2. 医学
  3. The elements other than hydrogen and helium exist in such small quantities that it is accurate to say that the universe is somew

The elements other than hydrogen and helium exist in such small quantities that it is accurate to say that the universe is somew

admin2013-12-14  42
问题     The elements other than hydrogen and helium exist in such small quantities that it is accurate to say that the universe is somewhat more than 75 percent hydrogen.
    Astronomers have measured the abundance of helium throughout our galaxy and in other galaxies as well. Helium has been found in old stars, in relatively young ones, in interstellar gas, and in the distant objects known as quasars. Helium nuclei have also been found to be constituents of cosmic rays that fall on the earth(cosmic "rays" are not really a form of radiation; they consist of rapidly moving particles of numerous different kinds). It doesn’t seem to make very much difference where the helium is found. Its relative abundance never seems to vary much. In some places, there may be slightly more of it; in others, slightly less, but the ratio of helium to hydrogen nuclei always remains about the same.
    Helium is created in stars. In fact, nuclear reactions that convert hydrogen to helium are responsible for most of the energy that stars produce. However, the amount of helium that could have been produced in this manner can be calculated, and it turns out to be no more than a few percent. The universe has not existed long enough for this figure to be significantly greater. Consequently, if the universe is somewhat more than 25 percent helium now, then it must have been about 25 percent helium at a time near the beginning.
    However, when the universe was less than one minute old, no helium could have existed. Calculations indicate that before this time temperatures were too high and particles of matter were moving around much too rapidly. It was only after the one-minute point that helium could exist. By this time, the universe had cooled sufficiently that neutrons and protons could stick together. But the nuclear reactions that led to the formation of helium went on for only a relatively short time. By the time the universe was a few minutes old, helium production had effectively ceased.
Most of the helium in the universe was formed______.
选项 A、in interstellar space
B、in a very short time
C、during the first minute of the universe’s existence
D、before most of the hydrogen
答案B
解析 文章的末尾有这样一句话：“But the nuclear reactions that led to the formation of helium wenton for only a relatively short time．”这句话的意思是，“但是导致氮形成的原子核反应持续的时间相对较短。”文章在最后指出，在宇宙形成的早期，氮并不存在，因为那时的温度太高。后来随着温度逐渐降低，中子和质子才得以粘在一起。原子核反应形成了氮，但这一过程进行的时间很短。到了the universe was a few minutes old的时候，氦的生产就停止了。
转载请注明原文地址:https://kaotiyun.com/show/iiU3777K
本试题收录于： 医学博士外语题库考研分类
0

医学博士外语

考研

相关试题推荐
随机试题
最新回复(0)