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  3. The Sun today is a yellow dwarf star. It is fueled by thermonuclear reactions near its center that convert hydrogen to helium. T

The Sun today is a yellow dwarf star. It is fueled by thermonuclear reactions near its center that convert hydrogen to helium. T

admin2014-01-13  77
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 Earth.
    By studying other stars, astronomers can predict what the rest of the Sun’s life will be like. About 5 billon years from now, the core of the Sun will shrink and become hotter. The surface temperature will fall. 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 state 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.
When the Sun becomes a red giant, what will conditions be like on Earth?
选项 A、Its atmosphere will freeze and become solid.
B、It will be enveloped in the expanding surface of the sun.
C、It will become too hot for life to exist.
D、It will be nearly destroyed by nova explosions.
答案D
解析 根据文章第三段第三句“such as when they are asked to offer a toast at a weddingreception or to participate in a televised debate or interview．”可知，太阳将会变成火红的星球时。地球上的生命，由于温度过高因此无法继续。据此判断，答案是D。
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