Chapter 20 AND 21 PDF

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CHAPTER 20 AND 21 1Q) The average density of the interstellar medium is: ANS: much less dense than the best vacuum on Earth.

2Q) The dust in the interstellar medium comes primarily from ANS: the stellar winds of main-sequence stars

3Q) . The coldest molecular clouds in our galaxy have temperatures of approximately ANS: 10 K.

4. If you could watch stars forming out of a gas cloud, which stars would form first? ANS: high-mass stars

5. When looking at the space between stars, what might you see? ANS: gas

6.If you wanted to observe heavy elements in the interstellar medium, where would be the best place to look? ANS

7. When radiation from an object passes through the interstellar medium, ANS: the object appears redder and dimmer.

8. Dust in the ISM appears dark in _________ wavelengths and bright in _________ wavelengths. ANS: visible; infrared

9. Dust reddens starlight because it ANS: preferentially affects light at visible and shorter wavelengths.

10. What is the most likely explanation for the dark area in the figure shown? ANS: It is a region with thick dust blocking the starlight coming from behind.

11. Sitting in a 100°F hot tub feels much hotter than standing outside on a 100°F day. This analogy illustrates ANS: an astronaut would feel cold standing in the 106 K intercloud gas

12. Warm ionized gas in the interstellar medium appears _________ when imaged in the optical region of the electromagnetic spectrum. ANS: red

13. An H II region signals the presence of ANS: all of the above

14. Interstellar clouds are ANS: regions where hydrogen tends to be denser than the surrounding gas.

15. What primarily makes it difficult to observe the process of star formation? ANS:

_ 16. A typical molecular cloud has a temperature of approximately ANS: 10 K.

17. Molecular cloud cores are places where you might find ANS: all of the above

18. For an object in hydrostatic equilibrium, if the temperature inside the object were to increase, the object would ANS: DECRESE

19. Because angular momentum must be conserved, as a gas cloud contracts due to gravity it will also ANS: increase in temperature

20. Stars forming in molecular clouds tend to form first in

ANS:

21. Of the following processes at work in molecular clouds, which is the one that inevitably dominates the clouds’ evolution? ANS: gravity

22. Magnetic fields inside a molecular cloud act to ANS:

23. 23. The entire process of star formation is really just an evolving balance between ANS: pressure and gravity

24. Which of the following traits does not help slow or prevent the collapse of a gas cloud? ANS: high mass

25. An accretion disk forms around a collapsing protostar because infalling material must conserve ANS: angular momentum.

26. As a protostar evolves, its temperature ANS: increases due to the kinetic energy of infalling material.

27. The source of energy for a contracting protostar comes from ANS: gravitational potential energy

28. What happens as a protostar contracts? ANS: All of the above are true

29. What critical event transforms a protostar into a normal main-sequence star? ANS: Nuclear fusion begins in the core

30. Stars with a mass from 0.01 M  to 0.08 M are very different from the Sun because they

ANS: . cannot successfully execute the proton-proton chain reactions.

31. A _________ is a failed star that shines primarily because of energy derived from its gravitational collapse rather than nuclear burning. ANS: brown dwarf

32. Brown dwarfs are considered failed stars because ANS: hydrogen fusion never begins in their cores.

33. A protostar’s evolutionary “track” in the H-R diagram traces out ANS: how the protostar’s luminosity, temperature, and radius change with time

34. Use the figure shown to complete the following statement. A high-mass protostar remains roughly constant in _________ and increases in _________ as it follows its evolutionary track. ANS:

35. Which of the following stars spend the longest time on their Hayashi tracks? ANS:

CHAPTER 16.1 AND 16.2 1. Hydrostatic equilibrium is a balance between ANS: pressure and gravity.

2.Where does hydrostatic equilibrium exist in the Sun? ANS: throughout the Sun

3. Density, temperature, and pressure increase as you move inward in the interior of the Sun. This means that the weight of the star pushing inward at a given radius _________ as you move inward toward the core. ANS: increases 4. The balance of energy in the solar interior means that ANS: energy production rate in the core equals the rate of radiation escaping the Sun’s surface.

5. The majority of the Sun’s energy comes from ANS: HYDROGEN FUSION.

6. What is the approximate temperature at the center of the Sun? ANS: 1.5  10^7 K

7. The net result of the proton-proton chain of nuclear reactions is that four protons are converted into ANS: one helium nucleus, as well as energy, positrons, and neutrinos.

8. What do astronomers mean when they say that the Sun makes energy by hydrogen burning? ANS: The Sun is fusing hydrogen into helium and releasing energy.

9. When two atomic nuclei come together to form a new species of atom, this is called ANS:

nuclear fusion

10. Suppose by some mysterious process that the nuclear fusion rate in the core of the Sun were to increase. What would happen to the appearance of the Sun? ANS: It would grow larger and hotter, making it more luminous

11. . If the core of the Sun were hotter than it is now, how would the Sun’s energy production change? ANS: It would produce more energy per second than it does now.

