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Astronomers theorize that a black hole forms when a massive object shrinkscatastrophically under its own gravity, leaving only a gravitational field so strong that nothing escapes it. Astronomers must infer the existence of black holes, which are invisible, from their gravitational influence on the visible bodies surrounding them. For example, observations indicate that gas clouds in galaxy M87 are whirling unusually fast about the galaxy’s center.Most astronomers believe that the large concentration of mass at the galaxy’s center is a black hole whose gravity is causing the gas to whirl. A few skepticshave argued that the concentration of mass necessary to explain the speed of the whirling gas is not necessarily a black hole: the concentration in M87 might be a cluster of a billion or so dim stars.
The same hypothesis might have been applied to the galaxy NGC 4258, but the notion of such a cluster’s existing in NGC 4258 was severely undermined when astronomers measured the speed of a ring of dust and gas rotating close to the galaxy’s center. From its speed, they calculated that the core’s density is more than 40 times the density estimated for any other galaxy. If the center of NGC 4258 were a star cluster, the stars would be so closely spaced that collisions between individual stars would have long ago torn the cluster apart.
天文学家提出黑洞的成因：大块物体由于自身重力灾难性收缩，遗留下重力场。天文学家一定是从黑洞重力对周围可见物体的影响来推断无形黑洞的存在。如银河M87绕银河中心异常快速地转动。大多数天文学家相信中心的大团块是一个黑洞。一些怀疑者觉得不一定，中心团可能是大约十亿暗星组成的星团。但银河NGC4258的现象显示怀疑者的观点站不住脚。NGC4258核心密度是其它银河估算密度的40倍有多。如果其核心是星团，它们那么密恒星之间的撞击很久以前就会把星团撕开。
Q12: Which of the following, if true, would most clearly undermine the possible explanation for the whirling gas in M87 that is mentioned in the last sentence of the first paragraph?
a. The stars in a star cluster at the center of M87 could exert a strong gravitational force without tearing the cluster apart.
b. A cluster of stars at the center would precludethe existence of certain other astronomical phenomena that have been observed at the center of M87.
c. The stars within many existing galaxies, such as NGC 4258, are more closely spaced than are the stars within the core of M87.
d. Only one other galaxy has been observed to contain gas clouds whirling about its center as they do about the core of M87.
e. The gravitational force of a cluster of a billion or so dim stars would be sufficient to cause a whirling ring of gas and dust to collect around the center of a galaxy.
答案是A or B? |
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发表于 2011-12-4 14:28:51
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