ASTR 1020 Stellar and Galactic Astronomy Professor Caillault Fall 2009 Semester Exam 1 Multiple Choice Answers (Each multiple choice question is worth 1.5 points) 1. The number of degrees in a full circle is (c) 360 2. An arcsecond is a measure of (d) angle. 3. How many arcseconds are there in a full circle? (d) 360 3600 4. The diameter of the hydrogen atom, 0.00000000011 meters can be written in scientific notation as (c) 1.1 10 10 m 5. 10 0 (10 to the power 0) is (b) 1 6. What is the approximate angular diameter of the Sun in arcseconds? (d) 1860 7. The average distance from Earth to the Sun, 149,600,000 km, can be written in scientific notation as (b) 1.496 10 8 km 8. Consider the angular diameter of the planet Saturn as viewed from Earth. Which of the following is a correct statement about this angular diameter? (c) This value changes. The angular diameter gets larger as the Earth s distance to Saturn becomes smaller, and smaller as the Earth s distance to Saturn becomes larger. 9. By what approximate factor in scientific notation is a human being (height about 2 m) larger than the nucleus of a hydrogen atom, or proton (diameter about 10 15 m)? (b) 2 10 15 10. One astronomical unit, or one AU is defined as the (b) mean distance between the Sun and Earth. 11. When observing planetary motions from Earth, the phrase retrograde motion refers to (d) a slow westward motion of the planet from night to night against the background stars. 12. Ptolemy's model for the solar system was (d) Earth-centered, with epicyclic planetary orbits.
13. In Ptolemy's description of the solar system, the deferent is (b) a circular path (around Earth) along which the center of a planet's epicycle moves. 14. The early Copernican system for planetary motions is (c) Sun-centered, with planets moving in perfect circles around the Sun. 15. When Mercury is at greatest elongation, what is the angle between the line from Earth to Mercury and the line from Mercury to the Sun? (b) 90 16. What is the angle between the line from Earth to Jupiter and the line from Earth to the Sun when Jupiter is at opposition? (c) 180 17. The synodic period of a planet is the time between (a) successive alignments of Sun, planet, and Earth (e.g., time between successive oppositions). 18. Which of the following statements correctly states the significance of Galileo's observation that Jupiter has satellites (moons)? (b) It showed that bodies can orbit an object other than Earth. 19. What did Galileo see when he observed Venus through his telescope? (d) Venus has phases like the Moon and has its largest angular diameter at crescent phase. 20. Kepler's first law states that the orbit of a planet about the Sun is (c) an ellipse with the Sun at one focus. 21. According to Kepler's second law, an object in an elliptical orbit around the Sun is traveling fastest when it is at what position? (c) perihelion, closest point to the Sun 22. Which of the following statements is true, according to Kepler's third law? (c) The larger the orbit, the longer it takes for the planet to complete one revolution. 23. According to Newton's first law, (d) if no force is acting on an object, then the object's speed and direction of travel will both be constant. 24. Newton's second law states that acceleration produced by a force is (d) inversely proportional to the mass on which the force acts. 25. Which of the following statements is a correct version of Newton's third law? (c) Whenever object A exerts a force on some other object B, B must exert a force of equal magnitude on A in the opposite direction.
26. The phenomenon of parallax is the (b) change in apparent position of a nearby object as the observer moves. 27. If the mass of the Sun were doubled, the gravitational force on Jupiter due to the Sun would be (b) twice its present value. 28. If the mass of the Sun were increased by a factor of four, the period of Jupiter s orbit around the Sun would be (a) half its present value. 29. Does the Sun or the Moon have a greater effect in causing the tides on Earth? (d) The Moon, because it has a larger difference between pulling on the near side and pulling on the far side of Earth. 30. When do spring tides occur? (d) whenever Earth, the Moon, and the Sun form a straight line, regardless of which side of Earth the Moon is on 31. Radio waves travel through space at what speed? (d) at the speed of light, 3 10 8 m/s 32. In 1675, Rømer measured the speed of light by (a) timing eclipses of Jupiter's satellites, which appeared to occur later when Earth was farther from Jupiter. 33. Around 1670, Isaac Newton performed a crucial experiment on the nature of light when he (d) demonstrated that the colors that make up white light are intrinsic, not produced by the glass through which the light passes. 34. Around 1801, Thomas Young in England showed that light behaves as a wave by (b) shining light through two closely spaced slits and observing the resulting pattern of light on a white screen. 35. Visible light occupies which position in the whole electromagnetic spectrum? (c) between infrared and ultraviolet 36. As a newly formed star continues to contract, its temperature increases while the chemical nature of the gas does not change. What happens to the peak wavelength of its emitted radiation? (a) It moves toward shorter wavelengths (e.g., IR to visible). 37. The range of temperatures in the Kelvin (absolute) scale between the freezing point and boiling point of water is (b) 100 degrees.
