Download PHYS 175 (2014) Final Examination Name: ___SOLUTION_____

PHYS 175 (2014) Final Examination Name: ___SOLUTION_____ All questions are worth 1 pt., unless otherwise noted. Select the best answer for each question. Please answer discussion/description questions in complete sentences. Your may use your own handwritten notes, homework assignments and quizzes as references during the exam. 1. The visible spectrum of the Sun a) contains dark absorption and bright emission lines on a continuous background. b) is a continuous spectrum, containing many absorption lines created by the atmosphere of the Sun. c) consists of only a few bright emission lines. d) consists of only a few bright absorption lines. e) is a bright continuous spectrum with no absorption lines. 2. The “fuel” of the Sun is ______, and the main products of the nuclear reactions include ______. a) hydrogen / helium, neutrinos, and gamma rays b) helium / only neutrinos and gamma rays c) hydrogen / neutrinos and microwaves d) helium / neutrinos and microwaves e) hydrogen / only neutrinos 3. At the distance of the Earth from the Sun (1 AU) the intensity of sunlight is 1370 watts/m2. What is the intensity at the distance of Saturn from the Sun (10 AU)? a) 13,700 watts/m2 b) 1370 watts/m2 c) 137 watts/m2 d) 13.7 watts/m2 e) 1.37 watts/m2 4. [3 pts.] On the blank H-­‐‑R Diagram axes sketch the evolutionary track of the Sun from the present day to it’s
eventual state. Label each major feature on its track. 5. The spectral classification of a star is closely related to the star’s
a) apparent brightness.
b) absolute magnitude.
c) luminosity.
d) surface temperature.
e) distance.
6. We see an emission nebula via
a) reflected blue light from a nearby star.
b) reflected red light from a nearby star.
c) blue light emitted by hot (excited) hydrogen atoms.
d) red light emitted by hot (excited) hydrogen atoms.
e) x-­‐‑rays emitted by hot (excited) hydrogen atoms.
7. Which stars on this H-­‐‑R diagram are on the main sequence? a) Vega, Sirius, and Mira b) Stars at letters A and B and Barnard’s Star c) Sirius A and Sirius B d) Rigel and Deneb e) Pollux and Barnard’s Star 8. Astronomers discover an object that emits most of its electromagnetic radiation at infrared wavelengths. What could this object be? a) A protostar b) A main-­‐‑sequence star of spectral class G c) A supergiant star of spectral class G d) A main-­‐‑sequence star of spectral class B e) More than one of the above 9. Which of the following are thought to be mechanisms that can cause a giant molecular cloud to collapse and form a protostar? a) The shock wave from a nearby supernova b) The shockwave from a newly formed high-­‐‑mass star that is nearby c) The shockwave experienced by the cloud as it passes through a spiral arm d) All of the above e) None of the above 10. Stars with larger masses have a) longer main-­‐‑sequence lifetimes than smaller mass stars because they have more fuel. b) shorter main-­‐‑sequence lifetimes than smaller mass stars because although they have more fuel, they use it much faster. c) longer main-­‐‑sequence lifetimes than smaller mass stars because they can fuse more elements. d) shorter main-­‐‑sequence lifetimes than smaller mass stars because they have a smaller core in which nuclear reactions can occur. 11. When hydrogen fusion in the core of the Sun ends, the Sun will move off the main sequence. As a result, the surface temperature of the Sun will ______ and the diameter of the Sun will ______. a) decrease / decrease b) increase / increase c) decrease / increase d) increase / decrease 12. Cepheid variable stars are very luminous and can be observed over very large distances. Why are such stars important to astronomers? a) They confirm the theory of nuclear fusion as the energy source for stars. b) They are used as distance indicators because their luminosity can be determined from their period. c) Such stars are unstable and are about to become supernovae. d) Their age can be determined directly from their period. 13. White dwarfs usually have surface temperatures well above 10,000 K, yet they have extremely low luminosity. Why is this? a) They are very far away. b) They have a very large surface area. c) They emit most of their radiation in the far infrared. d) They emit most of their radiation in the ultraviolet. e) They have a very small surface area. 14. All of the gold atoms in Fairbanks a) were formed during a supernova event as the Sun and Earth were forming. b) were formed at the core of the Sun by fusion. c) were present in the dust and debris of the solar nebula before the Sun and Earth formed. d) came from Hawking radiation escaping a nearby black hole. 15. Elements heavier than iron are produced by nuclear reactions a) in a white dwarf. b) in a neutron star. c) during a supernova explosion of a massive star. d) in the shells around the core of a massive star e) in the core of a massive star just before it explodes as a supernova. 16. If light is emitted by an object in the intense gravitational field near a black hole, and is viewed by an observer far away from the object, the observer will find that a) its frequency is decreased and its wavelength is redshifted because time passes more slowly in an intense gravitational field. b) its frequency is increased and its wavelength is blueshifted because time passes more slowly in an intense gravitational field. c) its frequency is decreased and its wavelength is redshifted because time passes more rapidly in an intense gravitational field. d) its frequency is increased and its wavelength is blueshifted because time passes more rapidly in an intense gravitational field. 