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The End

The End of Life on Earth The End The habitable zone on Earth will not last forever. Like most stars, the Sun exists in a stable configuration balanced between gravity pulling inward and pressure (maintained by heat from nuclear burning) pushing outward. This balance is self-regulating. Any slight change in one force will be off set by the other as long as there is enough fuel to maintain the burning. The solar system will undergo drastic change with the depletion of the hydrogen fuel. At the end of the evolution, the Earth may be still around, but any evidence that a biosphere once existed on the planet will have long since been melted

Figure 09-28 The End of Life on Earth [view large image]

Figure 09-29 The End [view large image]

and re-crystallized into oblivion. Figure 09-28 presents an updated history of life on Earth from the 2016 issue of New Scientist. The scenario combines fossil evidences in the past and the theory of solar evolution for the future.
Essentially, once life reaches its maximum prosperity, the decline is just the opposite of advance, i.e., from land to sea, from multicelluar to unicelluar, the final existence will linger hopelessly in underground cave to escape the scorching sun. The end of even such simple life would come about 7.5 billion years from now.
    As illustrated in Figure 09-29, evolution of the Sun can be divided into three phases :

  1. Main Sequence - Since the contraction from the pre-main sequence evolution, the Sun has settled into the main sequence for about 4.5 billion years. There will be another 7.5 billion years before the hydrogen fuel becomes exhausted. Meanwhile, its core grows denser, the central temperature climbs high enough that hydrogen fuses all the faster. During the next 1.2 billion years its luminosity will increase another 10% as it swells slightly and its surface temperature raised about 150oC hotter. The warmer climate will actually be a boon for life in the next 500 million years until the temperature reaches to the boiling point of water. Eventually, the Earth will reach a runaway moist greenhouse in the next 1.2 billion years, when very hot temperatures lead to greatly increased evaporation from the oceans, more water vapor in the atmosphere, thus even hotter temperatures, and so on until the oceans boil dry. The water vapor is then subject to photolysis (breaking up of the water molecule by ultraviolet light), and the hydrogen atoms lost to space for good.

  2. Red Giant 1 - By the end of the main sequence phase, hydrogen burning will shift to a shell outside the Sun's core and happens faster and faster. Energy production will increase dramatically, causing the Sun's outer layers to balloon in size. As the Sun nears the apex of its red giant 1 phase its size will increase than 200 times to engulf the present orbit of Venus and almost to the present orbit of Earth. Detailed study shows that the planets may spiral outward by tidal force. Thus, it is not yet clear whether the Earth will escape being engulfed by the red giant Sun. It is conceivable that the habitable zone would expand together with the Sun. Those now frozen moons such as Europa, and Titan may provide a shelter for life in the dying phase of the Solar system. The first red giant phase ends abruptly when the core temperature reaches 100 million degrees Celsius and helium begins to fuse into carbon, providing a fresh new energy source. The Sun responds by shrinking drastically and cutting its overall luminosity by a factor of nearly 100, providing a respite from a long string of disasters if the Earth is still intact.

  3. Red Giant 2 - This phase is the repeat of the previous one except that it is the exhaustion of helium instead of hydrogen. Once again the Sun becomes a serious threat to Earth's physical survival. During the second red giant phase it experiences several epochs of enormous energy output (helium shell flashes) that lead to massive, roughly 10,000 year long pulsations in size. It is distinctly possible that Earth could be briefly engulfed during these pulses. Then, roughly 100 million years after the red giant 2 phase begins, the Sun will throw off its massive outer layers completely to form a planetary nebula leaving behind a brilliantly hot but very tiny white dwarf. The surviving planets, including perhaps the cinders of Earth and Mars, will orbit the white dwarf quietly and stably for hundreds of billions of years as they and the Sun's little remnant cool ever closer to absolute zero - awaiting the bizarre ending of the cosmic expansion when everything is cut off from everything else (according to one of the many pre-bang theories).

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