Between the stars, there is a region, called the interstellar medium, which contains huge clouds of gases and solid particles. It is comprised of about 75% hydrogen, 25% helium, and traces of carbon, nitrogen and other elements. The clouds, called nebulae, are cool and relatively dense and are shaped by currents of hot, low-density gases. It is in these nebulae, that certain processes come together to form stars.Stars form when a shock wave passes through a nebula, compressing the clouds and causing the gases and dust there to clump together. Gravity pulls the material together in the center, accelerating the atoms, increasing the density and generating heat, creating what is calle ...view middle of the document...
The star contacts and its outer layers expand, become cool, and less luminous. The star gradually moves up and to the right on the H-R diagram. The duration of a star depends on its mass since the stars with the greatest mass consume fuel rapidly and have relatively short lives (millions of years). A low mass star conserves its energy and can live for billions of years. (Seeds)During its life, a star burns up its hydrogen in a nuclear fusion reaction. Once the hydrogen is consumed, if it has enough mass, it can then begin to fuse together its other elements, such as helium, for fuel. It will continue on this path until finally it cannot fuse together any of the remaining elements into fuel. It is then that the star begins to die. All stars evolve, get older, move around on the H-R chart, and eventually explode or die. The particular form it takes when it dies directly depends on the star's size at its birth.A very low mass star--a red dwarf--lower than half our sun's mass--consumes its hydrogen very slowly. It is totally convective, which means the rising hot gases from the core and the sinking cooler gases from the shell continuously stir the stellar materials. It is not hot enough to fuse helium or any of the other heavier elements, but consumes all of its hydrogen uniformly. Astronomers predict that Red Dwarf stars can live for a hundred billion years so no Red Dwarf has yet died.More massive stars that are higher up on the main sequence have a different fate. Their greater mass means that their core and shell are not totally conductive and they have different reactions. When the hydrogen at the core of these stars fuse, the star's core fills with the byproduct, helium. When the star eventually runs out of hydrogen at its core, it is not hot enough to fuse helium, so the core begins to cools. Once it begins to cool, the outward pressure that the heat had generated lessens, and the core begins to contract. This contraction will, in turn, cause the core to heat up as the gravitational energy of the contraction converts to thermal energy. Some of the thermal energy (heat) is passed to the unprocessed hydrogen outside of the core. When this heats up enough, a hydrogen fusion reaction begins in this shell. This reaction continues to expand outward, heating ...
