The sun today is a yellow dwarf star.
It is {{U}}fueled{{/U}} 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
{{U}}throw off{{/U}} 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 {{U}}there{{/U}}, it will have frozen onto the earth's
surface.