Astronomy 101.004 Test #3 Spring 2013 Instructor: D. Zirzow Name: ANSWER KEY SID: Please legibly write your name and student ID. There are 40 multiple choice questions and 5 short answer questions. For each multiple choice question, select the one best answer and write it legibly in the blank to the left of the question number. Each question is worth 2 points. B 1. How do the stars in a star cluster change with time? A) The stars with the greatest heavy-element content evolve the most rapidly. B) The highest-mass stars evolve the most quickly. C) The lowest-mass stars evolve the most quickly. D) All stars in it evolve at the same rate. C 2. What is the Sun's energy source? A) primordial heat left over from the release of gravitational energy when the Sun first formed. B) radioactivity (nuclear fission). C) thermonuclear fusion in the core. D) heat released by gravitational contraction. B 3. Black holes are so named because C 4. A pulsar is A) they emit a perfect blackbody spectrum. B) no light or any other electromagnetic radiation can escape from inside them. C) all their electromagnetic radiation is gravitationally redshifted to the infrared, leaving no light in the optical region. D) they emit no visible light, their only spectral lines being in the radio and infrared A) an interstellar beacon manufactured by little green persons (LGPs). B) a type of variable star, pulsating rapidly in size and brightness. C) a rapidly spinning neutron star. D) an accretion disk around a black hole, emitting light as matter is accumulated on the disk. D 5. The final remnant of the evolution of a red giant star that has ejected a planetary nebula is a A) blue supergiant. B) neutron star. C) supernova. D) white dwarf star
A 6. The evolution of a star is controlled mostly by its A) initial mass. B) location in the galaxy. C) surface temperature. D) chemical composition. B 7. The majority of the elements heavier than hydrogen and helium in the universe are believed to have originated in A) the original Big Bang. B) the central cores of stars. C) HII regions, under the action of Hα light. D) giant molecular clouds. C 8. New stars are formed A) within supernova remnants. B) from free space, out of pure energy. C) in huge, cool dust and gas clouds. D) by condensation of gas near black holes in the centers of galaxies. A 9. Which are the two most abundant elements in the universe? A) hydrogen and helium B) nitrogen and oxygen C) hydrogen and oxygen D) hydrogen and carbon D 10. Thermonuclear fusion reactions in the core of the Sun convert four hydrogen nuclei into one helium nucleus. The helium nucleus has A) more mass than the four hydrogen nuclei, because energy is produced in the reaction, and this energy adds the extra mass. B) the same mass as the four hydrogen nuclei, because the mass of any product has to equal the mass of the sum of its parts by the law of conservation of matter. C) an undetermined amount of mass that depends on the temperature at which the reaction occurs. D) less mass than the four hydrogen nuclei, the lost mass becoming energy in an amount given by E = mc2. A 11. The chemical makeup of a star's surface is usually determined by A) spectroscopy of the light emitted by the star. B) examining the chemicals present in a meteorite. C) theoretical methods, considering evolution of the star. D) taking a sample of the star's surface with a probe.
