2. 外语
  3. In addition to conventional galaxies, the universe contains very dim galaxies that until recently went unnoticed by astronomers.

In addition to conventional galaxies, the universe contains very dim galaxies that until recently went unnoticed by astronomers.

admin2019-11-17  66
问题 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.
According to the passage, conventional spiral galaxies differ from low-surface-brightness galaxies in which of the following ways?
选项 A、They have fewer stars than do low-surface-brightness galaxies.
B、They evolve more quickly than low-surface-brightness galaxies.
C、They are more diffuse than low-surface-brightness galaxies.
D、They contain less helium than do low-surface-brightness galaxies.
E、They are larger than low-surface-brightness galaxies.
答案B
解析 Supporting ideas
This question requires recognizing information that is provided in the passage. The first paragraph describes and compares two types of galaxies: conventional galaxies and dim, or low-surface-brightness, galaxies. It states that dim galaxies have the same approximate number of stars as a common type of conventional galaxy but tend to be larger and more diffuse because their mass is spread over wider areas (lines 4-10). The passage also indicates that dim galaxies take longer than conventional galaxies to convert their primordial gases into stars, meaning that dim galaxies evolve much more slowly than conventional galaxies (lines 10-14), which entails that conventional galaxies evolve more quickly than dim galaxies.
A The passage states that dim galaxies have approximately the same numbers of stars as a common type of conventional galaxy.
B Correct. The passage indicates that dim galaxies evolve much more slowly than conventional galaxies, which entails that conventional galaxies evolve more quickly.
C The passage states that dim galaxies are more spread out, and therefore more diffuse, than conventional galaxies.
D The passage does not mention the relative amounts of helium in the two types of galaxies under discussion.
E The passage states that dim galaxies tend to be much larger than conventional galaxies.
The correct answer is B.
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