritchman
Joseph Fritchman
Numerade Educator
03:03

Problem 30

In the direction of a particular star cluster, interstellar extinction allows only $15 \%$ of a star's light to pass through each kiloparsec $(1000 \mathrm{pc})$ of the interstellar medium. If the star cluster is $3.0$ kiloparsecs away, what percentage of its photons survive the trip to the Earth?

Mohit Khurana
Mohit Khurana
Texas A&M University
01:33

Problem 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.

Zachary Warner
Zachary Warner
Numerade Educator
02:26

Problem 32

Find the density (in atoms per cubic centimeter) of a Bok globule having a radius of 1 light-year and a mass of $100 \mathrm{M}_{\odot}$. How does your result compare with the density of a typical H II region, between 80 and 600 atoms per $\mathrm{cm}^{3}$ ? (Assume that the globule is made purely of hydrogen atoms.)

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:13

Problem 33

The Becklin-Neugebauer object is a newly formed star within the Orion Nebula. It is substantially more luminous than the other newly formed stars in that nebula. Assuming that all these stars began the process of formation of the same time, what can you conclude about the mass of the BecklinNeugebauer object compared with those of the other newly formed stars? Does your conclusion depend on whether or not the stars have reached the main sequence? Explain your reasoning.

Zachary Warner
Zachary Warner
Numerade Educator
00:24

Problem 34

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.

Zachary Warner
Zachary Warner
Numerade Educator
00:56

Problem 35

At one stage during its birth, the protosun had a luminosity of $1000 \mathrm{~L}_{\odot}$ and a surface temperature of about $1000 \mathrm{~K}$. At this time, what was its radius? Express your answer in three ways: as a multiple of the Sun's present-day radius, in kilometers, and in astronomical units.

Farhanul Hasan
Farhanul Hasan
Numerade Educator
05:44

Problem 36

. A newly formed protostar and a red giant are both located in the same region on the H-R diagram. Explain how you could distinguish between these two.

Sarah Mccrumb
Sarah Mccrumb
Numerade Educator
03:54

Problem 37

(a) Determine the radius of the circumstellar accretion disk in Figure 18-15. (You will need to measure this image with a ruler. Note the scale bar in this figure.) Give your answer in astronomical units and in kilometers. (b) Assume that the young star at the center of this disk has a mass of $1 \mathrm{M}_{\odot}$. What is the orbital period (in years) of a particle at the outer edge of the disk? (c) Using your ruler again, determine the length of the jet that extends to the right of the circumstellar disk in Figure $18-15$. At a speed of $200 \mathrm{~km} / \mathrm{s}$, how long does it take gas to traverse the entire visible length of the jet?

Andrew Duncan
Andrew Duncan
Numerade Educator
02:34

Problem 38

The star cluster NGC 2264 (Figure 18-18) contains numerous T Tauri stars, while the Pleiades (Figure 18-19) contains none. Explain why there is a difference.

Farhanul Hasan
Farhanul Hasan
Numerade Educator
01:27

Problem 39

The concentration or abundance of ethyl alcohol in a typical molecular cloud is about 1 molecule per $10^{8}$ cubic meters. What volume of such a cloud would contain enough alcohol to make a martini (about 10 grams of alcohol)? A molecule of ethyl alcohol has 46 times the mass of a hydrogen atom (that is, ethyl alcohol has a molecular weight of 46).

David Collins
David Collins
Numerade Educator
03:25

Problem 40

From the information given in the caption to Figure 18-24, calculate the angular diameter in arcminutes of Cassiopeia A as seen from Earth.

Prakash Hampole
Prakash Hampole
Numerade Educator
00:39

Problem 41

From the information given in the caption for Figure 18-24, calculate the average speed at which the shock wave has spread away from the site of the supernova explosion. Give your answer in kilometers per second and as a fraction of the speed of light. (Hint: There are $3.16 \times 10^{7}$ seconds in a year and the speed of light is $3.00 \times 10^{5} \mathrm{~km} / \mathrm{s}$.)

Sakib Sarker
Sakib Sarker
Numerade Educator
03:09

Problem 42

Some science-fiction movies show stars suddenly becoming dramatically brighter when they are "born" (that is, when thermonuclear fusion reactions begin in their cores). Discuss whether this is a reasonable depiction.

