Sunday, February 18, 2007

Sun-Life cycle

The Sun's current age, determined via computer models of stellar evolution and nucleocosmochronology, is thought to be about 4.57 billion years.
The Sun is about halfway through its main-sequence growth, during which nuclear fusion reactions in its core fuse hydrogen into helium. Each second, more than 4 million tonnes of matter are changed into energy within the Sun's core, producing neutrinos and solar radiation. The Sun will waste a total of approximately 10 billion years as a main sequence star.
The Sun does not have sufficient mass to explode as a supernova. Instead, in 4-5 billion years, it will enter a red giant phase, its outer layers growing as the hydrogen fuel in the core is consumed and the core contracts and heats up. Helium fusion will start on when the core temperature reaches around 100 MK, and will make carbon and oxygen. While it is likely that the expansion of the outer layers of the Sun will reach the current position of Earth's orbit, latest research suggests that mass lost from the Sun previous in its red giant phase will cause the Earth's orbit to move more out, preventing it from being engulfed. However, Earth's water will be boiled away and most of its atmosphere will run off into space.
Following the red giant phase, strong thermal pulsations will cause the Sun to throw off its outer layers, forming a planetary nebula. The only object that will stay after the outer layers are ejected is the really hot stellar core, which will gradually cool and fade as a white dwarf over many billions of years. This stellar evolution scenario is typical of low- to medium-mass stars.


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