Astro 115 Spring 20000
Homework # 5 DUE MAY 9
Put your answers on a standard scantron and hand it in on May 9. Turn in ONLY
The Scantron!
1. Protostars are difficult to observe because
a. the protostar stage is very short.
b. they are surrounded by cocoons of gas and dust.
c. they radiate mainly in the infrared.
d. all of the above
e. they are all so far away that the light hasn't reached us yet.
2. The nuclear reactions in a star's core remain under control so long as
a. luminosity depends on mass.
b. pressure depends on temperature.
c. density depends on mass.
d. weight depends on temperature.
e. temperature depends on mass.
3. The proton-proton chain needs high temperature because
a. of the ground state energy of the hydrogen atom.
b. of the presence of helium atoms.
c. the protons must overcome the Coulomb barrier.
d. of the need for low density.
e. the neutrinos carry more energy away than the reaction produces.
4. Which of the following have been suggested as explanations of the missing solar neutrinos?
I. The sun is fusing helium but not hydrogen.
II. Nuclear reactions do not produce neutrinos as fast as theory predicts.
III. The sun may contain WIMPs.
IV. Neutrinos may oscillate between three different flavors.
a. only I b. only II c. III & IV d. II, III, & IV
e. I, II, & IV
5. Interstellar gas clouds may collapse to form stars if they
a. have very high temperatures.
b. encounter a shock wave.
c. rotate rapidly.
d. are located near main sequence spectral type K and M stars.
e. all of these.
6. _____________ are small luminous nebulae excited by nearby young stars.
a. T Tauri stars b. Herbig-Haro objects c. O associations
d. WIMPs e. Giant molecular clouds
7. Opacity is
a. the balance between the pressure and force of gravity inside a star.
b. the force that binds protons and neutrons together to form a nucleus.
c. the force that binds an electron to the nucleus in an atom.
d. a measure of the ease with which photons can pass through a gas.
e. the temperature and density at which a gas will undergo thermonuclear fusion.
8. The region of the sun just below the photosphere
a. is undergoing thermonuclear fusion using the proton-proton chain.
b. is undergoing thermonuclear fusion using the CNO cycle.
c. is transporting energy to the photosphere by convection.
d. is not in hydrostatic equilibrium.
e. Both is undergoing thermonuclear fusion using the proton-proton chain, and is transporting energy to the photosphere by convection.
9. ______________ is the thermonuclear fusion of hydrogen to form helium operating in the cores of massive stars on the main sequence.
a. The CNO cycle b. The proton-proton chain
c. Hydrostatic equilibrium d. The neutrino process e. none of these
10. The Great Nebula in Orion
a. is a Herbig-Haro object. b. is a reflection nebula.
c. is an emission nebula. d. contains only young low mass stars.
e. is believed to be about 5 billion years old.
11. The capture of too few solar neutrinos by Davis in the solar neutrino experiment
a. has been disproven by the results of later experiments.
b. can be explained if the sun is not undergoing thermonuclear fusion of hydrogen in its core.
c. indicates that the sun's core is much hotter than expected.
d. indicates that the sun's core is convective.
e. none of these
12. If the sun produces energy by the proton-proton chain, then the center of the sun must have a temperature of at least
a. 104 K b. 107 K c. 1010 K d. 1013 K e. 1016 K
13. What causes the outward pressure that balances the inward pull of gravity in a star?
a. The outward flow of energy. b. The opacity of the gas.
c. The temperature of the gas d. The density of the gas
e. Both the temperature of the gas, and the density of the gas.
14. The free-fall contraction of a molecular cloud
a. can be initiated by shock waves from supernovae.
b. can be initiated by nearby spectral type G stars.
c. can be initiated by the rotation of the cloud.
d. causes the cloud to become transparent to ultraviolet radiation.
e. causes the particles in the cloud to decrease the speed with which they move.
15. Atoms of cool hydrogen emit 21-cm radiation when the
a. electrons move from the fourth to the second energy level.
b. electrons move from the second to the fourth energy level.
c. protons in neutral helium change their spin.
d. electrons in neutral hydrogen change their spin.
e. hydrogen atoms adds a second electron to form H-.
