托福 托福 50 - Star Death
题目
1 2 3 4 5 6 7 8 9 10 11 12 13 14
7.The word "exhausted" in the passage is closest in meaning to
  • A.used up.
  • B.released.
  • C.invisible.
  • D.broken up.
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    Until the early- to mid-twentieth century, scientists believed that stars generate energy by shrinking. As stars contracted, it was thought, they would get hotter and hotter, giving off light in the process. This could not be the primary way that stars shine, however. If it were, they would scarcely last a million years, rather than the billions of years in age that we know they are. We now know that stars are fueled by nuclear fusion. Each time fusion takes place, energy is released as a by-product. This energy, expelled into space, is what we see as starlight. The fusion process begins when two hydrogen nuclei smash together to form a particle called the deuteron (a combination of a positive proton and a neutral neutron). Deuterons readily combine with additional protons to form helium. Helium, in turn, can fuse together to form heavier elements, such as carbon. In a typical star, merger after merger takes place until significant quantities of heavy elements are built up.

    We must distinguish, at this point, between two different stellar types: Population I and Population ll, the latter being much older than the former. These groups can also be distinguished by their locations. Our galaxy, the Milky Way, is shaped like a flat disk surrounding a central bulge. Whereas Population I stars are found mainly in the galactic disk, Population II stars mostly reside in the central bulge of the galaxy and in the halo surrounding this bulge.

    Population II stars date to the early stages of the universe. Formed when the cosmos was filled with hydrogen and helium gases, they initially contained virtually no heavy elements. They shine until their fusible material is exhausted. When Population II stars die, their material is spread out into space. Some of this dust is eventually incorporated into newly formed Population I stars. Though Population I stars consist mostly of hydrogen and helium gas, they also contain heavy elements (heavier than helium), which comprise about 1 or 2 percent of their mass. These heavier materials are fused from the lighter elements that the stars have collected. Thus, Population I stars contain material that once belonged to stars from previous generations. The Sun is a good example of a Population I star.

    What will happen when the Sun dies? In several billion years, our mother star will burn much brighter. It will expend more and more of its nuclear fuel, until little is left of its original hydrogen. Then, at some point in the far future, all nuclear reactions in the Sun’s center will cease.

    Once the Sun passes into its "postnuclear" phase, it will separate effectively into two different regions: an inner zone and an outer zone. While no more hydrogen fuel will remain in the inner zone, there will be a small amount left in the outer zone. Rapidly, changes will begin to take place that will serve to tear the Sun apart. The inner zone, its nuclear fires no longer burning, will begin to collapse under the influence of its own weight and will contract into a tiny hot core, dense and dim. An opposite fate will await the outer region, a loosely held-together ball of gas. A shock wave caused by the inner zone's contraction will send ripples through the dying star, pushing the stellar exterior's material farther and farther outward. The outer envelope will then grow rapidly, increasing, in a short interval, hundreds of times in size. As it expands, it will cool down by thousands of degrees. Eventually, the Sun will become a red giant star, cool and bright. It will be so large that it will occupy the whole space that used to be the Earth's orbit and so brilliant that it would be able to be seen with the naked eye thousands of light-years away. It will exist that way for millions of years, gradually releasing the material of its outer envelope into space. Finally, nothing will be left of the gaseous exterior of the Sun; all that will remain will be the hot, white core. The Sun will have become a white dwarf star. The core will shrink, giving off the last of its energy, and the Sun will finally die.

    直到20世纪中叶以前,科学家们一直都相信恒星是通过收缩产生能量的。人们认为,随着恒星收缩,它们会变得越来越热,在这个过程中它们会发光。然而,这可能不是恒星发光的主要方式。假如是这样的话,它们几乎不可能存在一百万年,而据我们所知,它们一直存在了几十亿年。我们现在知道恒星是从核聚变中获得能量的。每当核聚变发生时,能量就会作为副产品被释放。这些能量,被喷射到太空中,就成了我们所看到的星光。核聚变的过程开始时,两个氢原子碰撞在一起,形成了一个被称作氘核(正质子和中性中子的结合)的原子。氘核很容易与其他的质子结合形成氦。氦,又会融合在一起形成更重的元素,比如碳。在一个典型的行星上,合并融合的过程会不停地上演,直到形成大量的重元素。 就此,我们必须区分两种不同的恒星类型:星族I和星族II,后者比前者的年龄要更大。这些恒星群也可以根据它们所在的位置来区分。我们的星系,银河系,它的形状就像一个围绕着中央凸起的扁平圆盘。星族I的恒星主要位于银盘之上,而星族II的恒星大部分都位于银河系中央凸起的部分,以及凸起部分周围的光晕中。 星族II的恒星可以追溯到宇宙的早期阶段。这些恒星是在宇宙充满氢气和氦气的情况下形成的,它们最初几乎不含重元素。它们闪闪发光直到耗尽易熔物质。当星族II的恒星死亡的时候,它们所含的物质会分散到宇宙中。其中一些尘埃物质最终并入了最新形成的星族I的恒星中。虽然星族I的恒星的组成物质大部分是氢气和氦气,它们也含有重元素(比氦更重),这些重元素约占其质量的1或2%。这些较重的物质是由恒星所积聚的较轻的元素融合而成的。因此,星族I的恒星包含了属于以前恒星的物质。太阳就是一个很好的星族I恒星的例子。 当太阳死亡的时候会发生什么?在几十亿年中,我们的母星将燃烧变得更亮。它将消耗越来越多的核燃料,直到它原来所含的氢都所剩无几。然后,在遥远未来的某个时刻,太阳中心所有的核反应都将停止。 一旦太阳进入“核后”阶段,事实上它就会分为两个部分:内部和外部。虽然内部区域不再有氢燃料,但在外层区域仍会留下少量的氢燃料。很快,变化将开始发生,这些变化将会把太阳撕裂。在内部区域,太阳核心不再燃烧,在自身重力的影响下它将会开始崩塌,并收缩为一个微小而炽热的核心,密度很高,颜色昏暗。而外部区域的变化却截然相反,外部区域将变成一个由气体组成的松散的球状物。由内区收缩引起的冲击波会使垂死挣扎的恒星发出涟漪,将恒星表面的物质向外推得更远。恒星的外层会迅速成长,在很短的时间内,体积就会增加数百倍。当它膨胀时,它的温度会下降几千度。最终,太阳将变成一颗红巨星,又冷又亮。 它将变得如此之大,以至于它将占据曾经是地球轨道的整个空间,它是如此明亮以至于在数千光年以外就可以用肉眼看到它。红巨星将以这种方式存在数百万年,逐渐将外层外壳的物质释放到太空中。最终,太阳的气体外层将不再释放任何物质;剩下的将是炽热的、白色的核心。太阳将变成一颗白矮星。它的核心会收缩,释放出最后的能量,最终太阳会“死亡”。