B 12. A white dwarf is A) an object intermediate between planets and stars, that will never become a star. B) a star at the end of its life, with a size close to that of the Earth. C) any main sequence star with a surface temperature between about 9000 K and 15,000 K. D) a star at the beginning of its life, with a size two to ten times that of the Sun. D 13. Which important stellar parameter can be determined by the study of binary stars? A) the age of the stars. B) the distance of the stars from the Earth. C) surface temperatures of the stars. D) stellar masses. C 14. All stars on the main sequence A) are at a late stage of evolution after the red giant stage. B) are changing slowly in size by gravitational contraction. C) generate energy by hydrogen fusion in their centers. D) have approximately the same age to within a few million years. A 15. At what stage of its evolutionary life is the Sun? A) main-sequence: middle age B) pre main-sequence: variable star C) post main-sequence: red giant (cool) phase D) just before supernova stage (perhaps 5 years): late evolutionary stage B 16. The space between stars is now known to contain A) large quantities of dust that absorb light but no gas, either atomic or molecular. B) gas, made up of atoms and molecules, and dust particles. C) a perfect vacuum. D) variable amounts of gas but no dust, because dust forms only in planetary systems near stars. C 17. At what location in the space around a black hole does the escape velocity become equal to the speed of light? A) at the point where escaping X rays are produced. B) at the point where clocks are observed to slow down by a factor of 2. C) at the event horizon. D) at the singularity. B 18. Which is the correct sequence for the following end-points of stellar evolution, in order of increasing maximum mass? A) neutron star, black hole, white dwarf B) white dwarf, neutron star, black hole C) black hole, neutron star, white dwarf D) white dwarf, black hole, neutron star
B 19. The relationship between mass and luminosity of stars on the main sequence is that A) the luminosity of stars rises to a peak at around a mass of 1 solar mass and decreases as mass increases beyond this limit. B) the greater the stellar mass, the larger the luminosity. C) luminosity is independent of the stellar mass. D) the greater the stellar mass, the less the luminosity. A 20. A white dwarf star is supported from collapse under gravity by A) degenerate-electron pressure in the compact interior. B) centrifugal force due to rapid rotation. C) pressure of the gas, heated by nuclear fusion reactions in a shell around its core. D) pressure of the gas, heated by nuclear fusion reactions in its core. B 21. The very strong magnetic field of a neutron star is created by A) a burst of neutrinos produced by the supernova explosion, because this would be the equivalent of a very large electrical current flowing for a short time. B) the collapse of a star, which significantly intensifies the original weak magnetic field of the star. C) differential rotation of the neutron star, its equator rotating faster than the poles, similar to sunspot formation. D) turbulence generated in electrical plasmas during the collapse of a star, even though this star had no magnetic field originally. C 22. The Sun has existed for a very long time without change in its size, appearance, or behavior. This means that it must be in hydrostatic equilibrium. Under these conditions, which two parameters must be in exact balance within the Sun? A) numbers of hydrogen and helium nuclei. B) hydrogen gas pressure and helium gas pressure. C) inward force of gravity and outward gas pressure. D) magnetic field and force of gravity. D 23. Stellar parallax is used to determine which of the following properties of a nearby star? A) its spectral type and surface temperature B) its rotation period C) its apparent magnitude D) its distance from the Sun A 24. The "surface" of the Sun, or the layer of the Sun that we see, is called the A) photosphere B) radiation zone C) corona D) tecate
C 25. Which of the following properties of a star depends on its distance from us? A) its luminosity B) its temperature C) its apparent brightness D) its mass D 26. Suppose that, at night, the brightness of a light bulb is measured from a certain distance and then the light bulb is moved to a distance twice as far away. Using the "inverse square law," how bright will the light appear compared to the earlier measurement? A) 1/16 as bright B) 1/2 as bright C) 1/8 as bright D) 1/4 as bright D 27. Which effect has been most useful (and successful) in the search for and identification of black holes in the universe? A) their magnetic fields and their influence on nearby matter. B) the effect of their angular momentum or spin on nearby matter. C) the influence of their intense gravitational field on atoms which are emitting light from the event horizons of the black holes. D) their gravitational influence on nearby matter, particularly companion stars. A 28. Measurements indicate that a certain star has a very high intrinsic brightness (100,000 times as bright as our Sun) and yet is relatively cool (3500 K). How can this be? A) The star must be very large. B) The star must be quite small. C) The star must belong to the main sequence. D) There must be an error in observation, since no star can have this form. C 29. An eclipsing binary system is A) two stars whose spectral lines move back and forth, indicating relative motion. B) a star that is periodically eclipsed by the Moon. C) two stars whose combined light output when measured from Earth appears to vary periodically as the two stars move in front of one another. D) two stars that are clearly seen as separate but associated in the sky. A 30. The effect of interstellar dust on starlight is A) to dim and redden distant stars by preferentially scattering their blue light. B) to scatter the red light from stars preferentially, making them appear bluer than expected. C) almost nonexistent, because light does not interact with dust. D) to make stars appear less bright than expected by absorbing light about equally at all wavelengths.