16. Molecular clouds can be observed
a. using infrared telescopes to detect ionized gas in the clouds.
b. using x-ray telescope to observe the x-ray radiation from the molecules in the cloud.
c. using radio telescopes to observe the CO emission from the clouds.
d. by looking for blue wispy regions near star clusters.
e. by looking for the 21-cm radiation from hydrogen.
17. The main sequence has a limit at the lower end because
a. low mass stars form from the interstellar medium very rarely.
b. low mass objects are composed primarily of solids, not gases.
c. pressure does not depend on temperature in degenerate matter.
d. the lower limit represents when the radius of the star would be zero.
e. there is a minimum temperature for hydrogen fusion.
18. There is a mass-luminosity relation because
a. hydrogen fusion produces helium.
b. stars expand when they become giants.
c. stars support their weight by making energy.
d. the helium flash occurs in degenerate matter.
e. all stars on the main sequence have about the same radius.
19. The lowest mass object that can initiate thermonuclear fusion of hydrogen has a mass of about
a. 1 Msun b. 60 Msun c. 0.5 Msun d. 0.08 Msun e. 0.001 Msun
20. What is the lifetime of a 10 <<<GRAPHIC>>> star on the main sequence?
a. 3.2 ´ 107 years b. 320 years c. 3.2 ´ 1012 years
d. 1 ´ 109 years e. 1 ´ 1011 years
21. Stars are born in
a. reflection nebulae. b. dense molecular clouds.
c. HII regions. d. the intercloud medium. e. the local bubble.
22. As a star exhausts hydrogen in its core, it
a. becomes hotter and more luminous. b. becomes cooler and more luminous.
c. becomes hotter and less luminous. d. becomes cooler and less luminous.
e. it becomes larger in radius and hotter.
23. A star will experience a helium flash if
a. it is more massive than about 6 solar masses.
b. its core contains oxygen and helium.
c. its mass on the main sequence was less than 0.1 solar masses.
d. it is a supergiant.
e. its core is degenerate when helium ignites.
24. In degenerate matter
a. pressure depends only on the temperature.
b. temperature depends only on density.
c. pressure does not depend on temperature.
d. pressure does not depend on density.
e. Both temperature depends only on density, and pressure does not depend on temperature.
25. Giant and supergiant stars are rare because
a. they do not form as often as main sequence stars.
b. the giant and supergiant stage is unstable.
c. the giant and supergiant stage is very short.
d. helium is very rare.
e. helium flash destroys many of the stars before they can become giants and supergiants.
26. The triple alpha process
a. controls the pulsations in Cepheid variable stars.
b. is the nuclear fusion of hydrogen to helium in massive stars.
c. is the process that produces the neutrinos we receive from the sun.
d. requires a temperature of about 5,000,000 K to operate.
e. occurs during helium flash.
27. Which of the following nuclear fuels does a one solar mass star use over the course of its entire evolution?
a. hydrogen b. hydrogen and helium
c. hydrogen, helium and carbon d. hydrogen, helium, carbon, and neon
e. hydrogen, helium, carbon, neon, and oxygen.
28. Helium flash occurs
a. because helium is very explosive and cannot be controlled when the nuclear reactions occur.
b. because degenerate electrons in the core do not allow the core to expand as it heats up.
c. in Cepheid variables. d. in stars with masses less than 0.4 Msun
e. none of these
29. Stars in a star cluster
a. all have the same age. b. all have the same chemical composition.
c. all have the same luminosity. d. all of these
e. Both all have the same age, and all have the same chemical composition.
30. The lowest-mass stars cannot become giants because
a. they do not contain helium. b. they rotate too slowly.
c. they cannot heat their centers hot enough. d. they contain strong magnetic fields. e. they never use up their hydrogen.
31. A planetary nebula is
a. the expelled outer envelope of a medium mass star. b. produced by a supernova explosion. c. produced by a nova explosion.
d. a nebula within which planets are forming. e. a cloud of hot gas surrounding a planet.
32. The Chandrasekhar limit tells us that
a. accretion disks can grow hot through friction.
b. neutron stars of more than 3 solar masses are not stable.
c. white dwarfs must contain more than 1.4 solar masses.
d. not all stars will end up as white dwarfs.
e. stars with a mass less than 0.5 solar masses will not go through helium flash.
33. A Type I supernova is believed to occur when
a. the core of a massive star collapses. b. carbon detonation occurs.
c. a white dwarf exceeds the Chandrasekhar limit.
d. the cores of massive stars collapse.
e. neutrinos in a massive star become degenerate and form a shock wave that explodes the star.