CHAPTER 17

1. Stars with a larger brightness must be ANS:

3.Star A and star B appear equally bright, but star A is twice as far away from us as star B. Which of the following is true? ANS: Star A is four times as luminous as star B

4. Two main-sequence stars have the same temperature. If star A is four times brighter than star B, then ANS: star B is two times farther away than star A

5. What is the difference between brightness and luminosity? ANS: . Brightness is how much light we see from a star; luminosity is how much light it emits

6. Star A is a red star. Star B is a blue star. You are able to determine that both stars are the same size. Which star is brighter? ANS: We also need to know the distance of the stars to determine their brightness.

7. Star A and star B both have the same temperature but different sizes and distances. As a result, star A is more luminous than star B, but star B is brighter than star A. Which of these statements about the absolute and apparent magnitudes of the two stars is correct? ANS: Star B has a larger apparent magnitude, while star A has a larger absolute magnitude.

8. You observe two stars in a visual binary system using a blue filter that is centered at a wavelength of 550 nm and a red filter that is centered at a wavelength of 650 nm. Star A has a temperature of 10,000 K, while star B has a temperature of 4000 K, and you know that both stars are the same size. Which star will be the brightest in each filter? ANS:

9. Star A is a red star. Star B is a blue star. Which star is hotter? ANS: Star B is hotter.

10. Star A is a red star. Star B is a blue star. You are able to determine that both stars are the same size. Which star is more luminous?

ANS: We also need to know the distance of the stars to determine their luminosity

11. What type of spectrum do most stars produce? ANS: an absorption spectrum on top of a blackbody spectrum

12. Which sequence correctly lists the spectral classes of stars in order from hottest to coolest? ANS:

OBAFGKM

13. The spectral class of a star is related to its ANS: temperature

14. What spectral class is the Sun? ANS: G2

15. Two stars with similar temperatures but different sizes will have ANS: similar spectral types but different luminosities.

16. Why do O- and B-type stars have weaker hydrogen absorption lines than A-type stars? ANS:

17. . If we know the temperature and luminosity of a star, we can also calculate its ANS: radius

18. Star C is a red star. Star D is a blue star. Which has a larger radius? ANS:

19. Star E is the same temperature as star F, but star E is four times as luminous as star F. How do the radii of the stars compare? ANS: The radius of star E is twice that of star F.

CHAPTER 18 AND 19 1. What advantage do you gain by having two eyes that are separated on your face, rather than being very close together? ANS: stereoscopic vision, which allows you to determine distances

2. To measure the parallax of the most distant stars measurable, we would make two measurements of the star’s position on the sky separated by ANS: 6 months.

3. Parallax is used to measure a star’s ANS: distance

4.How is the distance to a star related to its parallax? ANS: . Distance is inversely proportional to parallax.

5. A parsec is a measure of ANS: distance.

6. Stars with a larger brightness must be ANS:

7. . The fraction of the Sun’s mass that is made of heavy elements is Ans: 2 percent.

8. Which stars are the most common? ANS: Stars with a smaller mass and radius than the Sun’s are most common.

9. The faster-moving star in a binary is the

ANS: less massive star

10. Which of the following properties are NOT useful in determining the masses of stars in a typical binary system? ANS:

11. Binary star systems are extremely useful in studying stars because they allow us to determine ANS:

12. Astronomers can measure the speed of the stars in a binary system by measuring the _________ of the stars. ANS: spectra

13For which type of binary system are astronomers able to resolve each of the two stars individually? ANS: visual binary

14. Eclipsing binary systems ANS: . contain stars that pass in front of one another during their orbit

15. Main-sequence stars range in mass from approximately ANS: 0.08 to 150 M

16. The Hertzsprung-Russell diagram is a graph of _________ for stars. ANS: luminosity versus temperature

17. Any of the following properties could be plotted on the horizontal axis of an H-R diagram except for: ANS:

18. The figure shows an H-R diagram, with five stars labeled A through E. Which star has the highest temperature? ANS: A

19.The figure shows an H-R diagram, with five stars labeled A through E. Which star has the highest luminosity? ANS: B

20. The figure shows an H-R diagram, with five stars labeled A through E. Which star has the smallest radius? ANS: D

21. On a typical H-R diagram, where are the stars with the largest radii located? ANS: in the upper right corner

22. What type of star is most common in the solar neighborhood? ANS: main-sequence

23. Roughly what percentage of stars in our galaxy are main-sequence stars? ANS: 90 percent

24. A star’s position in the H-R diagram is determined by its ANS:

25. A star’s location on the main sequence is determined entirely by its ANS: mass

26. The stars that have the largest radii are classified as ANS: red supergiants.

27. The figure shows an H-R diagram, with five stars labeled A through E. Which of the mainsequence stars has the smallest mass? ANS: E

28. The one property of a main-sequence star that determines all its other properties is its

ANS: mass. 29. The stars that have the largest radii are classified as ANS: SUPERGAINTS

30.The brightest stars in the sky also tend to be ANS:very luminous

SHORT ANSWER QUESTION 1. How do astronomers determine the physical properties of stars? What do astronomers interpret from the physical properties of stars?

