In the 1980's, astronomer BohdanPaczynski proposed a way of determining whether the enormous dark haloconstituting the outermost part of the Milky Way galaxy is composed of MACHO's(massive compact halo objects), which are astronomical objects too dim to bevisible.  aczynski reasoned that ifMACHO's make up this halo, a MACHO would occasionally drift in front of a starin 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 otherwisediverge, to bend together so that, as observed from Earth, the star wouldtemporarily appear to brighten, a process known as microlensing. Because many individual stars are ofintrinsically variable brightness, some astronomers have contended that the brighteningof intrinsically variable stars can be mistaken for microlensing. However, whereas the different colors oflight emitted by an intrinsically variable star are affected differently whenthe star brightens, all of a star's colors are equally affected bymicrolensing. Thus, if a MACHO magnifiesa star's red light tenfold, it will do the same to the star's blue light andyellow light. Moreover, it is highlyunlikely that a star in the Large Magellanic Cloud will undergo microlensingmore than once, because the chance that a second MACHO would pass in front ofexactly the same star is minuscule.
Question #55. 639-01 (23881-!-item-!-188;#058&000639-01)
It can be inferred from the passagethat which of the following would constitute the strongest evidence of themicrolensing of a star in the Large Magellanic Cloud?
(A) The brightness of such a staris observed to vary at irregular intervals.
(B) The brightening of such a staris observed to be of shorter duration than the brightening of neighboringstars.
(C) The red light of such a star isobserved to be brighter than its yellow light and its blue light.
(D) The red light, yellow light,and blue light of such a star are observed to be magnified temporarily by thesame factor.
(E) The red light of such a star isobserved to have increased tenfold.
Question #56. 639-04 (23927-!-item-!-188;#058&000639-04)
According to the passage,Paczynski's theory presumes that if MACHO's constituted the Milky Way's darkhalo, occasionally a MACHO would
(A) drift so as to lie in a directline between two stars in the outer Milky Way
(B) affect the light rays of a starin the Large Magellanic Cloud with the result that the star would seem for atime to brighten
(C) become obscured as a result ofthe microlensing of a star in the Large Magellanic Cloud
(D) temporarily increase theapparent brightness of a star in the Large Magellanic Cloud by increasing thegravity of the star
(E) magnify each color in thespectrum of a star in the Large Magellanic Cloud by a different amount
Question #57. 639-06 (23973-!-item-!-188;#058&000639-06)
The passage is primarily concernedwith
(A) outlining reasons why aparticular theory is no longer credited by some astronomers
(B) presenting data collected by aresearcher in response to some astronomers' criticism of a particular line ofreasoning
(C) explaining why a researcherproposed a particular theory and illustrating how influential that theory hasbeen
(D) showing how a researcher'stheory has been used to settle a dispute between the researcher and someastronomers
(E) describing a line of reasoningput forth by a researcher and addressing a contention concerning that line ofreasoning
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发表于 2010-10-24 22:10:12
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