38. A blackbody is an idealized object that (d) reflects no light and emits light in a manner determined by its temperature. 39. Suppose we compare the energy flux emitted from two heated spheres. Both are at the same temperature, but one sphere has twice the radius of the other. The flux emitted from the larger sphere is (b) the same as the flux emitted from the smaller sphere. 40. In the revolution that overtook physics around 1900, the assumption that Max Planck made to solve the problem concerning the spectrum of radiation emitted by a hot blackbody was that (c) all radiation is emitted in small, discrete packets, or quanta, of energy, each quantum having an energy that is inversely proportional to the wavelength of the light. 41. The spectrum of sunlight, when spread out by a spectrograph, has what characteristic appearance? (c) continuous band of color, crossed by innumerable dark absorption lines 42. The physical force that holds the components of an atom together is the (d) electromagnetic attraction between the nucleus and the electrons. 43. Why is the sky blue? (d) The air molecules scatter blue light better than red light, so more blue light reaches our eyes. 44. The atomic number that designates the position of an element in the periodic table is equal to the (b) number of protons in the nucleus of the atom. 45. One neutral atom of 13 C has how many particles? (b) 19: 6 protons, 7 neutrons, 6 electrons 46. Consider an atom in which electron transitions can involve three energy levels: highest energy, middle, and lowest energy. In the resulting emission line spectrum the line with the shortest wavelength is produced by the transition from (b) highest to lowest. 47. If light from a hot, dense star is viewed through a cool cloud of gas (b) only specific wavelengths of light will be removed from the spectrum. 48. If two photons in a vacuum have different energies, what can we say about the wavelengths of these photons? (a) The higher-energy photon has the shorter wavelength.
49. An electron is in the n = 3 energy level in a hydrogen atom. What can you say about the spectral series in which it can participate? (d) If it gains energy it can participate in the Paschen Series; if it loses energy it can participate in the Lyman Series or the Balmer Series. 50. The spectrum of a star shows an equivalent set of dark absorption lines to those of the Sun, but with one exception: Every line appears at a slightly longer wavelength, shifted toward the red end of the spectrum. What conclusion can be drawn from this observation? (b) The star is moving away from Earth. ASTR 1020 Stellar and Galactic Astronomy Professor Caillault Fall 2009 Semester Exam 1 Problems Solutions (Each problem is worth 5 points; show all of your work!) 1. The Sun subtends about ½ (= 1800 arcseconds) and is about 1 AU from Earth. What is its approximate diameter in km? D = (αd)/(206,265) = (1800 1.496 10 8 km)/(206,265) = 1.3 10 6 km Suppose a newly discovered asteroid is in a circular orbit with a synodic period of 1.25 years. The asteroid lies between the orbits of Mars and Jupiter. 2. What is the asteroid's sidereal period? 1/P = 1/E 1/S = 1/1 1/1.25 = 1 0.8 = 0.2 P = 1/0.2 = 5 years 3. What is the distance of the asteroid from the Sun? P 2 = a 3 a = 3 (P 2 ) = 3 (5 2 ) = 2.9 AU
4. The human eye has evolved over time so that its peak wavelength sensitivity is about 0.5 µm (1 µm = 10 6 m). Use Wien's law to calculate the temperature of blackbody radiation to which the eye is most sensitive. λ max = (0.0029 K m)/t T = (0.0029 K m)/λ max = (0.0029 K m)/(0.5 10-6 m) = 5800 K 5. Hydrogen gas emits a strong spectral line of red light with a wavelength of 656.281 nm (the Hα or Balmer-α line). This emission line is seen in the spectrum of a distant quasar but at a wavelength of 721.918 nm. Applying Doppler's relation, how fast is this object moving with respect to Earth, in terms of the velocity of light, c? Is it moving towards us or away from us? Δλ/λ lab = (λ obs λ lab )/λ lab = (721.918 656.281)/(656.281) = 0.100 = ν/c ν = 0.100 c, away from us, since ν is positive resulting in a redshift.