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  3. Because of its precision, the Cepheid-variable method of correlating the luminosity of Cepheid stars with those of known

Because of its precision, the Cepheid-variable method of correlating the luminosity of Cepheid stars with those of known

admin2011-01-17  51
问题            Because of its precision, the Cepheid-variable method of correlating the
       luminosity of Cepheid stars with those of known types of ordinary stars is one of
       the most important tools that astronomers have for ascertaining distances to
Line     proximate galaxies. Fortunately, Cepheids are luminous enough to be observed
(5)      in other galaxies, not just in our own, such that scientists have established a
       benchmark for their true brightness. One first observes a Cepheid, notes the
       time span for its luminance to vary, and plots that information on an already
       established graph to derive its intrinsic luminosity.  Comparing intrinsic
       luminosity with apparent brightness allows one to calculate the star’s distance
(10)     as long as the period-luminosity relation for the star in question is first corrected
       for gravitational interference, parallax, and experimental error. Data gathered
       by this process would suggest that the universe is expanding, but the method is
       less reliable in the nether reaches of the universe, where the crucial evidence
       for cosmic expansion would likely be spotted.
It can be inferred from the passage that the Cepheid variable method of estimating galactic distances would be likely to yield experimentally flawed results in which of the following circumstances?
选项 A、If the luminosity of ordinary stars being compared with that of a Cepheid star were affected by parallax.
B、If the apparent brightness of the Cepheid star being studied required an unusually long period of time to vary.
C、If the gravitational interference surrounding the Cepheid star being observed were more negligible than in most cases.
D、If the Cepheid star being studied existed on the periphery of the universe, at an extremely large distance from the observer.
E、If the Cepheid star being studied were in the same galaxy as the observer.
答案D
解析
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