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  3. In addition to conventional galaxies, the universe contains very dim galaxies th

In addition to conventional galaxies, the universe contains very dim galaxies th

游客2024-01-12  21
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. [br] It can be inferred from the passage that the "longstanding puzzle" refers to which of the following?
选项 A、The difference between the rate at which conventional galaxies evolve and the rate at which low-surface-brightness galaxies evolve
B、The discrepancy between estimates of total baryonic mass derived from measuring helium and estimates based on measuring galactic luminosity
C、The inconsistency between the observed amount of helium in the universe and the number of stars in typical low-surface-brightness galaxies
D、Uncertainties regarding what proportion of baryonic mass is contained in intergalactic space and what proportion in conventional galaxies
E、Difficulties involved in detecting very distant galaxies and in investigating their luminosity
答案 B
解析 Inference
This question requires drawing an inference from information given in the passage. The second paragraph describes the long-standing puzzle of the missing baryonic mass in the universe. The passage states that baryons are the source of galactic luminosity, and so scientists can estimate the amount of baryonic mass in the universe by measuring the luminosity of galaxies (lines 17-21). The puzzle is that spectroscopic measures of helium in the universe suggest that the baryonic mass in the universe is much higher than measures of luminosity would indicate (21-25).
A The differences between the rates of evolution of the two types of galaxies is not treated as being controversial in the passage.
B Correct. The passage indicates that measurements using spectroscopy and measurements using luminosity result in puzzling differences in estimates of the universe’s baryonic mass.
C The passage does not suggest how helium might relate to the numbers of stars in dim galaxies.
D The passage indicates that astronomers have speculated that the missing baryonic mass might be discovered in intergalactic space or hard-to-detect galaxies but does not suggest that these speculations are constituents of the long-standing puzzle.
E The passage does not mention how the distance to galaxies affects scientists’ ability to detect these galaxies.
The correct answer is B.
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