ASTR 1010 Astronomy of the Solar System Professor Caillault Fall 2009 Semester Exam 2 Answers

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1 ASTR 1010 Astronomy of the Solar System Professor Caillault Fall 2009 Semester Exam 2 Answers 1. Radio waves travel through space at what speed? (d) at the speed of light, m/s 2. 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. 3. 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. 4. 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. 5. Visible light occupies which position in the whole electromagnetic spectrum? (c) between infrared and ultraviolet 6. 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). 7. The range of temperatures in the Kelvin (absolute) scale between the freezing point and boiling point of water is (b) 100 degrees. 8. A blackbody is an idealized object that (d) reflects no light and emits light in a manner determined by its temperature. 9. 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. 10. 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.

2 11. The physical force that holds the components of an atom together is the (d) electromagnetic attraction between the nucleus and the electrons. 12. 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 13. Why is the sky blue? (d) The air molecules scatter blue light better than red light, so more blue light reaches our eyes. 14. 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. 15. One neutral atom of 13 C has how many particles? (b) 19: 6 protons, 7 neutrons, 6 electrons 16. 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. 17. 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. 18. 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. 19. 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. 20. 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. 21. Light enters the smooth, flat surface of a glass from the vacuum of space (e.g., a spacecraft window). What is the speed of light inside the glass compared to that in a vacuum? (b) Since the glass is denser than the vacuum, it is slower.

3 22. What is the refraction of light? (d) The change in direction of a light ray as it crosses from a less dense, transparent material to a more dense one 23. When a light ray in air or a vacuum enters the surface of a piece of perfectly smooth, flat glass at an angle, it (c) bends toward the perpendicular to the surface. 24. A typical refracting telescope is made up of (a) a long-focal-length lens at the front and a short-focal-length lens at the rear (next to your eye as you look through the telescope). 25. For many years, the Palomar telescope (5 m diameter) in California was the largest fully steerable telescope in the world; in the 1990 s that honor fell to the first of the two Keck telescopes (each of diameter 10 m) in Hawaii. How many times larger is the lightgathering power of the Keck telescope than the Palomar telescope? (c) 4 times larger 26. A refracting telescope has an objective lens of focal length 80 cm, a diameter of 10 cm, and an eyepiece of focal length 5 cm and diameter 1 cm. What is the magnifying power of this telescope? (c) When light from the concave primary mirror of a telescope is reflected by a small secondary mirror through a hole in the primary, it is called a (c) Cassegrain focus telescope. 28. The prime focus cage of a telescope whose primary mirror is 3 m in diameter has a diameter of 0.5 meters. What fraction of the incoming light is obstructed by this cage? (a) about 3% 29. What is chromatic aberration in a telescope? (b) The light of different colors comes to a focus at different points inside the telescope. 30. The largest refracting telescope in the world is the 102-cm (40 in.) diameter telescope at Yerkes Observatory, built in Refracting telescopes with larger diameter have never been built because they would (a) sag too much under their own weight. 31. In the reflection of a beam of light from a flat surface, the relationship between the angle of incidence i between the perpendicular to the surface and the incident beam and the angle of reflection r between the perpendicular and the reflected beam is (b) r is equal to i.

4 32. A reflecting telescope in which light is reflected by one curved mirror and a second plane mirror at 45 to the original beam, to reach a focus at the side of the telescope, is being used at its (a) Newtonian focus. 33. To produce the sharpest images of very distant objects, the best shape for the crosssection of a large astronomical mirror should be (c) parabolic. 34. A spherical mirror suffers from spherical aberration because (c) different parts of the mirror focus the light at different distances from the mirror. 35. In telescopes, the angular resolution is worse for (d) smaller diameter lenses or mirrors and longer wavelength electromagnetic radiation. 36. If all effects caused by Earth's atmospheric variations (seeing) could be removed from the visible image of a star obtained with one of the 10-m diameter Keck telescopes on Hawaii, what would be the angular resolution achievable by this telescope in arcseconds for light of wavelength 500 nm? (1 nm = 10 9 m) (b) arcsec 37. What does the word seeing mean to an astronomer using a telescope? (a) The twinkling and blurring of the image due to air currents in Earth's atmosphere. 38. During the past few decades a number of techniques have been developed to enhance the performance of optical and radio telescopes. These include all of the following except one. Which one is the exception? (b) building refracting telescopes comparable in size to the largest reflecting telescopes 39. What is the main reason for combining many radio telescopes together into an interferometer with large distances between telescopes? (a) to obtain much sharper images of sources 40. Orbital eccentricity is a number that describes the (d) shape of the orbital ellipse. 41. The CCD (charge-coupled device) is (b) an array of small light-sensitive cells that can be used in place of photographic film to obtain a picture. 42. Astronomy from space vehicles is particularly useful because the telescope (b) is above Earth's absorbing and distorting atmosphere and can measure radiation over a very wide wavelength range.

