About 5.5 billion years ago, a passing star or galaxy disturbed a calm and placid cloud of gas and dust, called a nebula. The star or galaxy caused the cloud to swirl around, causing small eddies to form. The swirl caused the gas to start to coalesce together. Gravity, one of the universe's four fundamental forces, caused more and more gas and dust to gather onto these masses. The masses kept getting bigger and bigger. At this stage, they were called protostars. As gravity caused the material to pile on, it also caused them to condense, which increased their gravitational force. The condensation caused the pressure in their cores to rise, and their internal heat increased. When the heat reached a temperature of 10,000,000 °C, nuclear fusion started, and our Sun was born (Case Western Reserve University, 2006).

After a lifetime of 9 billion years as a main-sequence star, approximately 10% of the hydrogen in the Sun's core will have been converted into helium, and nuclear fusion reactions will cease producing energy. The equilibrium between the total pressure force directed outwards and the gravitational force directed towards the centre of the Sun will be disturbed. The core of the Sun starts to slowly collapse under its own gravity and the fusion reactions move out towards shells surrounding the core, where hydrogen-rich material is still present. The gravitational energy from the collapse will be converted into heat causing the shell to burn vigorously and the Sun's outer layers swell. The surface will be far removed from the central energy source, and it will cool and appear to glow red. The Sun will then have evolved into the stage of a red giant (Encyclopedia of Planetary Sciences, 1997).

For a few hundred million years, the expansion of the outer solar layers will continue, and the Sun, as a red giant, will engulf the planet Mercury. The temperature on Venus and Earth will rise tremendously. Hydrogen fusion in the shell continues to deposit helium "ash" onto the core, which becomes even hotter and more massive. In the Sun's core nuclear fusion of helium into carbon and oxygen will start to trigger even further expansion of its outer layers. The helium-rich core, unable to lose heat fast enough becomes unstable. In a very short time of a few hours, the core will get too hot and is forced to expand explosively. Outer layers of the Sun will absorb the core explosion but the core will no longer be able to produce energy by thermonuclear burning. Helium fusion will then continue in a shell and the structure of the Sun will look like an onion: An outer, hydrogen-fusion layer and an inner, helium-fusion layer, which surrounds an inert core of carbon and oxygen. The old Sun may repeat the cycle of shrinking and swelling several times. In this stage of evolution the Sun is called an asymptotic giant branch star. Finally enough carbon will accumulate in the core to prevent the core explosion. Helium-shell burning will add heat to the outer layers of the Sun, mainly containing hydrogen and helium. The asymptotic giant Sun will eventually generate an intense wind that will begin to carry off its outer envelope. The precise mechanism behind this phenomenon is not yet well understood. The Sun will expand a final time, and after about 30 million years it will swallow Venus and Earth, outer layers will keep expanding outward and as much as half of the Sun's mass will be lost into space (Encyclopedia of Planetary Sciences, 1997). 

07.07.2010 22:25:23 

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