2. 外语
  3. Until recently most astronomers believed that the space between the galaxies in our universe was a near perfect vacuum. This ort

Until recently most astronomers believed that the space between the galaxies in our universe was a near perfect vacuum. This ort

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问题     Until recently most astronomers believed that the space between the galaxies in our universe was a near perfect vacuum. This orthodox view of the universe is now being challenged by astronomers who believe that a heavy "rain" of gas is falling into many galaxies from the supposedly empty space around them. The gas apparently con- denses into a collection of small stars, each a little larger than the planet Jupiter. These stars vastly outnumber the other stars in a given galaxy. The amount of " intergalactic rainfall" into some of these galaxies has been enough to double their mass in the time since they formed. Scientists have begun to suspect that this intergalactic gas is probably a mixture of gases left over from the "big bang" when the galaxies were formed and gas was forced out of galaxies by supernova explosions.
    It is well known that when gas is cooled at a constant pressure its volume decreases. Thus, the physicist Fabian reasoned that as intergalactic gas cools, the cooler gas shrinks inward toward the center of the galaxy. Meanwhile its place is taken by hotter intergalactic gas from farther out on the edge of the galaxy, which cools as it is compressed and flows into the galaxy. The net result is a continuous flow of gas, starting as hot gases in inter galactic space and ending as a drizzle of cool gas called a "cooling flow," falling into the central galaxy.
    A fairly heretical idea in the 1970’s, the cooling-flow theory gained sup- port when Fabian observed a cluster of galaxies in the constellation Perseus and found the central galaxy, NGC 1275, to be a strange-looking object with irregular, thin strands of gas radiating from it. According to previous speculation, these strands were gases that had been blown out by an explosion in the galaxy. Fabian, however, disagreed. Because the strands of gas radiating from NGC 1275 are visible in optical photographs, Fabian suggested that such strands consisted not of gas blown out of the galaxy but of cooling flows of gas streaming inward. He noted that the wavelengths of the radiation emitted by a gas would changes as the gas cooled, so that as the gas flowed into the galaxy and became cooler, it would emit not x-rays, but visible light, like that which was captured in the photographs. Fabian’s hypothesis was supported by Canizares’ determination in 1982 that most of the gas in the Perseus cluster was at a temperature of 80 mil lion degrees Kelvin, whereas the gas immediately surrounding NGC 1275(the subject of the photographs)was at one-tenth this temperature.
According to the passage, Fabian believes that gas flowing into a central galaxy has which of the following characteristics?
选项 A、It is one-tenth hotter than it was in the outer regions of the galaxy cluster.
B、It emits radiation with wavelengths that change as the gas moves toward the center of the galaxy.
C、The total amount of radiation emitted diminishes as the gas cools.
D、It loses 90 percent of its energy as it moves to the center of the galaxy.
E、It condenses at a rate much slower than the rate of decrease in temperature as the gas flows inward.
答案B
解析 F相信向星系中心流动的气体有如下哪个特点?A．只有外层气体的1／10热。这是C观测的结果，而且这只对一个星系团适用，不能任意推广。B．正确。随着其进入星系中心，发出的射线波长也起相应的变化。原文L54—61，气体冷却时，其所发出的射线的波长也起变化，这些气体流向星系中心时，渐渐冷却，其辐射波长也就相应地变化。C．“total amount of radiation”文中无此概念。D．“loses 90 percent of its energy”在文中没有。E．文中没有比较气体的收缩速度。
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