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In addition to conventional galaxies, the universe contains very dim galaxies that until recently went unnoticed by astronomers. Possibly as numerous as conventional galaxies, these galaxies have the same general shape and even the same approximate number of stars as a common type of conventional galaxy, the spiral, but tend to be much larger. Because these galaxies' mass is spread out over larger areas, they have far fewer stars per unit volume than do conventional galaxies. Apparently these low-surface-brightness galaxies, as they are called, take much longer than conventional galaxies to condense their primordial gas and convert it to stars--that is, they evolve much more slowly.

These galaxies may constitute an answer to the long-standing puzzle of the missing baryonic mass in the universe. Baryons-subatomic particles that are generally protons or neutrons-are the source of stellar, and therefore galactic, luminosity, and so their numbers can be estimated based on how luminous galaxies are. However, the amount of helium in the universe, as measured by spectroscopy, suggests that there are far more baryons in the universe than estimates based on galactic luminosity indicate. Astronomers have long speculated that the missing baryonic mass might eventually be discovered in intergalactic space or as some large population of galaxies that are difficult to detect.

It can be inferred from the passage that the "long-standing puzzle" refers to which of the following?

  • AThe difference between the rate at which conventional galaxies evolve and the rate at which low-surface-brightness galaxies evolve
  • BThe discrepancy between estimates of total baryonic mass derived from measuring helium and estimates based on measuring galactic luminosity
  • CThe inconsistency between the observed amount of helium in the universe and the number of stars in typical low-surface-brightness galaxies
  • DUncertainties regarding what proportion of baryonic mass is contained in intergalactic space and what proportion in conventional galaxies
  • EDifficulties involved in detecting very distant galaxies and in investigating their luminosity
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正确答案: B
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  • 所属科目:阅读RC
  • 题目来源1:OG18-文本逻辑-84
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