Principles of Astronomy Exam
Solutions
Course Introduction
Principles of Astronomy introduces students to the fundamental concepts and methods used to study the universe beyond Earth. The course covers the structure and motion of celestial bodies, the nature of stars and galaxies, the evolution of the universe, and the tools and techniques astronomers use to observe and interpret cosmic phenomena. Emphasis is placed on understanding scientific models, physical laws governing the cosmos, and recent discoveries that shape our knowledge of space. This foundational course lays the groundwork for advanced studies in astrophysics and related sciences, fostering an appreciation for humanitys place in the universe.
Recommended Textbook
Astronomy Preliminary Edition by Adam Frank
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Page 2
Chapter 1: Getting Started: Science, astronomy, and Being Human
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Q1) Everyday materials like wood,water,and air are composed of:
A) dark matter.
B) atoms.
C) dust.
D) neutrinos.
Answer: B
Q2) Which of the following is closest to the diameter of our Galaxy?
A) 10<sup>21</sup> m
B) 10<sup>24</sup> m
C) 10<sup>27</sup> m
D) 10<sup>35</sup> m
Answer: A
Q3) Which of the following might be the diameter of a planet?
A) 100 m
B) 10<sup>3</sup> m
C) 10<sup>6</sup> m
D) 10<sup>10</sup> m
Answer: C
Q4) Identify the base,coefficient,and exponent in the number 4.07 × 10 .
Answer: The base is 10,the coefficient is 4.07,and the exponent is 6.
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Chapter 2: A Universe Made, a Universe Discovered
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Q1) A synodic month is about two days longer than a sidereal month.Why?
A) The Earth-Moon system is revolving around the Sun.
B) The Moon's orbit around Earth is tilted with respect to Earth's orbit around the Sun.
C) The Moon must follow the ecliptic.
D) The Moon's distance from Earth varies across its orbit.
Answer: A
Q2) In order to explain the varying speed of the planets across the night sky,Ptolemy's model introduced:
A) elliptical orbits.
B) the equant.
C) the celestial sphere.
D) synodic months.
Answer: B
Q3) The Sun's path across the celestial sphere over the course of a year is the:
A) zenith.
B) zodiac.
C) ecliptic.
D) analemma.
Answer: C
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Chapter 3: A Universe of Universal Laws
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Q1) Kepler's laws of planetary motion were formulated using data recorded by:
A) Tycho Brahe.
B) Nicolaus Copernicus.
C) Galileo Galilei.
D) Isaac Newton.
Answer: A
Q2) Who was the first astronomer to discover sunspots?
A) Tycho Brahe
B) Galileo Galilei
C) Isaac Newton
D) William Herschel
Answer: B
Q3) The gravitational force experienced by Earth due to the Moon is ________ the gravitational force experienced by the Moon due to Earth.(Note that the Moon is about 1% of the mass of Earth.)
A) 10-4 times
B) 10-2 times
C) equal to
D) 104
Answer: C
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Chapter 4: A Universe of Universal Laws
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Q1) An astronomer measures the luminosities of two stars with identical sizes,finding that star A is 256 times as luminous as star B.How are their temperatures related to each other?
A) Star A is twice as hot as star B.
B) Star A is 4 times hotter than star B.
C) Star A is 16 times hotter than star B.
D) Star A is 64 times hotter than star B.
Q2) Which of the following objects would emit the most radiation?
A) A 1 m2 block of molten iron at 6,000 K.
B) A 0.5 m2 block of molten nickel at 6,000 K.
C) A 2 m2 block of molten iron at 1,500 K.
D) A 6 m2 block of molten nickel at 3,000 K.
Q3) A telescope can barely detect a star situated at 1 kpc from Earth.If an identical star were placed 4 kpc from Earth,how much larger would the radius of the telescope have to be to observe it?
Q4) The radiation spectrum produced by a blackbody depends on its:
A) temperature.
B) composition.
C) mass.
D) thickness.
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Chapter 5: The Architecture and Birth of Planetary Systems
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Q1) What is a dwarf planet?
Q2) The most successful method to date in determining an exoplanet's radius is:
A) the transit method.
B) the radial velocity method.
C) direct imaging.
D) gravitational lensing.
Q3) Arrange the following types of planets in order of increasing distance from the Sun:
A) terrestrial planets, ice giants, gas giants.
