Questions

Review Questions

1. What is the difference between a hypothesis and a theory?

2. What is the difference between a theory and a law of physics?

3. How are scientific theories tested?

4. Describe the role that skepticism plays in science.

5. Describe one reason why it is useful to have telescopes in space.

6. What caused the craters on the Moon?

7. What are meteorites? Why are they important for understanding the history of the solar system?

8. What makes the Sun and stars shine?

9. What role do nebulae like the Orion Nebula play in the life stories of stars?

10. What is the difference between a solar system and a galaxy?

11. What are degrees, arcminutes, and arcseconds used for? What are the relationships among these units of measure?

12. How many arcseconds equal 1°?

13. With the aid of a diagram, explain what it means to say that the Moon subtends an angle of ½°.

14. What is an exponent? How are exponents used in powers-of-ten notation?

15. What are the advantages of using powers-of-ten notation?

16. Write the following numbers using powers-of-ten notation: (a) ten million, (b) sixty thousand, (c) four one-thousandths, (d) thirty-eight billion, (e) your age in months.

17. How is an astronomical unit (AU) defined? Give an example of a situation in which this unit of measure would be convenient to use.

18. What is the advantage to the astronomer of using the light-year as a unit of distance?

19. What is a parsec? How is it related to a kiloparsec and to a megaparsec?

20. Give the word or phrase that corresponds to the following standard abbreviations: (a) km, (b) cm, (c) s, (d) km/s, (e) mi/h, (f) m, (g) m/s, (h) h, (i) y, (j) g, (k) kg. Which of these are units of speed? (Hint: You may have to refer to a dictionary. All of these abbreviations should be part of your working vocabulary.)

21. In the original (1977) Star Wars movie, Han Solo praises the speed of his spaceship by saying, “It’s the ship that made the Kessel run in less than 12 parsecs!” Explain why this statement is obvious misinformation.

22. A reporter once described a light-year as “the time it takes light to reach us traveling at the speed of light.” How would you correct this statement?

Advanced Questions

Questions preceded by an asterisk (*) are discussed in the Boxes.

23. What is the meaning of the letters R I V U X G that appear with some of the figures in this chapter? Why in each case is one of the letters highlighted? (Hint: See the Preface that precedes Chapter 1.)

24. The diameter of the Sun is 1.4 × 10¹¹ cm, and the distance to the nearest star, Proxima Centauri, is 4.2 ly. Suppose you want to build an exact scale model of the Sun and Proxima Centauri, and you are using a ball 30 cm in diameter to represent the Sun. In your scale model, how far away would Proxima Centauri be from the Sun? Give your answer in kilometers, using powers-of-ten notation.

25. How many Suns would it take, laid side by side, to reach the nearest star? Use powers-of-ten notation. (Hint: See the preceding question.)

26. A hydrogen atom has a radius of about 5 × 10^-9 cm. The radius of the observable universe is about 14 billion light-years. How many times larger than a hydrogen atom is the observable universe? Use powers-of-ten notation.

27. The Sun’s mass is 1.99 × 10³⁰ kg, three-quarters of which is hydrogen. The mass of a hydrogen atom is 1.67 × 10^-27 kg. How many hydrogen atoms does the Sun contain? Use powers-of-ten notation.

28. The average distance from Earth to the Sun is 1.496 × 10⁸ km. Express this distance (a) in light-years and (b) in parsecs. Use powers-of-ten notation. (c) Are light-years or parsecs useful units for describing distances of this size? Explain.

29. The speed of light is 3.00 × 10⁸ m/s. How long does it take light to travel from the Sun to Earth? Give your answer in seconds, using powers-of-ten notation. (Hint: See the preceding question.)

30. When the Voyager 2 spacecraft sent back pictures of Neptune during its flyby of that planet in 1989, the spacecraft’s radio signals traveled for 4 hours at the speed of light to reach Earth. How far away was the spacecraft? Give your answer in kilometers, using powers-of-ten notation. (Hint: See the preceding question.)

31. The star Altair is 5.15 pc from Earth. (a) What is the distance to Altair in kilometers? Use powers-of-ten notation. (b) How long does it take for light emanating from Altair to reach Earth? Give your answer in years. (Hint: You do not need to know the value of the speed of light.)

32. The age of the universe is about 13.7 billion years. What is this age in seconds? Use powers-of-ten notation.

33. *Explain where the number 206,265 in the small-angle formula comes from.

34. *At what distance would a person have to hold a European 2-euro coin (which has a diameter of about 2.6 cm) in order for the coin to subtend an angle of (a) 1°? (b) 1 arcmin? (c) 1 arcsec? Give your answers in meters.

35. *A person with good vision can see details that subtend an angle of as small as 1 arcminute. If two dark lines on an eye chart are 2 millimeters apart, how far can such a person be from the chart and still be able to tell that there are two distinct lines? Give your answer in meters.

36. *The average distance to the Moon is 384,000 km, and the Moon subtends an angle of ½°. Use this information to calculate the diameter of the Moon in kilometers.

37. *Suppose your telescope can give you a clear view of objects and features that subtend angles of at least 2 arcsec. What is the diameter in kilometers of the smallest crater you can see on the Moon? (Hint: See the preceding question.)

38. *On April 18, 2006, the planet Venus was a distance of 0.869 AU from Earth. The diameter of Venus is 12,104 km. What was the angular size of Venus as seen from Earth on April 18, 2006? Give your answer in arcminutes.

39. *(a) Use the information given in the caption to Figure 1-7 to determine the angular size of the Orion Nebula. Give your answer in degrees. (b) How does the angular diameter of the Orion Nebula compare to the angular diameter of the Moon?

Discussion Questions

40. Scientists assume that “reality is rational.” Discuss what this means and the thinking behind it.

41. All scientific knowledge is inherently provisional. Discuss whether this is a weakness or a strength of the scientific method.

42. How do astronomical observations differ from those of other sciences?

Web/eBook Questions

43. Use the links given in the Universe Web site or eBook, Chapter 1, to learn about the Orion Nebula (Figure 1-7). Can the nebula be seen with the naked eye? Does the nebula stand alone, or is it part of a larger cloud of interstellar material? What has been learned by examining the Orion Nebula with telescopes sensitive to infrared light?

44. Use the links given in the Universe Web site or eBook, Chapter 1, to learn more about the Crab Nebula (Figure 1-8). When did observers on Earth see the supernova that created this nebula? Does the nebula emit any radiation other than visible light? What kind of object is at the center of the nebula?