34. Massive stars cannot generate energy through iron fusion because
a. iron fusion requires very high density.
b. stars contain very little iron.
c. no star can get hot enough for iron fusion.
d. iron is the most tightly bound of all nuclei.
e. massive stars supernova before they create an iron core.
35. The theory that the collapse of a massive star's iron core produces neutrinos was supported by
a. the size and structure of the Crab nebula.
b. laboratory measurements of the mass of the neutrino.
c. the brightening of supernovae a few days after they are first visible
d. underground counts from solar neutrinos.
e. the detection of neutrinos from the supernova of 1987.
36. Synchrotron radiation is produced by
a. objects with temperature below 10,000 K.
b. high-velocity electrons moving through a magnetic field.
c. cold hydrogen atoms in space.
d. the collapsing cores of massive stars.
e. helium flash.
37. A nova is almost always associated with
a. a very massive star. b. a very young star.
c. a star undergoing helium flash.
d. a white dwarf in a close binary system.
e. a solar like star that has exhausted its hydrogen and helium.
38. A white dwarf is composed of
a. hydrogen nuclei and degenerate electrons.
b. helium nuclei and normal electrons.
c. carbon and oxygen nuclei and degenerate electrons.
d. degenerate iron nuclei.
e. a helium burning core and a hydrogen burning shell.
39. The density of a neutron star is
a. about the same as that of a white dwarf.
b. about the same as that of the sun.
c. about the same as an atomic nucleus.
d. about the same as a water molecule.
e. smaller than expected because the magnetic field is so strong.
40. A neutron star is expected to spin rapidly because
a. they conserved angular momentum when they collapsed.
b. they have high orbital velocities.
c. they have high densities.
d. they have high temperatures.
e. the energy from the supernova explosion that formed them made them spin faster.
41. Although neutron stars are very hot, they are not easy to locate because
a. light does not escape from their event horizon.
b. most lie beyond dense dust clouds.
c. solid neutron material cannot radiate photons.
d. they are only found in other galaxies.
e. they have small surface areas.
42. In A. D. 1054, Chinese astronomers observed the appearance of a new star, whose location is now occupied by
a. a pulsar. b. a neutron star. c. a supernova remnant.
d. all of these e. Both a pulsar, and a neutron star.
43. Pulsars cannot be spinning white dwarfs because
a. white dwarfs are not that common.
b. white dwarfs are not dense enough.
c. white dwarfs do not have magnetic fields.
d. a white dwarf spinning that fast would fly apart.
e. all of these
44. Pulsars are believed to slow down because
a. they are converting energy of rotation into radiation.
b. they are dragging companions stars around in their magnetic field.
c. of friction with the interstellar medium.
d. of conservation of angular momentum.
e. their mass is decreasing.
45. The event horizon
a. is believed to be a singularity.
b. is a crystalline layer.
c. has a radius equal to the Schwarzschild radius.
d. marks the inner boundary of a planetary nebula.
e. is located at the point where synchrotron radiation is created around a pulsar.
46. An isolated black hole in space would be difficult to detect because
a. there would be no light source nearby.
b. it would not be rotating rapidly.
c. it would be stationary.
d. very little matter would be falling into it.
e. there would be very few stars behind it whose light it could block out.
47. Cygnus X-1 and LMC X-3 are black holes if the masses of the unseen companions are
a. less than 5 solar masses. b. more than 5 solar masses.
c. between 0.4 and 1.4 solar masses. d. less than 0.4 solar masses
e. not larger than the masses of the stars that we can see.
48. Which of the following objects is considered to possibly contain a black hole?
a. The central star of the Crab nebula b. The Orion nebula.
c. LMC X-3 d. Algol e. PSR 1257+12
49. The density of a _________ is greater than the density of a ____________.
a. white dwarf neutron star
b. neutron star black hole
c. pulsar neutron star
d. pulsar white dwarf
e. white dwarf black hole
50. The search for black holes involves searching for
a. single stars that emit large amounts of x-rays.
b. x-ray binaries where the compact companion has a mass in excess of 3 Msun
c. large spherical regions from which no light is detected.
d. pulsars with periods less than one millisecond.
e. pulsars that are orbited by planets.