ENERGY GENERATION IN STARS & STELLAR EVOLUTION ---------------------------------------------- Energy generation in stars: -------------------------- Why does fusion of hydrogen release energy? a) Fusion breaks the electromagnetic bonds between hydrogen atoms, releasing energetic photons. $b) The mass of a helium nucleus is smaller than the mass of four protons. c) The mass of a helium nucleus is larger than the mass of four protons. d) The velocity of four protons is larger than the velocity of a helium nucleus. e) None of the above are true. Why would two protons combine to form an atom of deuterium (heavy hydrogen) in the core of a star like the Sun? a) The electromagnetic force strongly attracts the protons. b) The gravitational force strongly attracts the protons. $c) The velocity of protons in the core of the Sun is very large. d) Protons never combine to form deuterium in the core of the Sun. e) Both a and c. Fusion in the core of a stable massive star cannot proceed beyond iron because: a) it would require temperatures that even stars cannot generate. b) the fusion of iron nuclei is impossible under any circumstances. $c) iron nuclei are the most tightly bound of all nuclei so iron fusion does not release energy. d) the Chandrasekhar limit has been reached. After the Sun's core hydrogen is depleted by nuclear fusion the core will consist primarily of a) carbon b) deuterium $c) helium d) oxygen The solar corona has temperatures roughly the same level as temperatures in the Sun's core, where nuclear fusion takes place. Why doesn't fusion occur in the corona? $a) The density in the corona is too low. b) The corona has too many free electrons. c) Atoms in the corona are mostly ionized. d) The corona has more heavy atoms than the core. e) Two of the above. Fusion in the core of a main sequence star changes the chemical composition in the core. What happens to the chemical composition outside the core? a) We have no way of finding out. $b) The chemical composition outside the core changes very little. c) The same changes occur outside the core as within the core. d) Hydrogen becomes more abundant outside the core. Which of the following would lengthen the amount of time a star is able to fuse hydrogen at its center? a) The temperature in the core of the star is increased. $b) The gas in the core of the star is enriched in hydrogen. c) The mass of the star is increased. d) none of the above About 25\% of the mass of a newborn star is initially in helium. Why doesn't helium also fuse on the main sequence? I. Helium nuclei travel more slowly on average than hydrogen nuclei at any temperature II. Helium nuclei repel each other with more force than do hydrogen nuclei. III. Helium fusion requires three helium nuclei to hit each other almost simultaneously. a) I b) II c) III d) I, II e) II, III $f) I, II, III If the rate of hydrogen fusion within the Sun were to increase, the core of the Sun would a) contract and decrease in temperature. b) expand and increase in temperature. $c) expand and decrease in temperature. d) stay the same size but increase in temperature. If the temperature in the core of the Sun increased, which of the following would occur? a) The rate of nuclear reactions in the core would increase. b) The radiation pressure in the core would increase. c) The core of the star would expand. d) The temperature in the core would decrease. $e) All of the above. If the core of the Sun were somehow kept extremely cold for a long time, the Sun would a) freeze into ice b) expand to about the radius of Earth's orbit $c) collapse to about the size of the Earth d) collapse to about the size of a large city A celestial body growing by accretion of material must surpass a certain mass before hydrogen fusion begins in the core, making it a star. If the strength of the charge on the proton were to be increased, then that minimum mass $a) must increase b) must decrease c) would not change Stellar evolution: ----------------- If two stars are on the main sequence, and one is more luminous than the other, we can be sure that the a) more luminous star will have the longer lifetime b) fainter star is the more massive $c) more luminous star is the more massive d) more luminous star will have the redder color When a star becomes a red giant it becomes much brighter because it is a) moving closer to us b) losing its outer envelope c) fusing iron in its core $d) increasing in size As a one solar mass star evolves into a red giant, its a) surface temperature and luminosity increase. b) surface temperature and luminosity decrease. c) luminosity decreases while the surface temperature increases. $d) luminosity increases while the surface temperature decreases. After hydrogen fusion stops in the core of a star, the core a) cools and the star as a whole expands. b) cools and the star