The correct answers are written in bold, italic and underlined. The most important questions to study for the exam are highlighted.

Transcription

1 Comins DEU 3e Ch 10 Quiz 1 The correct answers are written in bold, italic and underlined. The most important questions to study for the exam are highlighted. 1. A low-mass star expands and cools, becoming more luminous as it evolves from the main-sequence to the giant phase. In which direction will the star's position move on the Hertzsprung-Russell diagram? Horizontally, to the left, along the horizontal branch Up and to the right Up and to the left, along the main sequence Cooling moves the star to the right, while increased luminosity moves it upward. 2. Both red giant stars and asymptotic giant branch (AGB) stars are red and very large. Which statement below CORRECTLY describes the fundamental difference between them? Red giants are undergoing core helium fusion, while AGB stars are undergoing helium-shell fusion. They are in similar stages of their evolution, but red giants are low-mass stars, while AGB stars are high-mass stars. Red giants are undergoing hydrogen-shell fusion, while AGB stars are undergoing both hydrogen- and helium-shell fusion. Red giants go on to ignite core helium fusion and become horizontal branch stars; then after core helium fusion ends and a helium shell is ignited (with hydrogen shell fusion around it), they become AGB stars. 3. When a star ascends the red giant branch for the second time, it will have a quiescent core of helium and hydrogen fusion reactions occurring in a shell around this core. a quiescent core of carbon and oxygen and hydrogen and helium fusion reactions occurring in shells around this core. no fusion reactions going on anywhere in the star because it has used up its nuclear fuel. The hydrogen and helium originally in the core have been converted to carbon and oxygen, and gravitational energy has compressed the region outside the core and increased its temperature to the point where hydrogen and helium reactions can occur in shells surrounding the core. 4. In the entire lifetime of a low-mass star, how many kinds of thermonuclear reactions occur?

2 Two hydrogen and helium fusion One hydrogen fusion to produce helium Many hydrogen, helium, carbon, oxygen, and heavier element fusion to make even heavier nuclei The internal temperature in a low-mass star does not become great enough for fusion reactions to begin in the carbon and oxygen that result from helium fusion, so helium is the heaviest element that undergoes fusion. 5. Why do carbon fusion reactions never occur in the core of a low-mass star? The star explodes before carbon fusion can begin. The core never becomes hot enough. The required carbon is never produced in the core. Electron degeneracy carbon-oxygen core pressure prevents the core from becoming hot enough to start either carbon or oxygen fusion. 6. What happens after the asymptotic giant branch stage of a star's life? The star contracts relatively rapidly toward the main sequence. Helium fusion ends in the core, and fusion of both hydrogen and helium begins in shells around the core. Matter is ejected into space to form a planetary nebula. Shell helium flashes become strong enough to drive the matter off the star's surface, leaving behind a hot core that then forms a white dwarf. 7. How does a planetary nebula form? Episodes of thermal runaway in the helium-fusion shell of a low-mass star push the envelope of the star off into space. The expelled envelope forms a planetary nebula. The iron core of a high-mass star collapses, and the layers around it explode, driving the envelope off into space. The expelled envelope forms a planetary nebula. Dust and ice particles condense in the gas around a newly formed star and begin to accrete into planetesimals. This distribution of gas and accreting planetesimals forms a planetary nebula. This happens at the end of the life of a low-mass star like the Sun and is a slow expansion, not a violent explosion. 8. A planetary nebula appears as a glowing shell or cloud around a very hot white dwarf star. What are the physics and dynamics of this radiating region? The "shell" is simply the region of interstellar medium surrounding the star. The ultraviolet radiation from the star induces emission of atomic line radiation in the gas.

3 The shell is the original outer layers of the star being pushed into space. The intense ultraviolet radiation from the remaining core of this star ionizes the gas, exciting atoms to emit atomic line radiation. The shell is interstellar gas and dust that are slowly condensing onto the hot star. The ultraviolet radiation from the star is exciting the gas to emit visible radiation. The outer layers of the star are pushed off the star by radiation pressure and make up the shell of the planetary nebula. This process represents a late phase in the evolution of a low-mass star. 9. The visible "lifetime" of a typical planetary nebula is very long, billions of years, because it is a fixed shell of interstellar matter being illuminated by a white dwarf star whose age is this long. very short, about 100 years, because it represents the rapidly expanding shell of an exploding star, or supernova. relatively short, about 50,000 years, after which it fades from view as the gas of the expanding shell moves too far from the exciting central star. The planetary nebula shell expands slowly away from its source of excitation, the white dwarf star, and eventually ceases to emit light. 10. Where do white dwarf stars come from? They are the remnant left behind after mass loss during the red giant phase, before core helium ignition. They are the former cores of asymptotic giant branch stars that have pushed off their outer layers, leaving the core behind. They are the former cores of high-mass stars in which the core has collapsed, causing the outer layers to explode. The envelope is pushed off quite smoothly (not explosively), producing a planetary nebula around a white dwarf star. 11. As a white dwarf star slowly evolves over billions of years, its temperature remains fixed because it has stopped generating internal energy, but its size is slowly shrinking, and this leads to decreasing luminosity. it slowly cools and shrinks in size, while the release of gravitational potential energy maintains approximately constant luminosity. its temperature and its luminosity both decrease. Electron degeneracy prevents further reduction in size, but energy loss through radiation cools it, leading to decreased luminosity. 12. Which of the following household items is the closest analogy to a white dwarf star? A sodium vapor streetlight An incandescent light bulb

