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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  42
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] As a white dwarf, the sun will be ______ .
选项 A、the same size as the planet Mercury
B、thousands of times smaller than it is today
C、around 35 million miles in diameter
D、cold and dark
答案 B
解析 细节题型见第三段第一、二句：一旦太阳用尽了它作为一个红色巨星的热核能，它就开始缩小；等它缩小到地球的大小时就会变成一个白色的矮星。所以B为答案。
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