托福阅读真题Official 50 Passage 3（七）

托福阅读真题Official 50 Passage 3（七）

Star Death

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 II,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.

Question 13 of 14

Look at the four squares[■]that indicate where the following sentence could be added to the passage.Clearly,a more plausible mechanism was needed to explain how stars generate energy.Where would the sentence best fit?Click on a square[■]to add the sentence to the passage.

正确答案：D

题目详解

题型分类：插入句子题

题干分析：考察句子和句子之间的衔接判断。原文意为：很明显，需要一个更加合理的机制来解释太阳是如何产生能量的。a more plausible mechanism说明前文已经说过一个太阳产生能量的机制以及这个机制的问题了，而后文会给出一个新的机制。插入句是一个过渡句。

选项分析：

D选项前面几句都是在将太阳通过收缩产生能量以及这个机制的不合理性，而C选项后一句提出了新的机制——核聚变。fueled by nuclear fusion呼应a more plausible mechanism。

A选项前一句说收缩产能，后一句在解释如何收缩产能，中间不需要过渡句。

B选项后面半句本身就是一个过渡句，引出这个机制的问题，不需要再加入过渡句。

C选项后一句是在解释这个机制的问题，而不是提出新的机制，与a more plausible mechanism不对应。

Question 14 of 14

Directions:An introductory sentence for a brief summary of the passage is provided below.Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage.Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage.This question is worth 2 points.

Stars generate the energy that makes them shine as a by-product of nuclear fusion and not by shrinking,as scientists had once believed.

Answer Choices:

A.

The Sun is a good example of a Population I star because the Sun generates its energy through nuclear fusion rather than through contraction.

B.

Population II stars,the oldest stars,are formed from hydrogen and helium gases,and they shine until they exhaust their fusible material.

C.

In the Milky Way,Population I stars are found in and around the central bulge and Population II stars are found in the galactic disk.

D.

Population I stars,including the Sun,are relatively young stars that are mostly hydrogen and helium gas but also contain heavier elements.

E.

The Sun and stars like it will separate into inner cores and outer envelopes before all nuclear reactions in the cores stop and the stars finally die.

F.

The outer envelope of the Sun and stars like it will release their energy into space,and the inner cores will become white dwarfs before they finally give off their last energy.

正确答案：BDF

题目详解

题型分类：文章总结题

题干分析：选择概括性的正确选项。

选项分析：

B选项为第三段前半部分重要信息。the oldest stars,are formed from hydrogen and helium gases对应文章第三段第一句和第二句。they shine until they exhaust their fusible material对应文章第三段第三句。

D选项为第三段后半部分重要信息。relatively young stars出自第三段第五句Some of this dust is eventually incorporated into newly formed Population I stars。mostly hydrogen and helium gas but also contain heavier elements出自第六句consist mostly of hydrogen and helium gas,hey also contain heavy elements。

F选项为第五段重要信息。The outer envelope...will release their energy into space对应第五段倒数第三句gradually releasing the material of its outer envelope into space。become white dwarfs before they finally give off their last energy对应倒数第一句和第二句。

A选项The Sun is a good example对应文章第三段最后一句，但是前一句说的是Population I stars contain material that once belonged to stars from previous generations，没有提nuclear fusion rather than through contraction。

C选项Population II stars are found in the galactic disk与第二段的最后一句话Population I stars are found mainly in the galactic disk矛盾。

E选项对应文章第四、第五段。但是在第四段末提到Then,at some point in the far future,all nuclear reactions in the Sun’s center will cease，然后第五段才开始描述太阳分为内部和外部的情况，所以说是核聚变停止在先，然后太阳才开始分为2个部分。选项时间顺序颠倒。

>> 雅思 托福 免费测试、量身规划、让英语学习不再困难<<