托福閱讀真題Official 50 Passage 3（二）

2023-06-27 16:10:24 來源：中國教育在線

托福閱讀真題Official 50 Passage 3（二）

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 3 of 14

In paragraph 1,why does the author point out that stars are billions of years old?

A.To establish that starlight is produced by an ongoing process and not by a one-time event

B.To suggest that stars contract much more slowly than was previously believed

C.To argue that shrinking cannot be the main way stars generate energy

D.To argue that fusion in a star slows down as quantities of heavy elements build up

Paragraph 1 is marked with an arrow

正確答案：C

題目詳解

題型分類：修辭目的題

題干分析：考察句子之間的關(guān)系。

原文定位：根據(jù)billions定位到第一段第四句。

選項分析：

定位句中If it were表明是一個虛擬語氣——“假如是這樣的話”。需要往前看，說科學(xué)家們從前一直都相信恒星是通過收縮產(chǎn)生能量的，在這個過程中它們會發(fā)光。但第三句話說這可能不是恒星發(fā)光的主要方式。由此可以看出，第四句是想對前三句提出的觀點(diǎn)進(jìn)行反面論證，來支持其觀點(diǎn)即收縮不是恒星產(chǎn)生能量的主要方式。對應(yīng)C選項。

A選項a one-time event第一段前三句沒有提到。

B選項much more slowly第一段前三句沒有提到。

D選項fusion（聚變）第一段前三句沒有提到。

Question 4 of 14

According to paragraph 2,Population I stars and Population II stars differ from each other in terms of both

A.how old they are and where in their galaxies they are found

B.how old they are and whether they have a halo around them

C.where in their galaxies they are found and whether they bulge out in the center

D.whether they are at the center of a flat disk and whether they have a halo

Paragraph 2 is marked with an arrow

正確答案：A

題目詳解