B) gas giants, terrestrial planets, ice giants.
C) terrestrial planets, gas giants, ice giants.
D) ice giants, gas giants, terrestrial planets.
Q4) Venus is unusual within the Solar System because it:
A) rotates in the opposite sense from its orbit around the Sun.
B) is so massive.
C) has no evidence for active plate tectonics.
D) has a highly inclined orbit.
Q5) What is the key difference between the core accretion and hydrodynamic instability models for gas giant formation?
Q6) Explain the differences between meteoroids,meteorites,and meteors.
Page 7
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Chapter 6: Home Base: Earth and Moon
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Q1) Without an atmosphere,Earth's surface temperature would be below the freezing point of water.What is responsible for the heating that allows liquid oceans to form?
Q2) Half of the carbon-14 in a rock decays to nitrogen-14 after about 5,700 years.How long would it take before there is seven times as much carbon-14 as nitrogen-14 (i.e.the composition of the sample is 7/8 carbon-14)?
Q3) According to plate tectonics,where do most earthquakes occur?
Q4) "Snowball Earth" phases of geologic history were most likely caused by a(n):
A) decrease in the Sun's luminosity.
B) reduced concentration of atmospheric methane.
C) increased solar wind.
D) increase in volcanism.
Q5) Suppose that Earth's magnetic poles were along the equator.How would that affect the aurorae,and why?
Q6) The ultimate power source of plate tectonics comes from:
A) volcanoes.
B) radioactive decays.
C) convection.
D) friction.
Q7) What is responsible for heating the stratosphere? What about the ionosphere?
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Chapter 7: Sibling Worlds: Mercury, Venus, and Mars
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Q1) The creation of the Tharsis Bulge was also responsible for:
A) lava domes.
B) outflow channels.
C) the Danilova crater.
D) the Valles Marineris system.
Q2) Mercury's iron content indicates that it may have:
A) strong magnetic fields.
B) retained much of its internal heat.
C) undergone a significant impact early in its history.
D) formed before the other terrestrial planets.
Q3) How does the massive Olympus Mons volcano on Mars show that Mars does not undergo plate tectonics?
Q4) The origin of Mars's moons is:
A) capture of nearby asteroids.
B) ejecta from giant impacts.
C) fragmentation of a larger body.
D) currently an area of active investigation.
Q5) How are Mercury's orbital and rotation periods related to each other? Why?
Q6) How do astronomers use guiding principles to supplement the scientific method?
Q7) Name two ways in which Phobos and Deimos differ from Earth's Moon.
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Chapter 8: Gas, ice, and Stone: the Outer Planets
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Q1) Clumpy features in a planetary ring most likely indicate:
A) the presence of magnetic fields.
B) collisions within the ring.
C) recent capture of the ring material.
D) the presence of a shepherd moon.
Q2) Suppose that the freezing point of water was 73 K,instead of 273 K.How would the planets in our Solar System differ?
Q3) Which of the following most accurately describes the interior structure of Saturn,from its center to its edge?
A) rocky core, liquid metallic hydrogen, molecular hydrogen
B) liquid metallic hydrogen, ammonia/methane ice, molecular hydrogen
C) rocky core, liquid molecular hydrogen, gaseous molecular hydrogen
D) ammonia/methane ice, liquid molecular hydrogen, gaseous molecular hydrogen
Q4) Why do Uranus's moons have low densities?
Q5) Uranus was discovered by:
A) Isaac Newton.
B) Galileo Galilei.
C) William Herschel.
D) Hipparchus.
Q6) How does the atmosphere of Titan compare to Earth's?
Page 10
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Chapter 9: Life and the Search for Habitable Worlds
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Q1) Microorganisms and large animals differ in the:
A) nucleotides that make up their DNA.
B) physical structure of their DNA.
C) primary mechanism through which they pass genes to each other.
D) importance of natural selection to their evolution.
Q2) What element produces radiation with a wavelength of 21 centimeters?
A) hydrogen
B) helium
C) carbon
D) oxygen
Q3) What is the "cosmic watering hole"?
Q4) Suppose the inner edge of a star's habitable zone is at 2 AU.The star suddenly increases its luminosity by a factor of 4.Where is the inner edge of the habitable zone now?
A) 1 AU
B) 2 AU
C) 4 AU
D) 8 AU
Q5) What is sensitivity in the context of the definition of life?
