16) A supernova explosion occurs when A. the core of a massive star collapses in the attempt to ignite iron B. the core of a massive star begins to burn iron into iron into uranium C. a neutron star becomes a cepheid D. tidal forces from one of the stars in a binary tear the other apart E. the core of a massive star expands from rapid nuclear fusion

17) High-mass stars have ___________ lifetimes than low-mass stars because __________. A. longer; they have more fuel to burn B. shorter; although they have more fuel, they burn it a much higher rate C. shorter; nuclear reactions proceed at a lesser rate D. longer; nuclear reactions proceed at a higher rate E. none of these is correct

18) A white dwarf star is A. burning hydrogen B. burning iron C. burning helium D. burning uranium E. in the process of cooling

19) Stars with a cluster differ from each other mostly in their A. distance apart B. mass C. age D. chemical composition E. All of the above

20) During the main sequence stage of a star, the property of the star that changes the most is its A. mass B. radius C. surface temperature D. composition at the core E. composition at the surface

21) The more massive a star is the A. more luminous it is during its lifetime on the main sequence B. shorter its lifetime on the main sequence C. more likely it will end up as a black hole D. all of the above E. both A and B only

22) To initiate nuclear fusion, a high temperature is required near the center of a star to overcome the A. electrical repulsion between protons B. nuclear force between protons C. nuclear force between neutrons D. gravitational force between all the particles E. high pressure due to high particle density