as a whole contracts. $c) heats and the star as a whole expands. d) heats and the star as a whole contracts. A star is burning hydrogen to helium in its core and has ten times the mass of the Sun. Which of the following are true? a) The surface temperature of the star is smaller than that of the Sun. b) The star is redder, more luminous, and larger than the Sun. $c) The star is bluer, larger, and more luminous than the Sun. d) both a and b. e) both a and c. A star evolves off the main sequence when a) nuclear reactions begin in the core of the star. $b) hydrogen is exhausted in the core of the star. c) hydrogen is exhausted everywhere in the star. d) helium is exhausted in the core of the star. Giant stars are rare because a) they do not form as often as main sequence stars. b) giant stars are unstable. $c) the giant stage is very short compared to the main sequence stage. d) elements heavier than helium are relatively rare. Which of the following stars is probably oldest? a) A one solar mass main sequence star. $b) A one solar mass white dwarf. c) A ten solar mass main sequence star. d) A ten solar mass red giant. After the first 20 billion years of the Universe, the Sun will have evolved through red giant, main sequence, and white dwarf stages. Assuming a current radius of 1, what radii will the Sun have during these phases, in correct order? $a) 1, 400, 0.01 b) 0.01, 1, 400 c) 1, 1, 1 d) 1, 0.01, 400 After a star has evolved into a red giant, hydrogen burning a) ceases completely. b) happens only in the center of the star. $c) happens only in shells outside the core of the star. d) happens only during novae. Which of these stars will end its main sequence lifetime most rapidly? $a) A very massive star, since more massive stars consume their hydrogen more rapidly. b) A low-mass star, since less massive stars have less hydrogen to burn. c) A star like the Sun, since the combination of fuel-use rate and available fuel amount peaks at about spectral class G. d) none of the above; all stars have main sequence lifetimes of about 10 billion years. The event that marks the end of a star's evolutionary life before becoming a white dwarf is a) a nova. $b) a planetary nebula. c) the exhaustion of hydrogen in the core. d) a helium flash. The force of gravity acts to a) make a star larger. $b) make a star smaller. c) make a star cooler. d) none of these. When during a star's evolution its core gets smaller, the rest of the star typically a) also gets smaller b) stays the same size $c) gets larger d) explodes in a supernova Suppose in a given region of the sky, you see a red star and a blue star. The two are not parts of binary systems, and both stars look pretty typical for their colors. Which of the following is true? a) The red star is older than the blue star. b) The red star is younger than the blue star. $c) The stellar ages cannot be determined from the information given. The helium burning phase for a star of a given mass is much shorter than the hydrogen burning phase primarily because a) the helium mass fraction in the core is less than the hydrogen mass fraction was when the star was young. $b) helium releases less energy per reaction than hydrogen. c) the star becomes a white dwarf before it can use most of its helium. d) the temperature never rises high enough for complete helium burning. A star remains at constant size and temperature for a long period of time. Which of the following is most likely to be true? The star generates a) more energy than it radiates into space. $b) about as much energy as it radiates. c) less energy than it radiates into space. You are an immortal alien being, hiding in the photo archive room of a library on Earth. You can best learn about the life cycles of people by bringing home the drawer filled with photographs of a) individuals $b) crowds on the street c) people lined up at the voting both d) doctors Black Holes: ----------- A black hole is best defined as a) a star which sucks all matter into itself. b) a window to another Universe. $c) any object which is smaller than its event horizon. d) the final result of all stellar evolution. Which of the following can escape from inside the event horizon of a black hole? a) Particles of matter b) Particles of antimatter c) Visible light d) X-rays $e) None of the other answers is correct Isolated black holes slowly evaporate because they slowly leak mass via a) faster-than-light particles that can escape $b) virtual particles that form near the event horizon d) nuclear fusion near the event horizon c) holes in the event horizon The best current theories about an isolated black hole suggests that radiation a) is impossible because it a hole b) is impossible because the escape velocity exceeds the speed of light c) is lower from smaller black holes $d) is higher from smaller black holes