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In the 1980's, astronomer Bohdan Paczynski proposed a way of determining whether the enormous dark halo constituting the outermost part of the Milky Way galaxy is composed of MACHO's (massive compact halo objects), which are astronomical objects too dim to be visible.  aczynski reasoned that if MACHO's make up this halo, a MACHO would occasionally drift in front of a star in the Large Magellanic Cloud, a bright galaxy near the Milky Way. The gravity of a MACHO that had so drifted, astronomers agree, would cause the star's light rays, which would otherwise diverge, to bend together so that, as observed from Earth, the star would temporarily appear to brighten, a process known as microlensing. Because many individual stars are of intrinsically variable brightness, some astronomers have contended that the brightening of intrinsically variable stars can be mistaken for microlensing. However, whereas the different colors of light emitted by an intrinsically variable star are affected differently when the star brightens, all of a star's colors are equally affected by microlensing. Thus, if a MACHO magnifies a star's red light tenfold, it will do the same to the star's blue light and yellow light. Moreover, it is highly unlikely that a star in the Large Magellanic Cloud will undergo microlensing more than once, because the chance that a second MACHO would pass in front of exactly the same star is minuscule.
Question #57. 639-06 (23973-!-item-!-188;#058&000639-06)
The passage is primarily concerned with
(A) outlining reasons why a particular theory is no longer credited by some astronomers
(B) presenting data collected by a researcher in response to some astronomers' criticism of a particular line of reasoning
(C) explaining why a researcher proposed a particular theory and illustrating how influential that theory has been
(D) showing how a researcher's theory has been used to settle a dispute between the researcher and some astronomers
(E) describing a line of reasoning put forth by a researcher and addressing a contention concerning that line of reasoning
为什么选择E，不明白，怎么觉得这么抽象。 |
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发表于 2011-9-13 20:08:31
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