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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2025-02-25  16
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 which of the following is an accurate physical description of typical low-surface-brightness galaxies?
选项 A、They are large spiral galaxies containing fewer stars than conventional galaxies.
B、They are compact but very dim spiral galaxies.
C、They are diffuse spiral galaxies that occupy a large volume of space.
D、They are small, young spiral galaxies that contain a high proportion of primordial gas.
E、They are large, dense spirals with low luminosity.
答案 C
解析 Inference
This question requires drawing an inference from information given in the passage. The first paragraph compares dim galaxies and conventional galaxies. Dim galaxies are described as having the same general shape (lines 4-5) as a common type of conventional galaxy, the spiral galaxy, suggesting that dim galaxies are, themselves, spiral shaped. The passage also indicates that, although both types of galaxies tend to have approximately the same number of stars, dim galaxies tend to be much larger and spread out over larger areas of space (lines 4-10) than conventional galaxies.
A The passage states that the two types of galaxies have approximately the same number of stars.
B The passage indicates that dim galaxies are relatively large and spread out.
C Correct. The passage indicates that dim galaxies have the same general shape as spiral galaxies and that their mass is spread out over large areas of space.
D The passage indicates that dim galaxies are relatively large and spread out.
E The passage states that dim galaxies have few stars per unit of volume, suggesting that they are not dense but diffuse.
The correct answer is C.
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