Q6) What is the most important process in driving evolutionary adaptation?
Page 11
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Chapter 10: The Sun As a Star
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Q1) Sunspots are dark because:
A) dust particles congregate around magnetic fields.
B) magnetic fields decrease the gas temperature.
C) light is deflected by magnetic fields.
D) magnetic fields trigger line emission from the Sun's plasma.
Q2) Which of the following properties decreases from the Sun's core to its outermost layer?
A) temperature
B) density
C) helium fraction
D) hydrogen line opacity
Q3) What forces maintain hydrostatic equilibrium in an object?
A) centrifugal force and gravity
B) pressure and gravity
C) magnetism and gravity
D) radiation and pressure
Q4) An astronomer discovers a region on the Sun's surface,roughly the size of Earth,that produces 80% less light than the rest of the Sun's surface.What is the most likely cause of this feature?
Q5) Why does fusion occur only at the center of the Sun?
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Chapter 11: Measuring the Stars: the Main Sequence and Its Meaning
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Q1) Which of the following inputs are necessary for astronomers to compute the stellar evolutionary track of a star?
A) mass and surface temperature
B) mass and luminosity
C) mass and composition
D) luminosity and surface temperature
Q2) How are globular clusters used to test stellar models?
Q3) If the Sun spends 10 billion years on the main sequence,how long will a star 100 times more massive than the Sun spend there?
A) 10<sup>5</sup> years
B) 3 × 10<sup>6</sup> years
C) 10<sup>9</sup> years
D) 10<sup>11</sup> years
Q4) How do astronomers determine the composition of stars?
Q5) How does the inclination of a spectroscopic binary star system's orbit affect astronomers' mass estimates of the stars?
Q6) How do astronomers construct stellar evolutionary tracks on the HR diagram?
Page 13
Q7) Draw an HR diagram and sketch the location of the main sequence,red giant branch,and white dwarfs.
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Chapter 12: Nursery of the Stars: the Interstellar Medium and
Star Formation
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Q1) An interstellar gas cloud with a density of 100 particles per cubic centimeter and with a temperature of 100 K is in equilibrium with a medium with a density of 0.1 particles per cubic centimeter and 10 K.What is the most likely explanation?
A) supernova blast waves
B) photoionization
C) turbulence
D) the ideal gas law
Q2) What kind of equilibrium characterizes the relation between gas clouds in the interstellar medium?
Q3) What would happen to the Milky Way's interstellar medium if more supernovae occurred than usual?
Q4) A gas cloud with a temperature of 100 K is embedded in a medium of temperature 3 × 10 K and density 0.1 particle per cubic centimeter.If the gas cloud is neither expanding nor contracting,what is its density?
Q5) Why does the initial mass function of stars differ from the actual distribution of stars in the Milky Way at the present day?
Q6) Why is carbon important to the energy-generation process in a massive star?
Q7) What is the criterion for a collapsing cloud to become a protostar?
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Chapter 13: To the Graveyard of Stars: the End Points of Stellar Evolution
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Q1) What happens when a massive star's core becomes iron?
A) The star begins to fuse iron into nickel.
B) The star begins to pulsate as a variable star.
C) The star becomes a planetary nebula.
D) The star can no longer support itself against gravity.
Q2) Which of the following describes the evolutionary stages of a star with three times the mass of the Sun?
A) horizontal branch, main sequence, red giant, asymptotic giant branch
B) main sequence, asymptotic giant branch, red giant, horizontal branch
C) main sequence, horizontal branch, red giant, asymptotic giant branch
D) main sequence, red giant, horizontal branch, asymptotic giant branch
Q3) A star begins its time on the main sequence with 10 solar masses.As it enters the red giant phase,it transfers 5 solar masses to a binary companion.How will the star end its life?
Q4) If the initial mass function changed to form more high-mass stars,how would the overall elemental composition of the Universe be different?
Q5) Why can massive stars fuse heavier elements than less massive stars?
Page 15
Q6) What explodes in Type Ia and Type II supernovas?
Q7) Suppose that a white dwarf accretes mass.How does its radius change?
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Chapter 14: Down the Rabbit Hole: Relativity and Black Holes
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Q1) What kind of stars are most likely to form black holes?