ANS: We can determine an amazing number of physical properties of stars, usually based only on a little bit of light. Over the next several lectures, "we" will talk about how the following properties are determined for stars: 

 

  

DISTANCE. This is determined from trigonometric and spectroscopic parallaxes. Determining distances is CRUCIAL to understanding stars because we can use distances to figure out o the scale of things in the Galaxy and o how much energy stars produce and radiate away by using the inverse square law for light dimming along with apparent brightnesses. LUMINOSITY. This is the amount of energy generated in the star and released as electromagnetic radiation. BRIGHTNESS. This is not a fundamental property but a combination of the luminosity and distance to a star (and in some cases it is also dependent on the amount of absorption in the direction of a star). RADIUS. "Size" of the star - calculated from Stephan's Law. CHEMICAL COMPOSITION. This is determined from absorption line spectra; it is tied up in a semi-complicated way with temperature. TEMPERATURE. We have talked about Wien's Law and using colors to derive stellar temperatures, but there are some complications. To REALLY get to surface temperatures of stars, we need to learn about and understand stellar spectral types.

2. Show by a simple diagram the relationship between a star's distance and its parallax, noting the limitations imposed by the size of the earth-sun distance. ANS:

There is a simple relationship between a star's distance and its parallax angle: d = 1/p The distance d is measured in parsecs and the parallax angle p is measured in arcseconds.

3. Explain the difference between apparent brightness and actual brightness. ANS: A star's apparent brightness is its brightness seen from Earth. A star's absolute brightness is the brightness the star would have if it were at a standard distance from Earth. 4. Describe the properties of an average star and describe why the apparently bright stars in our sky are not typical stars ANS: 5. How do astronomers determine the temperatures of stars? ANS: Astronomers group stars into general types based on their temperature. Temperature is chosen because the color of a star depends on the temperature and color is a easily seen characteristic, regardless of the distance Wien's Law is the mathematical relationship between the dominant wavelength & the temperature of a blackbody. You can use it to determine the temperature of a star's surface by finding the peak wavelength of its' electromagnetic radiation because it is inversely proportionate to its temperature.

6. What methods do astronomers use to classify stars?

ANS: Characteristics used to classify stars include color, temperature, size, composition, and brightness. The Hertzsprung-Russell Diagram is a graphical tool that astronomers use to classify stars according to their luminosity, spectral type, color, temperature and evolutionary stage.

7. Describe the three types of binary stars according to the type of observations possible for each type.

ANS: As has already been mentioned, binary stars are generally classified according to their method of detection. These types are discussed in detail below:  



Visual binaries-INDIVIDUALLY RESOLVED THROUGH A TELESCOPE Spectroscopic binaries-If a binary system is unresolved into its components then the spectrum

obtained from it will actually be a combination of the spectra from each of the component stars. As these stars orbit each other one star, A, may be moving towards us whilst the other, B, may be moving away. The spectrum from A will therefore be blue-shifted to higher frequencies (shorter wavelengths) whilst B's spectrum will be redshifted. Eclipsing binaries-The third method of detecting a binary system depends upon photometric measurement.

8. Explain the importance of the study of binary stars in determining the fundamental parameters of stars. ANS: Binary

star systems provide the best means for scientists to determine the mass of a star. As the pair pulls on each other, astronomers can calculate the size, and from there determine characteristics such as temperature and radius. These factors help characterize single main sequence stars in the universe.

9. Show how the Hertzsprung-Russell diagram for many stars provides clues about the evolution of individual stars. ANS: The Hertzsprung-Russell diagram (HR diagram) is one of the most important tools in the study of stellar evolution. Developed independently in the early 1900s by Ejnar Hertzsprung and Henry Norris Russell, it plots the temperature of stars against their luminosity (the theoretical HR diagram), or the colour of stars (or spectral type) against their absolute magnitude (the observational HR diagram, also known as a colour-magnitude diagram). Depending on its initial mass, every star goes through specific evolutionary stages dictated by its internal structure and how it produces energy. Each of these stages corresponds to a change in the temperature and luminosity of the star, which can be seen to move to different regions on the HR diagram as it evolves. This reveals the true power of the HR diagram – astronomers can know a star’s internal structure and evolutionary stage simply by determining its position in the diagram.

10. Sketch a Hertzsprung-Russell diagram for stars, indicating the positions of sun, the main sequence, red giants, and white dwarfs ANS:

11. Use the Hertzsprung-Russell diagram to infer the relative luminosities, surface temperatures, and sizes of stars represented on it. ANS:

CHAPTER 20 AND 21...