01 Mei 2008


Stars are born from huge clouds of dust and gas within the galaxies. As a cloud shrinks under the gravitational forces of the particles from which it is made, some areas will become denser than others. These collapse into globules of matter that form the future stars, and as the particles collide with one another at an increasing rate, the resulting friction causes this matter to glow hotter and hotter. Eventually, after perhaps millions of years, the temperature rises to the point where nuclear reactions begin.
Most of the atoms of matter in the growing star are hydrogen atoms, and the enormous pressures and temperatures convert these to helium, in a similar reaction to the explosion of a hydrogen bomb. Once this reaction reaches a stable level, the star can go on burning as a thermonuclear furnace for thousands of millions years. How long this stable stage last depends on the size of the star, as larger stars burn more fiercely and more quickly than smaller ones.

The smallest stars never achieve a fierce enough reaction to burn with the brightness of the Sun, and their small size and dull glow earns the name of red dwarfs, which continue burning for millions of millions of years. The life of larger stars is limited when the reserves of unburned hydrogen at the core begin to run out. The fiercest reactions move outward from the center to the outer layersof the star, where hydrogen reserves still exits.

The star grows hotter and larger, to become a red giant. When the Sun reaches this stage, in 5,000 million years, it will expand to a hundred times its present size, and its output of heat and light energy will increase by a thousandfold. Some smaller stars will the fade away as their final reserves of hydrogen burn out.                                                             
Other are large enough for the outer shell of hydrogen gas to continue heating the central core, which is now made up almost entirely of helium. When the temperature and pressure are high enough, another nuclear reaction breaks down the helium atoms, releasing still more and converting there core into carbon. This reaction then spreads outward in another red giant stage as the helium is used up. More and more of its matter spreads out into space as the central core cools, turning the star into a white dwarf. Finally the star dies down into a black dwarfs of intensely dense ash.

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