A 31. What are the main general features that make clusters of stars useful to astronomers? A) The stars are at the same distance from Earth, were formed at approximately the same time, and were made from same chemical mix. B) The stars are all at the same distance from Earth, have the same surface temperature, and joined the cluster at various times. C) The stars all have the same apparent magnitude, the same surface temperatures, and the same sizes. D) The stars all have the same intrinsic brightness but differ in size and surface temperature. B 32. The period of variability of a Cepheid variable star, which is easily measured, is directly related to which stellar parameter, thereby providing a reliable method for the measurement of distance to stars? A) velocity away from Earth B) luminosity C) surface magnetic field D) surface temperature B 33. What is the mass limit above which the self-gravity of stars can overcome electron degeneracy pressure i.e. the approximate maximum mass of a white dwarf star? A) 0.05 solar masses B) 1.44 solar masses C) 14 solar masses D) There is no limit, because nothing in nature can overcome this quantum mechanical limit. A 34. The explosion of a supernova appears to leave behind A) a rapidly expanding shell of gas and a central neutron star. B) a rapidly rotating shell of gas, dust, and radiation, but no central object. C) a rapidly expanding shell of gas and a compact white dwarf star at its center. D) nothing, the explosion changes all the matter completely into energy, which then radiates into space at the speed of light. B 35. Type II supernovae show prominent lines of hydrogen in their spectra, whereas hydrogen lines are absent in spectra of Type I supernovae. Why is this? (HINT: Think about the type of star that gives rise to each of the two types of supernova.) A) Massive stars have burned all of their hydrogen into heavier elements, whereas low-mass stars still have large hydrogen-rich envelopes. B) Massive stars contain large amounts of hydrogen, whereas white dwarfs are mostly carbon and oxygen. C) White dwarfs have a thick surface layer of hydrogen, whereas neutron stars contain no hydrogen at all. D) Massive stars contain large amounts of hydrogen, whereas neutron stars contain no hydrogen at all.
B 36. A neutron star will be detected from Earth as a pulsar by its regular radio pulses ONLY if A) Earth lies in the plane of the neutron star's magnetic equator, halfway between its magnetic poles. B) Earth lies almost directly above the magnetic axis of the neutron star at some time during the star's rotation C) Earth lies directly above the rotation axis of the rotating neutron star. D) Earth lies in the neutron star's "equator," the plane perpendicular to its spin axis. A 37. List these important regions of the Sun in order from the center outwards A) core, radiation zone, convection zone, photosphere, corona B) convection zone, core, photosphere, solar wind, corona C) core, photosphere, convection zone, solar wind, radiation zone D) radiation zone, convection zone, core, photosphere, corona D 38. Which of the following is TRUE about emission nebulae? A) They have a black-body spectrum, which gives them their reddish color. B) They account for most of the mass of the interstellar medium. C) They are never associated with molecular clouds. D) The gas is mostly ionized, and emits light at specific wavelengths, which gives them their reddish color. A 39. Which of the following statements about the rate of stellar evolution is true? A) The more massive the original star, the faster the evolution. B) Star mass has no bearing upon stellar evolution, because all stars evolve at the same rate, controlled by nuclear fusion and core temperature. C) The chemical makeup of the original nebula is the major factor in deciding the rate of evolution. D) The more massive the original star, the slower the evolution, because there is more material for thermonuclear burning. C 40. At what point in its evolution will a protostar stop shrinking and stabilize into a star? A) when gravitational contraction has heated up the gas to the point where radiation pressure opposes gravity for the first time. B) when contraction leads to an increase in spin rate as a result of the conservation of angular momentum and the resulting centrifugal force begins to oppose the gravitational contraction. C) when thermonuclear processes generate enough energy and internal pressure to resist further gravitational contraction. D) when the buildup of helium in the core stops the nuclear furnace.
