Mayan mythology says that we have had four different suns. Each one has been a god who was chosen to climb into the sky, and each one has been destroyed by the fighting of the gods Quetzalcoatl and Tezcatlipoca (gods of the evening star and shadows, respectively). Our sun, the fifth, will follow the same path and eventually be knocked out of the sky in a destructive battle. Today, we know our sun is not in danger of ever being knocked out of the sky by anyone or anything, but the Mayans did get one thing right; the sun will eventually die and it will take our world with it.
Over the next five billion years, the sun will slowly turn its remaining hydrogen into helium. When it runs out, it will becomes a red giant and eventually a white dwarf. First, fusion in the core will stop and gravity will start to compress the core of helium and a surrounding shell of hydrogen until the hydrogen gains enough thermal energy to reignite. The shell will actually fuse at a higher rate than the core did, so it will create a pressure buildup that will push out the surface of the sun. For around a billion years, the sun will continue to grow in size and luminosity, while the core and the hydrogen shell will continue to be compressed and heated until the previously inert helium core gains enough thermal energy to ignite. The sun will release a huge amount of energy in what is called a helium flash and the core will expand with the new thermal energy. The sun will actually contract after this as fusion rates stabilize. Though, as it uses up its helium, the sun will start to go through thermal pulses because helium fusion is not as stable as hydrogen fusion. When all the helium in the core has been fused to carbon, the core will start to condense, and a shell of helium fusion will drive the surface back out, just as it did with the hydrogen shell, except this time the sun will expand to a much larger size. Once the helium and hydrogen in the shells are used up, the sun will begin to collapse again. The carbon core will heat up, but degeneracy pressure (the repulsion of electrons) will stop it before it gets to the 600 million Kelvin that is needed to start carbon fusion. In this last stage, the sun's gravitational pull on its outer layers is weak enough that most of them will be carried away by solar winds to form a solar nebula. The leftover core of carbon will become a white dwarf at the center and spend the rest of eternity there radiating away all of its remaining thermal energy.
This is a process that can destroy the earth several times over. The sun will increase in luminosity near the end of its life and that combined with the greenhouse effect will bake the earth; the temperature will rise to 1000 K as the sun expands into a red giant. Then powerful solar winds will rip into the atmosphere, causing great damage. Eventually the sun’s outer layers will grow big enough to engulf the earth totally, and what's left of Earth will freeze over.
Clearly it is not a question of “if” but of “when” we will be leaving Earth. If we could wait for the sun, we would have nearly 5 billion years, but humankind will leave Earth long before that. Factors like overpopulation, resource depletion and pollution that are already becoming problems today will drive us.
Two of the main ideas that scientists and engineers have looked at for interstellar travel are ramjet fusion and antimatter reactions. Fusion is one of the most powerful reactions we know of, but it is only about 1% efficient (Kaku 287). Antimatter reactions are 100% efficient. When a particle, such as an electron meets its antimatter equivalent, a positron in this case, they annihilate each other, turning completely into energy. It is estimated that 4 milligrams of antimatter could fuel a rocket to Mars (Kaku 287). Today antimatter is too expensive to be economically viable, but in the future we could create particle accelerators specifically designed to make large quantities or the cost might be worth it for the more wealthy to escape Earth.
The other option, ramjet fusion, works similarly to regular ramjets today, which scoop air from in front of them, mix it with jet fuel and ignite that to give them thrust. Fusion engines would harvest hydrogen and compress and heat it inside the ship until it fuses and creates thrust. Since hydrogen is almost everywhere in the universe, this design would theoretically enable travel forever. Robert Bussard in 1960 calculated that if a fusion ramjet could create enough power to go 32 ft/s/s. (This is equivalent to the force of gravity felt on Earth.) Then it could get up to 77% lightspeed in a year. That means it could reach another solar system in 23 years (as felt by the astronauts; it would be longer for observers on earth) (Kaku 285).
Be it in a hundred years, or a thousand, or maybe billion, it’s inevitable that we will one day fly to other planets, and then other stars, and then other galaxies. Even if there were no other reasons, curiosity would not let us stay here, because every time we looked at the stars, our sense of wonder would not let us forget that there is more out there than we can possible imagine. Other galaxies, other worlds, other life, even. We won’t be able to stop ourselves until we know everything, and we won’t be able to know anything until we go. Biology is important, and geology, but astronomy studies more. It is the study of everything in our known world, while biology is the study of a phenomenon found only once, that we know of, on a tiny rock. The heavens are the realms of sun gods and dreams and science fiction, and soon they will be the realms of humans and science reality.
~Sarah P.
No comments:
Post a Comment