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

admin2013-12-06  51
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.
Why does the author mention "cosmic rays" in Paragraph 2?
选项 A、As part of a list of things containing helium.
B、As an example of an unsolved astronomical puzzle.
C、To explain how the universe began.
D、To explain the abundance of hydrogen in the universe.
答案A
解析 文章第二段中介绍了许多包含氦的物质，包括in old stars，in relatively young ones，ininterstellar gas，and in the distant objects known as quasars，同时作者还提到了宇宙射线，并告诉我们宇宙射线中也包含氦。因此，作者在这里也提到了cosmic rays的目的是要使人们知道另外一种包含氦的物质，具体而言，就是说宇宙射线也是众多包含氦元素的物质中的一部分。
转载请注明原文地址:https://www.kaotiyun.com/show/svU3777K
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)