GRE阅读高频机经原文及答案：Supernova

　　16. Supernova D 89-12

　　Astronomers observe a supernova that is dimmer than expected because of dust screening，If a supernova 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 distribute 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.

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　　If a supernova 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 螺旋星云,旋涡星云).

　　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.

　　16.1. The primary purpose of the passage is to

　　describe what results when a supernova triggers the creation of chains of star-forming regions

　　propose a modification in the most widely accepted theory of spiral-galaxy structure

　　compare and contrast the roles of clouds of gas and dust in two theories of spiral-galaxy structure

　　describe a new theory of spiral-galaxy structure and contrast it with the most widely accepted theory

　　describe a new theory of spiral-galaxy structure and discuss a reason why it is inferior to the most widely accepted theory

　　16.2. 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

　　producing an underlying density wave

　　affected by a density wave of spiral form

　　distributed in a spiral pattern

　　located in the central plane of a galaxy

　　located in a differentially rotating galaxy

　　16.3. Which of the following, if true, would most discredit the new theory as described in the passage?

　　The exact mechanism by which a star becomes a supernova is not yet completely known and may even differ for different stars.

　　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.

　　The most massive stars formed from supernova explosions are unlikely to evolve into supernovas.

　　Computer simulations of supernovas provide a poor picture of what occurs just before a supernova explosion.

　　A density wave cannot compress clouds of gas and dust to a density high enough to create a star.

　　答案：DEC