Sarah Mccrumb
Sarah Mccrumb
Numerade Educator
04:52

Problem 43

Suppose that the electrons in hydrogen atoms were not as strongly attracted to the nuclei of those atoms, so that these atoms were easier to ionize. What consequences might this have for the internal structure of main-sequence stars? Explain your reasoning.

Sarah Mccrumb
Sarah Mccrumb
Numerade Educator
01:23

Problem 44

What do you think would happen if our solar system were to pass through a giant molecular cloud? Do you think the Earth has ever passed through such clouds?

Donald Albin
Donald Albin
Numerade Educator
02:08

Problem 45

Many of the molecules found in giant molecular clouds are organic molecules (that is, they contain carbon). Speculate about the possibility of life-forms and biological processes occurring in giant molecular clouds. In what ways might the conditions existing in giant molecular clouds favor or hinder biological evolution?

Christina Sorrentino
Christina Sorrentino
Numerade Educator
02:27

Problem 46

Speculate on why a shock wave from a supernova seems to produce relatively few high-mass $\mathrm{O}$ and $\mathrm{B}$ stars, compared to the lower-mass $A, F, G$, and $K$ stars.

Farhanul Hasan
Farhanul Hasan
Numerade Educator
02:14

Problem 47

In recent years astronomers have been able to learn about the character of the interstellar medium in the vicinity of the Sun. Search the World Wide Web for information about aspects of the nearby interstellar medium, including features called the Local Interstellar Cloud and the Local Bubble. How do astronomers study the nearby interstellar medium? What makes these studies difficult? Is the interstellar medium relatively uniform in our neighborhood, or is it clumpy? If the latter, is our solar system in a relatively thin or thick part of the interstellar medium? How is our solar system moving through the interstellar medium?

Rodger Claar
Rodger Claar
Numerade Educator
01:29

Problem 48

Search the World Wide Web for recent discoveries about how brown dwarfs form. Do they tend to form in the same locations as "real" stars? Do they form in relatively small or relatively large numbers compared to "real" stars? What techniques are used to make these discoveries?

Rodger Claar
Rodger Claar
Numerade Educator
02:06

Problem 49

Measuring a Stellar Jet. Access the animation "A Stellar Jet in the Trifid Nebula" in Chapter 18 of the Universe Web site or eBook. (a) The Trifid Nebula as a whole has an anglar diameter of 28 arcmin. By stepping through the animation, estimate the angular size of the stellar jet shown at the end of the animation. (b) The Trifid Nebula is about $2800 \mathrm{pc}(9000 \mathrm{ly})$ from Earth. Estimate the length of the jet in light-years. (c) If gas in the jet travels at $200 \mathrm{~km} / \mathrm{s}$, how long does it take to traverse the length of the jet? Give your answer in years. (Hint: There are $3.16 \times 10^{7}$ seconds in a year.)

Sarah Mccrumb
Sarah Mccrumb
Numerade Educator
09:15

Problem 50

Use a telescope to observe at least two of the H II regions listed in the following table. In each case, can you guess which stars are probably responsible for the ionizing radiation that causes the nebula to glow? Can you see any obscuration or silhouetted features that suggest the presence of interstellar dust? Draw a picture of what you see through the telescope and compare it with a photograph of the object. Take note of which portions of the nebula were not visible through your telescope.

Khoobchandra Agrawal
Khoobchandra Agrawal
Numerade Educator
02:17

Problem 51

On an exceptionally clear, moonless night, use a telescope to observe at least one of the dark nebulae listed in the following table. These nebulae are very difficult to find, because they are recognizable only by the absence of stars in an otherwise starry part of the sky. Are you confident that you actually saw the dark nebula? Does the pattern of background stars suggest a particular shape to the nebula?

