3. READING
Question 1: (10pts)
Choose the answer (A, B,C, D) which you think fits best .
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 billion 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 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.
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