If a supernova (the explosion of a massive star) triggered star formation from dense clouds of gas and dust, and if the most massive star to be formed from the cloud evolved into a supernova and triggered a new round of star formation, and so on, then a chain of star-forming regions would result. If many such chains were created in a differentially rotating galaxy, the distribution of stars would resemble the observed distribution in a spiral galaxy. This line of reasoning underlies an exciting new theory of spiral-galaxy structure. A computer simulation based on this theory has reproduced the appearance of many spiral galaxies without assuming an underlying density wave, the hallmark of the most widely accepted theory of the large-scale structure of spiral galaxies. That theory maintains that a density wave of spiral form sweeps through the central plane of a galaxy, compressing clouds of gas and dust, which collapse into stars that form a spiral pattern. 17. The primary purpose of the passage is to (A) describe what results when a supernova triggers the creation of chains of star-forming regions (B) propose a modification in the most widely accepted theory of spiral-galaxy structure (C) compare and contrast the roles of clouds of gas and dust in two theories of spiral-galaxy structure (D) describe a new theory of spiral-galaxy structure and contrast it with the most widely accepted theory (E) describe a new theory of spiral-galaxy structure and discuss a reason why it is inferior to the most widely accepted theory 18. The passage implies that, according to the new theory of spiral-galaxy structure, a spiral galaxy can be created by supernovas when the supernovas are (A) producing an underlying density wave (B) affected by a density wave of spiral form (C) distributed in a spiral pattern (D) located in the central plane of a galaxy (E) located in a differentially rotating galaxy 19. Which of the following, if true, would most discredit the new theory as described in the passage? (A) The exact mechanism by which a star becomes a supernova is not yet completely known and may even differ for different stars. (B) Chains of star-forming regions like those postulated in the new theory have been observed in the vicinity of dense clouds of gas and dust. (C) The most massive stars formed from supernova explosions are unlikely to evolve into supernovas. (D) Computer simulations of supernovas provide a poor picture of what occurs just before a supernova explosion. (E) A density wave cannot compress clouds of gas and dust to a density high enough to create a star. 20. The author’s attitude toward the new theory of spiral-galaxy structure can best be described as (A) euphoric (B) enthusiastic (C) concerned (D) critical (E) disputatious
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发表于 2011-10-31 13:12:57
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