Short Answer Directions: Please write answers to the questions in the space provided. Use clear English sentences to explain your answers. You may also use pictures to clarify your answers if necessary. 1. (4 points) Why do thermonuclear reactions only take place in the cores of stars? What conditions must be true for thermonuclear reactions to take place? Thermonuclear fusion reactions can only take place in the cores of stars (generally speaking) because this is the only location in which the temperature and density is high enough to allow fusion to occur. In the proton-proton chain two protons must be forced close enough for the nuclear forces to bind them. Since protons have a positive charge they repel each other. The repulsive force can only be overcome if the protons are rammed into each other with high enough energy. This condition is only met when the temperature is high enough. This idea also holds true for fusion of elements heavier than hydrogen, the only difference being that in those cases the temperature must be even higher. 2. (4 points) Explain the difference between a star's apparent brightness and luminosity. If we know these two quantities for a star, what additional important property of a star can be determined using the inversesquare law? The luminosity of a star is a measure (in units of Watts) of how much total energy is emitted from the surface. Apparent brightness is a measurement (in units of Watts/Area) of how bright a star appears to an observer on Earth. These two quantities are not the same thing, but are related by the distance to the star in the inverse-square law. The luminosity of a star is a fixed quantity that DOES NOT CHANGE with distance. On the contrary the apparent brightness of a star is determined by the luminosity and distance from the star in which we are making the measurement. = 4
3. (4 points) What is a Hertzsprung-Russell diagram? What does it mean when an astronomer says that a star "moves" from one place to another on the Hertzsprung-Russell diagram? Give one example of when a star would "move" from one place to another on a Hertzsprung-Russell diagram. A Hertzsprung-Russell diagram (HRD for short) is a plot of luminosity against temperature ( L increasing on y-axis, T decreasing on x-axis). This plot is tremendously useful for understanding properties of stars and how they evolve. As stars evolve and change structure, their radii and temperatures also change. Since a star's luminosity is dependent on both temperature and radius, the luminosity also can change as a star evolves. If we plot a single star in an H-R diagram at several different stages of evolution we will find that it is located in different regions of the diagram. In this way, we speak of a star "moving" in the H-R diagram because of structural changes as the star evolves. The path that a star moves on in the H-R diagram is called an evolutionary track. Examples of when stars move from one location to another are: Pre-main-sequence evolution before the ignition of H- burning in the core, End of core hydrogen fusion and the ascent of the red giant branch (RGB) Typical Hertzsprung-Russell Diagram 4. (4 points) What is a planetary nebula and how do we think they form? Which stars form a planetary nebula (high or low mass)? What is left behind after the planetary nebula phase? A planetary nebula is a phase of evolution for low-mass stars that are evolving from red giants into their last stage of evolution, white dwarfs. The nebula itself is a shell of ionized gas that is the left over red giant atmosphere after it was ejected by powerful stellar winds. As the core of the star heats up and evolves into a white dwarf the UV photons ionize the surrounding shell of gas and makes it glow, which is what we see. We think the nebulae themselves are formed by the stellar winds of the red giant star as it ages. At the center of the nebula is the hot central star. The central star will evolve into a white dwarf and the nebula will disperse into the interstellar medium.
5. (4 points) How are neutron stars able to produce pulses of radiation that can be seen by an observer on Earth? (Hint: you can use a sketch of a pulsar to explain what is happening) Neutron stars are characterized by their very small sizes and powerful magnetic fields. The radiation emitted by a neutron star is along to oppositely directed beams that lie along the magnetic axis of the star. Because of their small size and the conservation of angular momentum, neutron stars are also rotating very rapidly, and the axis of rotation does not have to coincide with the magnetic axis. As a result these beams of radiation sweep across the sky like a lighthouse. If we happen to be present in the path of one of the beams we see pulses of light that occur at the same interval as the neutron star's rotation period. Lighthouse model for pulsars. Extra Credit (4 points each): What observational evidence to astronomers have for the existence of black holes? Why is the detection of black holes so challenging? See sections 22-3, 22-4, 22-5 in the text