Astronomy Ch 15 Practice Test – Flashcards
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What is the approximate chemical composition (by mass) with which all stars are born?
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three quarters hydrogen, one quarter helium, no more than about 2% heavier elements
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The total amount of power (in watts, for example) that a star radiates into space is called its _________.
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luminosity
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According to the inverse square law of light, how will the apparent brightness of an object change if its distance to us triples?
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Its apparent brightness will decrease by a factor of 9.
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Assuming that we can measure the apparent brightness of a star, what does the inverse square law for light allow us to do?
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Calculate the star's luminosity if we know its distance, or calculate its distance if we know its luminosity.
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If star A is closer to us than star B, then Star A's parallax angle is _________.
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larger than that of Star B
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Ten parsecs is about _________.
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32.6 light-years
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Star A has an apparent magnitude of 3 and star B has an apparent magnitude of 5. Which star is brighter in our sky?
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Star A
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From hottest to coolest, the order of the spectral types of stars is _________.
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OBAFGKM
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Our Sun is a star of spectral type _________.
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G
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Astronomers can measure a star's mass in only certain cases. Which one of the following cases might allow astronomers to measure a star's mass?
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The star is a member of a binary star system.
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Which of the following terms is given to a pair of stars that we can determine are orbiting each other only by measuring their periodic Doppler shifts?
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spectroscopic binary
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The axes on a Hertzsprung-Russell (H-R) diagram represent _________.
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luminosity and surface temperature
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What can we infer, at least roughly, from a star's luminosity class?
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its size (radius)
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On an H-R diagram, stellar radii _________.
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increase diagonally from the lower left to the upper right
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On an H-R diagram, stellar masses _________.
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can be determined for main sequence stars but not for other types of stars
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How is the lifetime of a star related to its mass?
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More massive stars live much shorter lives than less massive stars.
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Each choice below lists a spectral type and luminosity class for a star. Which one is a red supergiant?
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Spectral type M2, luminosity class I
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What is the common trait of all main sequence stars?
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They generate energy through hydrogen fusion in their core.
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Suppose our Sun were suddenly replaced by a supergiant star. Which of the following would be true?
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Earth would be inside the supergiant.
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What is a white dwarf?
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the remains of a star that ran out of fuel for nuclear fusion
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Which of the following statements comparing open and globular star clusters is not true?
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Open and globular clusters each typically contain a few hundred stars.
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What do we mean by the main-sequence turnoff point of a star cluster, and what does it tell us?
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It is the spectral type of the hottest main sequence star in a star cluster, and it tells us the cluster's age.
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All stars are born with the same basic composition, yet stars can differ greatly in appearance. Which two basic factors are most important in determining the current appearance of a star?
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its mass and its stage of life
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Based on the definition of apparent brightness, which units are appropriate for its measurement?
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watts per square meter
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Star A is identical to Star B, except that Star A is twice as far from us as Star B. Therefore:
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Both stars have the same luminosity, but the apparent brightness of Star B is four times that of Star A.
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A star with a parallax angle of 1/20 arcsecond is _________.
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20 parsecs away
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A star's color is related to its surface temperature because _________.
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stars emit thermal radiation
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Which of the following statements about spectral types of stars is not generally true?
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The spectral type of a star can be used to determine its distance.
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Sirius is a star with spectral type A star and Rigel is a star with spectral type B star. What can we conclude?
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Rigel has a higher surface temperature than Sirius.
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To calculate the masses of stars in a binary system, we must measure their _________.
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orbital period and average orbital distance
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Careful measurements reveal that a star maintains a steady apparent brightness at most times, except that at precise intervals of 73 hours the star becomes significantly dimmer for about 2 hours. The most likely explanation is that _________.
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the star is a member of an eclipsing binary star system
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You observe a star and you want to plot it on an H-R diagram. You will need to measure all of the following, except the star's _________.
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mass
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The approximate main-sequence lifetime of a star of spectral type O is _________.
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3 million years
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How did astronomers discover the relationship between spectral type and mass for main sequence stars?
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By measuring the masses and spectral types of main-sequence stars in binary systems.
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The choices below each describe the appearance of an H-R diagram for a different star cluster. Which cluster is the youngest?
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The diagram shows main-sequence stars of every spectral type except O, along with a few giants and supergiants.
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The choices below each describe the appearance of an H-R diagram for a different star cluster. Which cluster is most likely to be located in the halo of our galaxy?
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The diagram shows main-sequence stars of spectral types G, K, and M, along with numerous giants and white dwarfs.
