2. Energizing the Sun
The Sun's remote past and distant future
Nothing in the cosmos is fixed and unchanging, and nothing escapes the ravages of time. Everything moves and evolves, and that includes the seemingly constant and unchanging star of our solar system. The Sun and planets in our solar system formed 4.5 billions of years ago when a spinning cloud of dust and gas in space fell in on itself. The center got denser and denser, until it became so packed, so tight and so hot that protons came together and fused into helium, making the Sun glow. Ever since then, the Sun has slowly grown in luminous intensity with age, a steady, inexorable brightening that is a consequence of the increasing about of helium accumulating in the Sunís core.
The luminosity, effective temperature and radius of the Sun have all slowly increased with time (Fig. 3.9). The Sun is now 30 percent brighter than it was 4.5 billion years ago. The brightening is enough to make the visible solar disk 300 degrees Kelvin hotter and its radius 6 percent greater than when the Sun first shone. The luminosity has only increased by a miniscule 0.0000023 (2.3 x 10-6) percent during the past 350 years, and there is no way that this small change will ever be directly measured. Yet, it has profound implications over cosmic periods of time.
In the end, our prospects are not all that great anyway. The Earthís self-regulating thermostat will eventually go out of control, and the Sun will eventually consume the life it once nurtured. We can anticipate an additional 7 billion years of slow luminosity increase (Fig. 3.9), but terrestrial life will be wiped out well before then. In just 1 billion years the Sun will have brightened by another 10 percent. Calculations suggest that the Earthís oceans could then evaporate at a rapid rate, resulting in a hot, dry uninhabitable Earth. And if that doesnít do us in, any Earthly life is doomed to fry in about 3 billion years from now. The Sun will then be hot enough to boil the Earthís oceans away, leaving the planet a burned-out cinder, a dead and sterile place.
The Sun cannot shine forever, because it will eventually use up the hydrogen fuel in its core. Although it has converted only a trivial part of its original mass into energy, the Sun has processed a substantial 37 percent of its core hydrogen into helium during the past 4.5 billion years. There will be no hydrogen left at the very center of the Sun 4.8 billion years form now. Slow evolution will continue burning hydrogen near the center, but it is too cool and tenuous for nuclear fusion outside the hot, dense core. The Sun will have used up all its available core hydrogen in about 7 billion years, and will then balloon into a red giant star with a dramatic increase in size and a powerful rise in luminosity (Fig. 3.9).
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Copyright 2010, Professor Kenneth R. Lang, Tufts University