by L A Myers-Campos

Created on: September 05, 2009

The length of a star's life and what takes place in that life span depends mainly on its total mass. The more massive the star, the longer and more dramatically it lives, and dies. The smallest stars go out with a quiet unseen whimper, while the biggest ones never really die but become black holes.

Regardless of mass, all stars share the same general birth, formed within diffuse nebulae, molecular clouds sometimes referred to as stellar nurseries; the nearest example of which is Orion's Nebula in the constellation Orion. These nebulae possess a size and density conducive to the formation of molecules, primarily hydrogen with a tiny amount of helium. Within this nebular haze of gas and dust, gravity exerts an unequal force, forming the molecules into clumps. These clumps possess their own gravitational force. This gravity attracts more atoms to its mass. As the clumps grow, gravity increases, causing molecular collapse within the clump, which in turn, increases the mass of the core and subsequently, its gravity, and more atoms are gathered. This gathering of molecular mass is called accretion and results in the formation of the protostar.

As the protostar eliminates binding gravitational energy, heat is lost through the shell via radiant energy from gas expansion. This loss of energy causes the core of the protostar to continue to collapse, resulting in increased core temperature, pressure, and density.

In order for a protostar to become a star, it must reach equilibrium between these opposing forces of expansion and contraction before nuclear fusion can begin. Eventually, the core is so dense that it becomes opaque. When gas molecules fall toward this opacity, shock waves create additional heat. When the core reaches 2000 Kelvin, nuclear fusion of hydrogen into helium begins, and lasts until the hydrogen is depleted.

If it cannot do this, then it becomes a brown dwarf. The brownish red shade of failed protostars is remnant radiant energy from the failed fusion.

Thus begins the battle of equilibrium that will last for the duration of the star's energetic life. The major portion of any star's life, spent in fusion, is the main sequence phase. When there is no more hydrogen, helium is fused into carbon. The more massive stars continue the process of fusion, creating increasingly heavier elements according to the periodic table, while maintaining equilibrium.

Toward the end of the fuel supply, the core contracts further while the atmosphere inflates and expands, transforming

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