5 43. Our planetary system consists of (c) large and small planets, some accompanied by moons as large as the smaller planets. 44. Compared to the orbital distance of the Earth from the Sun, the equivalent orbital distances for the Jovian planets are (b) more than 5 times greater. 45. Most of the planets orbit the Sun on or close to the (d) ecliptic plane. 46. The best way to measure the mass of a planet is to measure the (a) gravitational pull of the planet on an orbiting satellite or a nearby spacecraft. 47. The Jovian planets have high masses and generate powerful gravitational fields, yet they have low average densities. What does this indicate about their interiors? (d) They are composed mainly of very light elements, such as H and He. 48. Which of the following general statements about all of the planets in the planetary system is true? (b) They orbit the Sun in the same direction. 49. The next planet beyond Saturn is Uranus. The distance from the orbit of Saturn to the orbit of Uranus is about the same as (a) the distance from Saturn to the Sun. 50. Consider the seven largest satellites in the solar system. The presence of an atmosphere correlates with (d) none of the above. 51. A nitrogen molecule (N 2 ) has a mass of kg, and a carbon dioxide molecule (CO 2 ) has a mass of kg. On a day when the temperature is 18 C (64 F), the N 2 molecules will have a (d) greater average speed than the CO 2 molecules but the same average kinetic energy. 52. On Mars, the temperature can reach 290 K and the escape speed is 5.0 km/sec. What is the average speed of a carbon dioxide molecule (mass = kg) at this temperature, and what can you say about the retention of carbon dioxide on Mars? (a) The average speed is 0.4 km/sec, so Mars should retain its carbon dioxide. 53. The asteroid Vesta, located in the main asteroid belt between the orbits of Mars and Jupiter, is spherical in shape with a radius of 265 km and a mass of kg. What is the minimum speed at which you could propel yourself upward and not fall back down? (c) 388 m/s 54. The asteroid belt exists between the orbits of which planets? (c) Mars and Jupiter

6 55. The trans-neptunian objects (such as Eris, Pluto, Sedna, Quaoar, etc.) are (c) small worlds of rock and ice, most of which orbit within the Kuiper belt. 56. What is the basic difference between comets and asteroids? (c) Comets are mostly composed of ices, while asteroids are mainly composed of rocks. 57. Planets and satellites with a large amount of surface cratering generally have insignificant atmospheres. Why are these two conditions correlated? (b) A large amount of cratering implies a small size, and this, in turn, implies a low escape speed. 58. The age of the solar system has been dated rather precisely to 4.56 billion years. What method was used to determine this number? (d) determining the age of meteorites by radioactive dating 59. In general, small bodies in the solar system are less likely than large bodies to possess a planet-wide magnetic field. Why should we expect size and magnetism to be correlated? (a) A small body cools more rapidly and is less likely to possess a molten, liquid interior one requirement for planet-wide magnetism. 60. A theory of the origin of the solar system must take into account all important general properties of the planets. These include three of the four properties listed below. Which one is not an important general property of the planets? (b) The magnetic fields of the planets are produced by a variety of mechanisms. 61. Together, hydrogen and helium account for what percentage of the total mass of all matter in the universe? (a) 98% 62. Where in the universe are heavy elements with masses greater than that of helium being produced at this time? (c) central cores of stars 63. The most likely mechanism for the solar system's formation appears to be (d) that a cloud of gas and dust condensed to form the Sun, while planets formed later by condensation and accretion within the nebular disk. 64. Our solar system seems to have formed about how long ago? (c) 5 billion years ago 65. The half-life of radiogenic 14 C is 5730 years. How long must you wait until 90% of the original sample remains? (a) less than 5730 years

7 66. It is found by the radioactive age-dating technique that only one-quarter of the original sample of a radiogenic element taken from a rock remains, so the time is calculated to be two half-lives. What does this time represent? (d) the time since the rock was formed 67. The three common substances believed to have been important in planet formation are (d) rocks, ices, and gas. 68. The process of accretion in planetary formation is the (a) slow accumulation of solid particles by gravity and collision into larger, solid objects. 69. The steps in the process of formation of the large, outer planets were (d) accretion of planetesimals to form a core, followed by gravitational capture of hydrogen and helium gas. 70. How are many of the satellites of the Jovian planets thought to have formed? (c) From a disk of material around the planet, similarly to the way the planets formed around the Sun 71. A T Tauri wind is (d) a rapid expulsion of the tenuous outer layers of a young star. 72. What type of search technique has discovered the largest number of planets around the stars other than the Sun? (b) Looking for tiny variations in the star's radial velocity, caused by the gravitational pull of one or more planets orbiting the star. 73. We measure the mass of an extrasolar planet by (d) using Newton's law of gravity, using the measured distance from the star and its gravitational pull on the star. 74. What properties of an extrasolar planet can be learned from the transit method? (a) mass, radius, chemical composition, and temperature 75. What is surprising about the extrasolar planets that have been discovered? (c) Many of them are giant planets like Jupiter, orbiting at distances characteristic of terrestrial planets.