4 A hot metal bar The metal bar has no internal energy source and its "luminosity" and temperature are falling steadily. 13. What is the Chandrasekhar limit? The maximum mass that a star can have on the main sequence without eventually exploding as a supernova The minimum mass that a star can have to produce helium shell flashes and a planetary nebula The maximum mass that a white dwarf can have without collapsing under its own gravity A white dwarf cannot be more massive than about 1.4 solar masses, or it will collapse. 14. The Chandrasekhar limit is an important quantity in astronomy, equal to 4 solar masses, the minimum mass of a star that can begin carbon nuclear reactions in its core. 1.4 solar masses, the maximum mass of a white dwarf star. 8 solar masses, the maximum mass a main sequence star can have and still end its life quietly (without becoming a supernova). If the mass of a star, or even the electron-degenerate core of a more massive star, exceeds the Chandrasekhar limit, then electron degeneracy cannot prevent core collapse. The star then proceeds to the supernova stage (immediately, for a white dwarf, or after some intervening stages for a more massive star). 15. When a nova occurs, we see a star suddenly brighten dramatically and then fade again. What causes this to happen? A white dwarf is destroyed by a cataclysmic explosion after mass is transferred to it from a companion in a binary star system. The outermost layer of a white dwarf is blasted into space by nuclear reactions, without harming the white dwarf itself. Mass is blasted into space from the surface of a neutron star after being transferred there from a companion in a binary star system. The exploding layer consists of material that was transferred to the white dwarf from a companion in a binary star system. The explosion occurs only in the outermost layer and leaves the white dwarf intact. 16. As intermediate- and high-mass stars evolve, their cores undergo stages of nuclear fusion reactions involving successively heavier and heavier elements (helium, carbon, oxygen, silicon). How do the lifetimes of these stages compare to each other? The lifetimes become longer as the nuclei become heavier. The lifetimes are approximately the same for all stages of nuclear fusion. The lifetimes become shorter as the nuclei become heavier.

5 Because of core contraction, each stage occurs at a higher temperature than the previous stage, and therefore the fusion occurs more rapidly. Also, the number of nuclei available to react is smaller, which further reduces the lifetime of the stage. 17. When do core oxygen fusion reactions occur in the evolution of a massive star? After carbon fusion and before silicon fusion After silicon fusion After helium fusion and before carbon fusion Oxygen nuclei are heavier than carbon nuclei and have a higher charge. Therefore, they require a higher temperature to react. Because the core heats up over time, the carbon will react first (at a lower core temperature) and the oxygen will react later, when the core temperature is higher. 18. For which of the following atomic species can energy generation NOT occur by nuclear fusion? Carbon, C Iron, Fe Silicon, Si Iron nuclei do not produce energy when they fuse together. In fact, they need extra energy to do so. 19. During a supernova explosion involving a high-mass star, what happens to the iron in the core of the star? The iron nuclei are broken up by high-energy photons into their individual protons and neutrons (photodisintegration). Iron is inert and does not react, so it remains as a dense iron core after the explosion. The iron is thrown out into space, enriching the interstellar medium. The temperature reaches about 5 billion K during the core collapse that initiates the supernova explosion, and at this temperature the gamma-ray photons are energetic enough to photodisintegrate the iron nuclei into protons and neutrons. 20. What causes a massive star to explode as a supernova? Fusion reactions produce a core of iron in the star, and this core collapses when its mass gets too large. Oxygen in the core of the star ignites at a temperature of over 1 billion K, and undergoes explosive fusion reactions, disrupting the star. Thermal flashes in the helium-fusion shell cause disruption of the star's envelope, with the core being left behind as a white dwarf. Iron does not react to produce heavier elements by fusion, so the amount of iron continues to increase until instability occurs.