A) stars that can only initiate hydrogen and helium fusion
B) stars with low-mass cores
C) stars with masses greater than 30 MSun
D) stars with an overabundance of heavy elements
Q2) An astronaut falls into a black hole that has 100 times the mass of the Sun.The astronaut sends signals to her partner,who sits at rest a large distance from the black hole.How far from the black hole's center is the astronaut when her signals can no longer reach her partner?
Q3) Why do the most massive stars leave black hole remnants after they die?
Q4) Which of the following is essential for galactic jets?
A) a starburst
B) strong magnetic fields
C) a molecular cloud
D) an elliptical galaxy
Q5) What does the "no-hair" theorem state?
A) Black hole formation cannot be observed.
B) Nothing can escape a black hole.
Page 16
C) Black holes can be described by just three numbers.
D) Black holes can only be observed through gravitational lensing.
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Chapter
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Q1) Which of the following helps prove that spiral structure is not a result of the winding action of galactic rotation?
A) the appearance of molecular clouds inside spiral arms
B) the 21-cm line
C) the orbital period of the Sun around the Milky Way
D) the presence of a bulge
Q2) Which component of the Milky Way has the most stars?
A) disk
B) bar
C) bulge
D) halo
Q3) Where is most of the Milky Way's dark matter? How do astronomers know?
Q4) How thick is the Milky Way's disk as a percentage of its radius?
A) 0.02 percent
B) 2 percent
C) 20 percent
D) It varies from 0.02 percent to 20 percent, depending on radius.
Q5) In 1906,Jacobus Kapteyn measured the size of the Milky Way.Why were his estimates inaccurate?
Q6) What is the shape of the Milky Way's central component?
Page 17
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Chapter 16: A Universe of Galaxies
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Q1) In the context of the cosmic distance ladder,to what does calibration refer?
A) measuring Hubble's constant
B) tuning one distance determination method to another
C) measuring the distance to Cepheid variable stars
D) defining units
Q2) Which of the following quantities is most useful to astronomers when measuring the dark matter content of galaxies?
A) color
B) age
C) stellar velocities
D) luminosity
Q3) Why were Cepheid variables essential to the discovery of galaxies outside the Milky Way?
Q4) An astronomer discovers an unusual active galactic nucleus without any orbiting clouds.How would such a source differ from a typical quasar?
A) It would lack emission lines.
B) It would lack a jet.
C) It would not have variability.
D) all of the above
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Chapter 17: The Cosmic Web: the Large-Scale Structure of the Universe
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Q1) Which of the following astronomical objects has the largest look-back time?
A) a galaxy with z = 1
B) a supernova with z = 3
C) a quasar with z = 6
D) a gamma-ray burst with z = 8
Q2) An astronomer would like to study a region of the Universe 100 Mpc across and 1 Gpc away from Earth,situated behind an obscuring cloud of dust,so he or she cannot directly measure light from the region.How could he or she measure the mass contained in this region?
Q3) Suppose a star produces an ultraviolet photon during the Dark Ages.The photon is emitted from a continuum source with a wavelength between the absorption lines of neutral hydrogen,but it is absorbed as it travels a long distance through the neutral hydrogen.Why?
Q4) The Hubble Deep Field has revealed that:
A) galaxy clusters assembled early in the Universe's history.
B) starbursts were more common in the early Universe.
C) quasars were more common in the early Universe.
D) all of the above
Q6) Why are distant galaxies' spectra redshifted? Page 19
Q5) Why were quasars most common 10 billion years ago?
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Chapter 18: Cosmology
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Q1) Of the following,who is responsible for creating the Big Bang model?
A) Robert Wilson
B) Fred Hoyle
C) Edwin Hubble
D) George Gamow
Q2) Consider two universes with identical Hubble constants.Universe A has a higher density than universe B.If neither universe has dark energy,this implies that:
A) universe A is older than universe B.
B) universe A is the same age as universe B.
C) universe A is younger than universe B.
D) This situation is not possible according to the Big Bang model.
Q3) If dark energy continues to dominate the expansion of the Universe,it will: A) recollapse.
B) decelerate until it stops expanding.
C) accelerate for a time and then recollapse.
D) enter another inflationary era.
Q4) An astronomer measures the properties of an alternate universe.He or she discovers that the angles of a triangle in this universe add up to more than 180 degrees.What can he or she deduce about the universe's density?
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