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    最新提问
    • wx_6697
      觉得B C 意思一样,不知道选哪个
    • wx_5576
      这道题C为什么对,E为什么不对?
    • wx_5576
      B为什么不能选啊?
    • wx_6697
      TPO30 passage 2 Q5我选的 D,不明白为啥不对?
    • wx_6697
      鑫哥,TPO6passage3Q5 答案是给错了吗?好多人都选A
    • wx_6697
      这题也很容易选错选成了D
    • wx_6697
      这道题A为什么错了
    • 芊儿
      为什么这道题不选c??a中的variety不是应该对应文中的differentiating 吗??求解!
    • wx_1000
      这道题不选E是因为太细节了吗
    • 王金阁
      这个题为什么不选C啊。。。
    • 芊儿
      这道题的D选项不是和文中的better able to reproduce in open settings相对应么??
    • 风荨火
      有大佬解释一下这个为啥选D嘛?
    • 以沫
      请问这个D 在哪里提现?为什么D错?
    • 芊儿
      第六题 的C选择为什么不对,感觉A是明显驳斥啊...
    • wx_6697
      鑫哥,这道题D是从哪里看出来的
    • wx_6697
      这题选的A,根据是Joly’s calculations clearly supported those geologists who insisted on an age for Earth far in excess of a few million years.想问鑫哥为啥不选A
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      这题我选的是C依据是into a new habitat outside of its natural range, it may adapt to the new environment and leave its enemies behind.C为啥错了呢?鑫哥
    • wx_8861
      F选项的weather-related destruction在哪里体现了呢?原文最后一段的开头Among the costs里的costs是不是打错了?应该是coast?
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      与托福的斗争史 去解答 去解答
      这题为什么选C?
    • 小雨淅沥哗啦的下
      小雨淅沥哗啦的下 去解答 去解答
      B哪里错了
    • 小雨淅沥哗啦的下
      小雨淅沥哗啦的下 去解答 去解答
      B为啥不对
    • 李浩然
      B选项错误,是因为残缺么?
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    • wx xxxxx
      请问鑫哥,这段开头有写As one pesticide replaces another为什么不是对应a new pesticide is developed?
    • wx_7695
      鑫哥,从哪里看出来这个masks 不是use呀,原文说了wear呀
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      @鑫哥,这题the damage will continue 不应该对应前面的 the target species evolves resistance to it,然后As one pesticide replaces another,不应该是结束了time cycle 吗
    • wx_2065
      鑫哥,想知道E错在哪里?
    • wx_7695
      鑫哥,B选项 cannot extended to earlier geological periods. 原文说的意思是后来的进化无法估计吧
    • wx_2163
      B为什么不选
    • wx_7780
      鑫哥,这个哪里看不use了。BD是修饰错,C是无中生有,怎么能直接选出A?
    • 100
      看到第一句话,以为是中心句就选了A... 为什么不能选A呢
    • 100
      为什么选b?
    • gu33
      请问下 这里选D的原因是 因为 evolutionary approach 对应着 原文的 Rates of evolution 嘛? 这里我选了C。。不是很懂 插入句和 D的关系 求解答
    • 我是啦啦啦
      我是啦啦啦 去解答 去解答
      这个题A哪里错了?是因为主语不对吗?这个C比A多一步推理啊
    • haiyuqiao
      鑫哥,D选项里的19世纪出现了很多假设,原文中并没有提到啊
    • wx_7060
      为什么选a 呢。我觉得a是细节。F哪错了?
    • wx_1105
      我想问一下,这道题为什么不能选A呢?
    • wx_8122
      D为什么不选
    • wx_1655
      f选项哪里说了
    • chaulaw
      鑫哥,原文是below经济损害水平,D是一触发经济损害就用,这也对吗?
    • chaulaw
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    • wx_6697
      鑫哥,这道题答案是不是错了,好多人选D 我也选的D求解答
    • wx_6697
      这道题应该是一道易错题,每个选项的都有,然而我选的A错了,求解
    • wx_6697
      鑫哥,这题的C是怎么得到的?B也没有找到啊?难道不是应该选B
    • wx_6697
      鑫哥,还有这个,好多人选A答案是不是错了
    • wx_6697
      求问D是从哪里得出来的,我选的B呀哎呀
    • wx_4185
      it is difficult to say how far they were intended to be portraits rather than generalized images 这句话怎么理解呢
    • 此楠楠
      请问下这个插入题怎么选的呢?
    • 此楠楠
      求鑫哥讲解下A选项。。。 Even though in error, Joly’s calculations clearly supported those geologists who insisted on an age for Earth far in excess of a few million years.