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’ s theory makes use of which of the following principles?
选项 A、Gas emanating from an explosion will be hotter the more distant it is from the origin.
B、The wavelength of radiation emitted by a gas as it cools remains constant.
C、If pressure remains constant, the volume of a gas will decrease as it is cooled.
D、The volume of a gas will increase as the pressure increases.
E、As gas cools, its density decreases.
答案C
解析 F的理论用到了下面哪个原理?A．爆炸中产生的气体离爆炸源越远就越热。原文无，且荒唐。B．气体发出辐射当其冷却时波长不变。和原文L54—61所叙相反。C．正确。如果压力恒定，气体冷却时其体积会缩小。简单的热胀冷缩原理。见原文L23—25，F推理是以此作前提的。D．未提温度因素，此推论不成立。文中未讲压力和体积关系。E．未提压力因素。
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