17. The Schwarzschild radius of a non-­‐‑rotating black hole is the a) distance from the center of a black hole to the point at which the escape velocity is the speed of light. b) radius of the solid matter part of a black hole. c) inner radius of the black hole’s accretion disk. d) outer radius of the black hole’s accretion disk. e) radius of the progenitor star of the black hole. 18. According to experiments, the speed of light a) is the same for all observers independent of whether they or the light source are moving. b) appears to be faster than 3 × 108 m/s if you are moving toward the light source. c) appears to be slower than 3 × 108 m/s if the light source is moving toward you. d) is 6 × 108 m/s if the relative motion between the source and the observer is 3 × 108 m/s. 19. William Herschel tried to locate the center of our Galaxy by counting the number of stars in different directions. This did not work because a) stars are not uniformly distributed. b) more distant stars are obscured by dust and gas. c) there are very few stars near the center of our Galaxy. d) most of the brighter stars are in the outer regions of our Galaxy. 20. Which of the following correctly describes our current understanding of our Galaxy’s spiral arms? a) The spiral arms are created by density waves that sweep around the Galaxy. b) The spiral arms are the sites for the formation of new stars. c) Stars that are formed in a spiral arm remain in that spiral arm. d) A and B are correct, but not C. e) All of A, B, and C are correct. 21. Most astronomers think that there is dark matter in our Galaxy because a) stars in the outer edges of our Galaxy move faster than expected. b) stars in the outer edges of our Galaxy move slower than expected. c) large amounts of matter can be seen at infrared wavelengths. d) large amounts of matter can be seen at radio wavelengths. e) large amounts of matter can be seen at x-­‐‑ray wavelengths. 22. What observation suggests that a supermassive black hole is at the center of our Galaxy? a) We do not see any light emanating from the center of our Galaxy. b) We see large amounts of gravitational radiation coming from the center of our Galaxy. c) Stars close to the center of our Galaxy are moving at high speeds. d) Light from the center of our Galaxy has a large gravitational blueshift. 23. How did Edwin Hubble show that M31 in Andromeda is a distant galaxy and not part of the Milky Way? a) By measuring the distance to M31 using Cepheid variables. b) By measuring the distance to M31 using RR Lyrae variables. c) By precisely measuring the parallax of M31. d) By observing a nova in M31. e) By observing a supernova in M31. 24. In which of the following types of galaxies would you be least likely to find a newly-­‐‑formed star? a) Elliptical b) Spiral c) Irregular 25. Which of the following best describes the Hubble law in its simplest form? a) The recessional velocity of a galaxy is proportional to its distance from us. b) The recessional velocity of a galaxy is inversely proportional to its distance from us. c) The brightness of a galaxy is proportional to its distance from us d) The brightness of a galaxy is inversely proportional to its distance from us. e) The brightness of a galaxy is inversely proportional to its color. 26. Quasars are known to be distant objects (more than 100 Mpc away) because a) they emit tremendous amounts of energy and thus cannot be nearby. b) they have very small parallax angles. c) the Cepheid variables in them are very dim. d) they have large redshifts. e) they have very low temperatures. 27. An active galactic nucleus (AGN) is thought to be powered by a) a supermassive black hole. b) supernovae. c) novae. d) luminous O and B stars. e) smaller dwarf galaxies within the AGN. 28. What does it mean to say the universe is expanding? a) Galaxies are moving through space away from each other. b) Space is expanding, carrying galaxies along with it. c) All galaxies are moving away from a point at the center of the universe. d) Space is expanding but galaxies are not carried along, so the separation of galaxies does not change. e) The galaxies are all getting larger. 29. Which of the following statements about the cosmic background radiation is incorrect? a) It was found by accident. b) It has the spectrum of a blackbody at 2.725 K. c) It confirms Einstein’s assumption that the universe is isotropic (roughly the same in all directions, on large scales). d) Its spectrum has always been the same. e) Its intensity shows very minor fluctuations in different directions. 30. This graph (below, for which the 2011 Nobel Prize was awarded) shows the apparent magnitude versus redshift for supernovae in distant galaxies. The data points are mostly in the blue region, which means that the expansion of the universe a) was slower in the distant past. b) was faster in the distant past. c) has always been at the same rate. d) is non-­‐‑isotropic. e) ceased about 13 billion years ago. 31. [3 pts.] Briefly discuss the importance of the overlap in the distance ranges of the various techniques shown below in the “distance ladder”. The overlap of the techniques is crucial for calibrating them. For an object whose distance can be estimated by two (or more) techniques, we should get roughly the same result from each technique. 32. [3 pts.] Describe the phenomenon of gravitational lensing and draw a sketch to illustrate the effect. How can this effect be used to detect the presence of dark matter in the universe? Massive objects warp spacetime. If a massive object is aligned between an observer and a light source, some rays of light from the source, which would not normally be directed toward the observer, can be deflected toward the observer. The effect may manifest itself as a halo, double image or even just an intensity fluctuation.
33. [3 pts.] Briefly discuss something you learned about astronomy during this course. This one is pretty open-­‐‑ended…