Ch. 16 Practice Test Questions – Flashcards
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Which two processes can generate energy to help a star or gas cloud maintain its internal thermal pressure?
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nuclear fusion and gravitational contraction
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About what percentage of the mass of a molecular cloud is in the form of dust?
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1%
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How do we learn the chemical composition of the interstellar medium?
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By studying spectra of interstellar gas clouds.
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What happens to the visible light radiated by stars located within a dusty gas cloud?
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It is absorbed by dust, which heats the dust grains so that they emit the absorbed energy as infrared light.
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Under which circumstances can you be sure that the thermal pressure within a gas cloud is increasing?
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the cloud's temperature and density are both increasing
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Which process is required to allow a gravitationally-collapsing gas cloud to continue to collapse?
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The cloud must radiate away much of its thermal energy.
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According to current understanding, how did the first generation of stars differ from stars born today?
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They were much more massive than most stars born today.
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Angular momentum plays an important role in star formation. Which of the following characteristics of a protostellar system is probably not strongly affected by the star's angular momentum?
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the onset of core hydrogen fusion
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Close binary star systems are thought to form when _____.
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gravity pulls two neighboring protostars quite close together, but angular momentum causes them to orbit each other rather than colliding.
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Generally speaking, how does the surface temperature and luminosity of a protostar compare to the surface temperature and luminosity of the main-sequence star it becomes?
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A main-sequence star is hotter and dimmer than it was as a protostar.
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Where does a 1-solar-mass protostar appear on an H-R diagram?
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to the right of the main sequence, and higher up than the Sun
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Why does the rotation of a protostar slow down over time?
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Magnetic fields can transfer angular momentum to the protostellar disk and protostellar winds can carry angular momentum away.
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During the time before fusion begins, a protostar's core gradually ________.
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shrinks and heats
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The core of a protostar that will eventually become a brown dwarf shrinks until ______.
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the type of pressure called degeneracy pressure becomes important
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If a star is extremely massive (well over 100 solar masses), why isn't it likely to survive for long?
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It may blow itself apart because of radiation pressure.
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Consider a large molecular cloud that will give birth to a cluster of stars. Which of the following would you expect to be true?
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A few massive stars will form, live, and die before the majority of the star's clusters even complete their protostar stage.
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We do not know for certain whether the general trends we observe in stellar birth masses also apply to brown dwarfs. But if they do, then which of the following would be true?
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Brown dwarfs would outnumber all ordinary stars.
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Where would a brown dwarf be located on an H-R diagram?
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below and to the right of the lowest part of the main sequence
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What do we mean by the interstellar medium?
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the gas and dust that lies in between the stars in the Milky Way galaxy
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The interstellar clouds called molecular clouds are _______.
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the cool clouds in which stars form
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Which of the following types of molecule is the most abundant in an interstellar molecular cloud?
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H2
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Interstellar dust consists mostly of _____.
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microscopic particles of carbon and silicon
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Which part of the electromagnetic spectrum generally gives us our best views of stars forming in dusty clouds?
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infrared
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Suppose you look at a star that can be seen through the edge of a dusty interstellar cloud. The star will look _______ than it would if it were outside the cloud.
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dimmer and redder
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Most interstellar clouds remain stable in size because the force of gravity is opposed by _______ within the cloud.
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thermal pressure
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What kind of gas cloud is most likely to give birth to stars?
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a cold, dense gas cloud
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How are magnetic fields thought to affect star formation in molecular clouds?
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They can help resist gravity, so that more total mass is needed before the cloud can collapse to form stars.
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Which of the following statements is probably true about the very first stars in the universe?
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They were made only from hydrogen and helium.
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What is a protostar?
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a star that is still in the process of forming
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Which of the following phenomena is not commonly associated with the star formation process?
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intense ultraviolet radiation coming from a protostar
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What law explains why a collapsing cloud usually forms a protostellar disk around a protostar?
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conservation of angular momentum
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What can we learn about a star from a life track on an H-R diagram?
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the surface temperature and luminosity the star will have at each stage of its life
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When does a protostar become a main-sequence star?
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when the rate of hydrogen fusion becomes high enough to balance the rate at which the star radiates energy into space
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Approximately what core temperature is required before hydrogen fusion can begin in a star?
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10 million K
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Which star spends the longest time in the protostellar phase of life?
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a 1 solar mass star
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What is the approximate range of masses that newborn main sequence stars can have?
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0.1 to 150 solar masses
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The vast majority of stars in a newly formed star cluster are ______.
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less massive than the Sun
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Which of the following statements about brown dwarfs is not true?
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Brown dwarfs eventually collapse to become white dwarfs.
