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Age (billions of years)

FIG. 2.7 The Sun's fate In about eight billion years the Sun will become much brighter (top) and larger (bottom). The time scale has been expanded near the end of the Sun's life to show relatively rapid changes. (Courtesy of I-Juliana Sackmann and Arnold I. Boothroyd.)

Age (billions of years)

FIG. 2.7 The Sun's fate In about eight billion years the Sun will become much brighter (top) and larger (bottom). The time scale has been expanded near the end of the Sun's life to show relatively rapid changes. (Courtesy of I-Juliana Sackmann and Arnold I. Boothroyd.)

brighter than it is now, resulting in a substantial rise in temperature throughout the Solar System. It will become hot enough to melt the Earth's surface.

Moreover, terrestrial life will be wiped out well before then. In just 1 billion years from now the Sun will have brightened by 10 percent. Calculations suggest that the Earth's oceans will then evaporate at a rapid rate, resulting in a hot, dry uninhabitable Earth. And if that doesn't do us in, any Earthly life is doomed to fry in about 3 billion years from now. tte Sun will then be hot enough to boil the Earth's oceans away, leaving the planet a burned-out cinder, a dead and sterile place.

Meanwhile, during the apocalyptic period of planetary destruction, the core of the Sun will continue to contract until the central temperature is hot enough to ignite helium burning - at about a hundred million kelvin. But this conversion of helium into carbon does not last very long, compared to the Sun's 12 billion years of hydrogen burning. In about 35 million years, the core helium will have been used up, and there will be no heat left to hold up the Sun. In a last desperate gasp of activity, the Sun will shed the outer layers of gas to produce an expanding "planetary" nebula around the star, while the core collapses into a white dwarf star.

By this time, intense solar winds will have stripped the Sun down to about half its original mass, and gravitational collapse will squeeze that into an insignificant cinder about the size of the present-day Earth. Nuclear reactions will then be a thing of the past, and there will be nothing left to warm the Sun or planets, tte former Sun will just gradually cool down and fade away into old age, plunging all of the planets into a deep freeze.

Such events are in the very distant future, of course. For now, the Sun provides us with an up-close laboratory of the subatomic particles known as neutrinos. Astronomers have discovered fundamental properties of the neutrinos by observing those emitted from the Sun's energy-generating core.

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