Zachary Warner
Zachary Warner
Numerade Educator
View

Problem 52

A few fine objects cover such large regions of the sky that they are best seen with binoculars. If you have access to a high-quality pair of binoculars, observe the North American Nebula in Cygnus and the Pipe Nebula in Ophiuchus. Both nebulae are quite faint, so you should attempt to observe them only on an exceptionally dark, clear, moonless night. The North America Nebula is a cloud of glowing hydrogen gas located about $3^{\circ}$ east of Deneb, the brightest star in Cygnus. While searching for the North America Nebula, you may glimpse another diffuse H II region, the Pelican Nebula, located about $2^{\circ}$ southeast of Deneb. The Pipe Nebula is a $7^{\circ}$-long, meandering, dark nebula to the south and to the east of the star $\theta$ (theta) Ophiuchi, which is in a section of Ophiuchus that extends southward between the constellations of Scorpius and Sagittarius. Located about $12^{\circ}$ east of the bright red star Antares, you can locate $\theta$ Ophiuchi using the Starry Night Enthusiast ${ }^{\mathrm{TM}}$ software on the CD-ROM that accompanies certain printed copies of this book.

Lainey Roebuck
Lainey Roebuck
Numerade Educator
08:27

Problem 53

Use the Starry Night Enthusiast ${ }^{\mathrm{TM}}$ program to investigate a star-forming region. Use the Find ... command in the Edit menu to find and center on M20 (the Trifid Nebula, shown in the figure that accompanies Question 31) as seen from your location. Zoom out as far as possible using the Zoom controls at the righthand end of the toolbar. Set the Time appropriately and adjust the Month and Day in the Date to answer the following questions. (Hint: You may want to remove daylight and display the local meridian to provide precise answers.) (a) On what day is M20 highest in the sky at noon? Explain how you determined this. (b) On what day is M20 highest in the sky at midnight, so that it is best placed for observing with a telescope? Explain how you determined this.

Ravindra Yadav
Ravindra Yadav
Numerade Educator
04:18

Problem 54

Use the Starry Night Enthusiast ${ }^{\mathrm{TM}}$ program to examine the Milky Way Galaxy. Open the Favourites pane and click on Stars $>$ Sun in Milky Way to display our Galaxy from a position $0.150$ million light-years above the galactic plane. (You can remove the astronaut's feet from this view if desired by clicking on View $>$ Feet.) You can zoom in or out on the Galaxy using the + and - buttons at the upper right end of the toolbar. You can move the Galaxy by holding down the mouse button while moving the mouse. You can also rotate the Galaxy by putting the mouse cursor over the image and holding down the Shift key while holding down the mouse button and moving the mouse. (a) You can identify H II regions by their characteristic magenta color. Describe where in the Galaxy you find these. Are most found in the inner part of the Galaxy or in its outer regions? (b) Where do you find dark lanes of dustin the inner part of the Galaxy or in its outer regions? Do you see any connection between the locations of dust and of H II regions? If there is a connection, what do you think causes it? If there is not a connection, why is this the case? You can examine the location of Galaxy in relation to neighboring galaxies by turning the Milky Way edge-on and by increasing the distance from the Earth using the up key below the Viewing Location on the toolbar.

Rashmi Sinha
Rashmi Sinha
Numerade Educator
01:38

Problem 55

Imagine that your group walks into a store that specializes in selling antique clothing. Prepare a list of observable characteristics that you would look for to distinguish which items were from the early, middle, and late twentieth century. Also, write a paragraph that specifically describes how this task is similar to how astronomers understand the evolution of stars.

Farhanul Hasan
Farhanul Hasan
Numerade Educator
03:43

Problem 56

Consider advertisement signs visible at night in your community and provide specific examples of ones that are examples of the three different types of nebulae that astronomers observe and study. If an example doesn't exist in your community, creatively design an advertisement sign that could serve as an example.

Donald Albin
Donald Albin
Numerade Educator
04:53

Problem 57

The pre-main-sequence evolutionary tracks shown in Figure 18-10 describe the tracks of seven protostars of different masses. Imagine a new sort of $\mathrm{H}-\mathrm{R}$ diagram that plots a human male's increasing age versus decreasing hair density on the head instead of increasing luminosity versus decreasing temperature. Create and carefully label a sketch of this imaginary HaiR diagram showing both the majority of the U.S. male population and a few oddities. Finally, draw a line that clearly labels your sketch to show how a typical male undergoing male-pattern baldness might slowly change position on the HaiR diagram over the course of a human life span.

Bryan Lynn
Bryan Lynn
Numerade Educator