Notes for Wednesday, July 16; Sample questions start on page 2 7/16/2008 Wed, July 16 MW galaxy, then review. Start with ECP3Ch14 2 through 8 Then Ch23 # 8 & Ch 19 # 27 & 28 Allowed Harlow Shapely to locate center Of the galaxy. Ch 23 # 9 12 leads to giant molecular clouds in spiral arms; # 22 is most important because it shows that the dimmer stars fill the entire disk; only very bright stars on the spiral arms; Dark matter discussion starts on # 23 # 27 is rotation curve. Good graphic on p 625 Disk of Galaxy is populated with metal rich Population I stars, because new generations of of stars formed in spiral arms are likely to be more metal rich than their ancestors. See Evans Ch14, # 15; great sequence in Evans Ch 14 through # 33; except skip # 18 # 19; Eagle nebulae, dark columns of gas are molecular clouds; arrows indicate two of the locations where dense knots of stars are currently forming. Evans # 21 is great viewgraph showing MW in several different wavelengths After Evans # 24; go to Ch23 # 28 Spiral arms and density waves; Then Evans # 27; history of MW revisit dark matter, # 41 & 42. Except Ch 23 # 40 is good view of stars orbiting Sagittarius A * ; black hole of 3.7 million solar masses.
SAMPLE Questions: Chapter 20 A planetary nebula is A) a contracting spherical cloud of gas surrounding a newly formed star, in which planets are forming. B) the expanding nebula formed by the supernova explosion of a massive star. C) an expanding gas shell surrounding a hot, white dwarf star. D) a disk-shaped nebula of dust and gas from which planets will eventually form, easily photographed around relatively young stars. The event that follows the asymptotic giant branch (AGB) phase in the life of a lowmass star is A) the ejection of a planetary nebula. B) core collapse and a supernova explosion. C) helium flash and the start of helium burning in the core. D) the onset of hydrogen burning in the core. Stars that have ejected a planetary nebula go on to become A) red giants. B) white dwarfs. C) protostars. D) supernovae. A white dwarf star, the surviving core of a low-mass star toward the end of its life, can be found on the Hertzsprung-Russell diagram A) below and to the left of the main sequence. B) at the bottom end of the main sequence, along which it has evolved throughout its life. C) at the upper left end of the main sequence, because its surface temperature is extremely high. D) above and to the right of the main sequence, because it evolved there after its hydrogen-burning phase. A white dwarf star is generating energy from what source? A) It no longer generates energy but is cooling slowly. B) nuclear fusion of heavy elements in the central core C) gravitational potential energy as the star slowly contracts D) nuclear fusion of hydrogen into helium Which physical phenomenon keeps a white dwarf star from collapsing inward on itself? A) electron degeneracy or quantum crowding B) normal gas pressure C) convection currents or updrafts from the nuclear furnace D) the physical size of the neutrons
The sequence of thermonuclear fusion processes inside massive stars can transform elements such as carbon, oxygen, etc. into heavier elements AND generate excess energy, until iron has been produced. Why is it not possible for fusion reactions to release energy from iron nuclei? It has been found that iron nuclei never undergo fusion reactions. The magnetic properties of iron produce extra repulsion which, along with the electrostatic repulsion, prevents fusion of iron nuclei with protons. Iron is the heaviest possible element in nature and so, fusion of heavier elements is not possible. The electrostatic repulsion between the iron nucleus and the proton is so great that fusion requires extra energy, rather than releasing it. Carbon fusion in massive stars combines helium and carbon to produce oxygen. This is followed by oxygen fusion in which oxygen is burned to produce sulfur. Why is a higher temperature required for oxygen fusion than for carbon fusion? A) Because of extensive mass loss between the carbon fusion and oxygen fusion stages, higher temperatures are required for nuclear reactions in the relatively rarified stars in which oxygen fusion takes place. B) Larger nuclei, like oxygen, have more protons and are therefore repelled more strongly from other nuclei. Thus faster speeds (at higher temperatures) are required to bring these nuclei together than are required for smaller nuclei. C) Free neutrons are required in greater numbers to enable the oxygen reaction, and this requires higher temperatures to produce them. D) The enormous neutrino flux in the core of a massive star inhibits nuclear reactions. High temperatures are necessary to force these neutrinos out of the star so nuclear reactions can proceed. Chapter 21 The diameter of a typical neutron star of 1 solar mass is predicted to be approximately A) 1 km. C) that of an average city, about 30 km. B) that of the Sun. D) that of Earth, 12,800 km. What stage of the evolutionary life of a star does the Crab Nebula represent? A) late stage, the remnant of a stellar explosion B) a black hole, the very late stages of evolution of a massive star C) a middle-age, main-sequence star, surrounded by an extended atmosphere D) the early, star-forming stage, surrounded by its protostar nebula The first pulsar was detected using A) the infrared satellite IRAS B) a British radio telescope C) the unaided eye, by Chinese astronomers D) the 200-inch telescope at Mount Palomar Pulsars are
