The sun today is a yellow dwarf star. It is powered by thermonuclear reactions near its centre that change hydrogen into 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 fall. The higher temperature of the centre 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. 76. It can be inferred from the passage that the sun _______ . A. is approximately halfway through its life as a yellow dwarf B. has been in existence for 10 billion years C. is rapidly changing in size and brightness D. will continue as a yellow dwarf for another 10 billion years 77. When the sun becomes a red giant, what will conditions be like on earth? 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 destroyed in nova explosions. 78. As a white dwarf, the sun will be_______. 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 79. According to the passage, which of the following best describes the sequence of stages that the sun will probably pass through? A. Yellow dwarf, white dwarf, red giant, black giant. B. Red giant, white dwarf, red dwarf, nova explosions. C. Yellow dwarf, red giant, white dwarf, black dwarf. D. White dwarf, red giant, black dwarf, yellow dwarf. 80. The word “there” in the last line refers to _______. A. our own planet B. the outer surface of the sun C. the core of a black dwarf D. the planet Mercury