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.

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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.
The primary purpose of the passage is to
选项 A、describe a phenomenon and consider its scientific significance
B、contrast two phenomena and discuss a puzz ing difference between them
C、identify a newly discovered phenomenon anc explain its origins
D、compare two classes of objects and discuss the physical properties of each
E、discuss a discovery and point out its inconsistency with existing theory
答案A
解析 Main idea
This question requires understanding, in broad terms, the purpose of the passage as a whole. The first paragraph describes a phenomenon: the discovery of dim galaxies and some of their general attributes. The second paragraph describes how this discovery may help astronomers to solve a long-standing puzzle about the baryonic mass of the universe.
A Correct. The passage describes the phenomenon of dim galaxies and describes their significance in solving the longstanding puzzle of the missing baryonic mass in the universe.
B Although the passage discusses the puzzling difference between the two estimates of baryonic mass, this option does not account for the broader topic of dim galaxies.
C While the passage identifies the newly discovered phenomenon of dim galaxies, it does not offer a significant explanation for these galaxies’ origins.
D Although the passage compares dim an 1 conventional galaxies in the first paragraph, this option does not account for the important detail that dim galaxies may help solve a long-standing puzzle.
E The discovery of dim galaxies discussed in the passage is not said to be inconsistent with any existing scientific theory.
The correct answer is A.
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