A) pulsating white dwarfs. C) pulsating black holes. B) pulsating neutron stars. D) rotating neutron stars. We have seen that the generation of magnetic fields requires both a rapid rotation and a fluid interior for the flow of currents of charged particles. But neutrons are electrically neutral. So how can a neutron star generate a magnetic field? A) Neutrons in the neutron degeneracy situation within a neutron star acquire an electric charge. B) Because the neutrons in a neutron star flow through a superconducting medium, they do not need to carry an electric charge in order to constitute a current. C) The enormous amounts of friction which degenerate neutrons experience cause electric charges to build up on their surfaces, like static electricity. D) Neutron stars actually contain some protons and electrons, and these carry the electric currents. Pulsating X-ray sources with periods of a few seconds are caused by A) the pulsation in radius, temperature, and hence luminosity of a hot Cepheid variable star with a surface temperature hot enough to emit X-rays. B) the eclipsing of an X-ray-emitting star with a very hot surface by a cool companion in a close binary system. C) matter falling onto the surface of a very hot, rotating white dwarf star from an ordinary companion star in a binary system, producing an X-ray-emitting hot spot that disappears periodically behind the white dwarf. D) matter falling violently onto the surface of a rotating neutron star from a close companion in a binary star system, causing an X-ray hot spot that disappears periodically behind the neutron star. The X-ray bursts from an X-ray burster are caused by A) explosive hydrogen burning on the surface of a neutron star. B) hot spots caused by material falling onto the poles of a rotating neutron star. C) explosive photodisintegration of iron nuclei on the surface of a neutron star. D) explosive helium burning on the surface of a neutron star. What is the upper limit to the mass of a neutron star beyond which neutron degeneracy pressure is unable to withstand the force of gravity and the neutron star is crushed out of existence into a black hole? 1.4 solar masses C) about 100 solar masses 20 solar masses D) about 3 solar masses Chapter 22
You are standing on the gangplank of your spaceship on Mars when you see an identical spaceship go past Mars at 90% of the speed of light. When you look closely at this spaceship, how do you find that it compares to your own spaceship? A) The moving spaceship looks shorter, and time on it appears to run more slowly than on yours. B) The moving spaceship looks longer, and time on it appears to run faster than on yours. C) The moving spaceship looks shorter, and time on it appears to run faster than on yours. D) The moving spaceship looks longer, and time on it appears to run more slowly than on yours. In what way is the general theory of relativity more general (deals with more situations) than the special theory? A) It includes accelerated motion but not gravitation. B) It includes accelerated motion and gravitation. C) It includes only constant, unaccelerated motion. D) It includes only motion at the speed of light. How does a gravitational field affect the passage of time? A) Gravity has no effect on the passage of time. B) Clocks in a gravitational field run faster than clocks outside the field. C) Gravity makes time stop. D) Clocks in a gravitational field run slower than clocks outside the field. Black holes are so named because 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. Suppose that a neutron star of 2.8 solar masses is part of a binary star system in which the other star is a normal giant star. What would happen if half a solar mass of material were transferred onto the neutron star from its companion? A) The neutron star would collapse and become a black hole. B) The increased gravitational force would transform the neutrons into quarks, and the neutron star would re-establish equilibrium as a quark star of smaller diameter. C) The neutron degeneracy pressure inside the neutron star would increase to balance the increased gravitational force within the neutron star. D) The neutron star would explode as a supernova.
What appears to happen to a clock as it approaches and reaches the event horizon around a black hole, when viewed by a remote observer? A) It speeds up because of the intensified gravitational field. B) It appears to slow down and stop. C) Time appears to pass at a much faster rate, the rate becoming infinitely fast at the event horizon. D) It ticks uniformly, because nothing changes the progress of time. Chapter 23 Where is the solar system located in our galaxy? A) It is not in a galaxy, but in intergalactic space. B) in the galactic halo C) in the galactic disk D) in the galactic nucleus Where in space would you look for a globular cluster? A) in the Milky Way halo, orbiting the galactic center in a long elliptical orbit B) in the galactic disk, moving in a circular orbit around the galactic center C) in the asteroid belt D) only in elliptical galaxies, because they are composed of old stars Where would you look in our galaxy to find older, metal-poor stars? A) in the disk and spiral arms B) everywhere in the galaxy C) in the globular clusters in the galactic halo D) only at the galactic center In our galaxy, young metal-rich stars are found A) in the disk and spiral arms. B) everywhere in the galaxy. C) only at the galactic center. D) in the globular clusters, in the galactic halo. The stellar components of the galaxy that act as tracers for the mapping of spiral arm structure in the Milky Way are A) supernova explosions, because they are very luminous and can be seen through considerable dust and gas. B) old, red giant stars and white dwarfs. C) globular clusters. D) bright, population I stars and emission nebulae surrounding them.
Much of the mass of our galaxy appears to be in the form of dark matter of unknown composition. At present this matter can be detected only because A) it absorbs light from distant galaxies and quasars and obscures them. B) it emits synchrotron radiation. C) its gravitational pull affects orbital motions of matter in the galaxy. D) it blocks out the light from distant stars in the plane of our galaxy.