#3 – Space

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The space between stars is known to contain
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gas, both atomic and molecular, and dust.
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Which of the following objects is NOT an abundant physical component of the interstellar medium?
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radioactive elements
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Similar star clusters at different distances from the Sun appear to be fainter than expected by distance alone because
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light is scattered and absorbed by interstellar dust and gas between distant clusters and the Earth.
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Interstellar extinction is the
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reduction of the apparent brightness of stars by scattering and absorption of their light by intervening interstellar clouds.
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What do a reflection nebula surrounding a star and Earth’s atmosphere have in common?
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The reflection nebula and Earth’s atmosphere both appear blue because of preferred scattering of this color of light.
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The apparent reddening of light from stars after its passage through the interstellar medium (ISM) is caused by
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preferential scattering of blue starlight by fine dust grains.
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The light from a distant cloud of gas and dust looks distinctly red to the unaided eye. When a spectrum is taken, the red color is found to come from a single, bright spectral line. Thus the red color in this situation is due to
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the Balmer spectrum of hydrogen.
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How much of the visible mass of the Milky Way Galaxy is in the form of gas and dust spread out between the stars?
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10%
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What are the two most abundant elements in the universe?
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hydrogen and helium
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In which part of the electromagnetic spectrum are molecules most easily detected?
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radio waves
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What fraction of the mass of a typical interstellar cloud is hydrogen?
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74%
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What fraction of the mass of a typical interstellar cloud is helium?
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25%
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Which of the following molecules is likely to be the most common in interstellar space?
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H2, molecular hydrogen
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Which of the following common molecules found in interstellar space contains nitrogen atoms but no oxygen?
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ammonia (NH3)
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How is gas distributed in interstellar space?
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in clumps, concentrated in interstellar clouds
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What is the typical mass of a giant molecular cloud?
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100,000 to 1 million solar masses
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The mass of a particular giant molecular cloud in interstellar space is 2 million solar masses. What is the mass of the helium in this cloud?
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500,000 solar masses
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What is the characteristic color of a reflection nebula?
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blue
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Evidence of massive amounts of hydrogen gas surrounding some stars comes from
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observation of emission of characteristic red Balmer light from nebulosity around them.
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Long-exposure color photographs of the night sky often show regions that glow red, such as the Rosette Nebula. This distinctive red color is caused by the
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ionization and subsequent recombination of hydrogen atoms.
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What is the ultimate fate of an open star cluster?
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The stars in the cluster escape one by one until the cluster no longer exists.
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New stars are formed from
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huge, cool dust and gas clouds.
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How does the temperature of an interstellar cloud affect its ability to form stars?
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Higher temperatures inhibit star formation.
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Which of the following mechanisms is NOT considered to be a way in which star formation is triggered or started?
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heating of an interstellar cloud by radiation from embedded young stars
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Which of the following mechanisms is thought to be ineffective and inefficient in the triggering of star birth in molecular clouds?
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gravitational contraction of a hot gas cloud
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What is a protostar?
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sphere of gas after collapse from an interstellar cloud but before nuclear reactions have begun
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At what stage in its life does a star pass through the protostar phase?
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after condensation but before nuclear reactions begin in its core
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Where are protostars most likely to form?
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dark nebulae
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Protostars are
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very young objects still contracting before becoming true stars.
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The source of a protostar’s heat is
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gravitational energy released as the star contracts.
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What is believed to be the most important factor determining whether a collapsing region (dense core) in an interstellar cloud becomes a single-star or a multiple-star system?
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amount of rotation (spin)
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Accretion of matter in an interstellar cloud leads to
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a protostar.
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What is a protostar called in the stage after it has finished accreting mass?
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pre-main-sequence star
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What point defines the end of the pre-main-sequence phase of a star’s life and the start of the main-sequence phase?
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Nuclear reactions begin in its core.
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In which region of the Hertzsprung-Russell diagram does a newly formed protostar first appear when it begins to shine at visible wavelengths?
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right side; relatively large luminosity because of its size but cool
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A main-sequence star can be no smaller than 0.08 solar mass. The reason is that
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the temperature in a contracting protostar of less than 0.08 solar mass does not become high enough for nuclear reactions to start.
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A brown dwarf is a(n)
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object intermediate between a planet and a star, with not enough mass to begin nuclear reactions in its core.
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The T Tauri phase of a protostar is characterized by
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strong spectral emission lines.
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What is believed to be the maximum mass a star can have?
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about 150 solar masses
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What is believed to prevent stars from being larger than a few hundred solar masses?
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The temperature becomes so high that the excess mass is pushed back into space by radiation from the star.
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What is the difference between an extrasolar planet and a sub-brown dwarf?
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An extrasolar planet orbits a star; a sub-brown dwarf does not. Otherwise, they are the same.
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Which of the following properties is NOT a characteristic of T Tauri stars?
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nuclear reactions in the core
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Which of the following facts referring to stars in a cluster is NOT particularly useful for interpreting the evolution of these stars?
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The majority of the material in these stars is hydrogen.
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The stars in an open cluster are useful for studying the early stages of stellar evolution because all the stars in a cluster have the same
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age.
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The definition of a main-sequence star is a star
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in which nuclear fusion reactions generate sufficient energy to oppose further condensation of the star.
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The stars that last longest are the stars
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with the smallest mass.
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Which of the following statements about the rate of stellar evolution is true?
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The more massive the original star, the faster is the evolution.
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Because all the stars in a cluster begin forming at the same time, the stars arrive on the main sequence at
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different times, more massive stars arriving first.
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If we plotted the stars in a young star cluster on a Hertzsprung-Russell diagram, we would expect to see
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the more massive stars on the main sequence and the less massive stars above the main sequence.
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In the Hertzsprung-Russell diagram, how does the position of a typical star change while it is at the main-sequence phase of its evolution?
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A star’s position on the main sequence is determined only by its mass, not its age, so stars do not move along the main sequence during evolution.

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