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  3. The sun today is a yellow dwarf star. It is fueled by thermonuclear reaction

The sun today is a yellow dwarf star. It is fueled by thermonuclear reaction

游客2023-12-27  21
问题     The sun today is a yellow dwarf star. It is fueled by thermonuclear reactions near its center that convert hydrogen to helium. The sun has existed in its present state for about 4 billion, 600 million years and is thousands of times larger than the Earth.
    By studying other stars, astronomers can predict what the rest of the sun’s life will be like. About 5 billion years from now, the core of the sun will shrink and become hotter. The surface temperature will fail. The higher temperature of the center will increase the rate of thermonuclear reactions. The outer regions of the sun will expand approximately 35 million miles, about the distance to mercury, which is the closest planet to the sun. The sun will then be a red giant star. Temperatures on the earth will become too hot for life to exist.
    Once the sun has used up its thermonuclear energy as a red giant, it will begin to shrink. After it shrinks to the size of the earth, it will become a white dwarf star. The sun may throw off huge amounts of gases in violent eruptions called nova explosions as it changes from a red giant to a white dwarf.
    After billions of years as a white dwarf, the Sun will have used up all its fuel and will have lost its heat. Such a star is called a black dwarf. After the Sun has become a black dwarf, the earth will be dark and cold. If any atmosphere remains there, it will have frozen onto the earth’s surface. [br] What is the primary purpose of this passage?
选项 A、To alert people to the dangers posed by the sun.
B、To discuss conditions on earth in the far future.
C、To present a theory about red giant stars.
D、To describe changes that the sun will go through.
答案 D
解析 主旨题型本文主要谈论了：如今的太阳是个黄色的矮星，由靠近它中心的高热原子核反应将氢转化为氦来提供燃料。……天文学家通过研究其它星球，能够预测太阳的余生/未来会是什么样子。距今大约五十亿年后，太阳的核会缩小并且会变得更热，而太阳表面的温度会降低……然后太阳将会成为一个红色的巨星。地球上的温度会变得过热，以至没有生命能够存活。一旦太阳耗尽了红色巨星的高热原子核反应的能量，它会开始缩小；等它缩小到地球的大小时，它将会变成个白色的矮星；当它从一个红色巨星变成一个白色矮星时，太阳会以剧烈喷发的形式释放大量的被称之为“新星爆发”的气体。在变成白色矮星的几十亿年后，太阳会耗尽它所有的燃料，热度消失；这样的一个星被称之为黑色矮星……由此得知本文的主要目的是谈论太阳所要经历的变化，因此D为答案。
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