托福閱讀真題Official 50 Passage 3（七）

托福閱讀真題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.

正確答案：D

題目詳解

題型分類：插入句子題

題干分析：考察句子和句子之間的銜接判斷。原文意為：很明顯，需要一個更加合理的機(jī)制來解釋太陽是如何產(chǎn)生能量的。a more plausible mechanism說明前文已經(jīng)說過一個太陽產(chǎn)生能量的機(jī)制以及這個機(jī)制的問題了，而后文會給出一個新的機(jī)制。插入句是一個過渡句。

選項分析：

D選項前面幾句都是在將太陽通過收縮產(chǎn)生能量以及這個機(jī)制的不合理性，而C選項后一句提出了新的機(jī)制——核聚變。fueled by nuclear fusion呼應(yīng)a more plausible mechanism。

A選項前一句說收縮產(chǎn)能，后一句在解釋如何收縮產(chǎn)能，中間不需要過渡句。

B選項后面半句本身就是一個過渡句，引出這個機(jī)制的問題，不需要再加入過渡句。

C選項后一句是在解釋這個機(jī)制的問題，而不是提出新的機(jī)制，與a more plausible mechanism不對應(yīng)。

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.

正確答案：BDF

題目詳解

題型分類：文章總結(jié)題

題干分析：選擇概括性的正確選項。

選項分析：

B選項為第三段前半部分重要信息。the oldest stars,are formed from hydrogen and helium gases對應(yīng)文章第三段第一句和第二句。they shine until they exhaust their fusible material對應(yīng)文章第三段第三句。

D選項為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選項為第五段重要信息。The outer envelope...will release their energy into space對應(yīng)第五段倒數(shù)第三句gradually releasing the material of its outer envelope into space。become white dwarfs before they finally give off their last energy對應(yīng)倒數(shù)第一句和第二句。

A選項The Sun is a good example對應(yīng)文章第三段最后一句，但是前一句說的是Population I stars contain material that once belonged to stars from previous generations，沒有提nuclear fusion rather than through contraction。

C選項Population II stars are found in the galactic disk與第二段的最后一句話Population I stars are found mainly in the galactic disk矛盾。

E選項對應(yīng)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，然后第五段才開始描述太陽分為內(nèi)部和外部的情況，所以說是核聚變停止在先，然后太陽才開始分為2個部分。選項時間順序顛倒。

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