Astronomy ~ Chapter 16 – Flashcards
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Astronomers estimate that new stars form in our galaxy at present at the rate of about
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a few per year
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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 galaxies.
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By mass, the interstellar medium in our region of the Milky Way consists of:
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70% Hydrogen, 28% Helium, 2% heavier elements.
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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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The interstellar clouds that are called molecular clouds are:
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the cool clouds in which stars form.
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The typical density and temperature of molecular clouds are:
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300 molecules per cubic centimeter, 10-30 Kelvin.
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The most abundant molecule in molecular clouds is
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molecular hydrogen
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What percentage of a molecular cloud's mass is interstellar dust?
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1%
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Interstellar dust consists mostly of
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particles of carbon and silicon.
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The typical size of an interstellar dust grain is
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about the size of soot and smoke particles.
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What is interstellar reddening?
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Interstellar dust absorbs more blue light than red light, making stars appear redder than their true color.
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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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If you wanted to observe stars behind a molecular cloud or stars forming within it, in what wavelength of light would you most likely observe?
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infrared
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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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What is a protostar?
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a star that is still in the process of forming
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What 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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What happens to the surface temperature and luminosity when gravity first assembles a protostar from a collapsing cloud?
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Its surface temperature and luminosity increase.
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About how many times more luminous than our Sun is a young solar mass protostar just beginning convective contraction?
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10-100
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Where does a 1-solar-mass protostar first 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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The surface of a protostar radiates energy while its core
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shrinks and heats
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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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When does a protostar become a true star?
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when nuclear fusion begins in the core
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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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How long does the protostellar stage last for a star like our Sun?
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30 million years
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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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Which of the following may be caused by a protostellar disk? - protostellar jets - protostellar winds - accretion of material onto the star - relatively slow protostellar rotation - all of the above
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all of the above
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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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Why does the rotation of a protostar slow down over time?
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Magnetic fields and stellar winds can transfer angular momentum to the protostellar disk and protostellar winds can carry angular momentum away.
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What happens to the rotation of a molecular cloud as it collapses to form a star?
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The rotation rate increases and results in a disk of material around a protostar.
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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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A binary star systems may 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 collide.
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What is the approximate range of masses that newborn main-sequence stars can have?
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0.08 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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No stars have been found with masses greater than 300 times our Sun because
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they would generate so much radiation pressure that they would blow themselves apart.
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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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What prevents a brown dwarf from undergoing nuclear fusion?
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Degeneracy pressure halts the contraction of a protostar so the core never becomes hot or dense enough for nuclear fusion.
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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 is the eventual fate of a brown dwarf?
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It gradually cools down and becomes ever dimmer.
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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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Which of the following statements is probably true about the very first stars in the universe? - They were made only from hydrogen and helium. - They were made from pure energy. - They were probably orbited only by terrestrial planets, but no jovian planets. - They were made approximately of 98% hydrogen and helium, and 2% of heavier elements.
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They were made only from hydrogen and helium.
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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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Why do we think the first generation of stars would be different from stars born today?
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Without heavy elements, the clouds could not reach as low a temperature as today and had to be more massive to collapse.
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What is the likely reason that we cannot find any examples of the first generation stars?
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The first generation stars were all very massive and exploded as supernova long time ago.
