The Life Cycle of a Sun-Like Star
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How does a star evolve? This question can have a very complicated answer. In order to simplify this answer we shall predominantly discuss the evolution of a star with a mass approximately the same as the mass of the sun.
The star begins its formation in a cloud of hydrogen and helium gas referred to as a nebula. Some anomaly within the gas causes gravitational collapse. This anomaly can be random motion, a supernova explosion, or several other things. As the gas collapses under gravitational forces the speed of the particles increases. As the speed of molecules increases the temperature of the gas rises. We know from blackbody radiation that as the temperature rises the gas will give off more and more light and that light will shine in the infrared portion of the spectrum and eventually move toward the visible portion of the spectrum or beyond. This large shining ball of hot gas is referred to as a protostar. Although the protostar is very bright due to its large diameter, it is generally very difficult to see because it is usually buried within the nebula from which it is forming.
During the protostar stage, the star's position on the HR diagram is in the mid to upper right side of the diagram. A star of about one solar mass will travel down and to the left on the HR diagram as it evolves. (See the diagram below.) Stars with other masses will evolve toward the main sequence via different paths.
As time goes on the center of the protostar continues to increase in temperature generating heat from gravitational contraction. At some point in time the core temperature will reach 10 million Kelvin or higher. At this point hydrogen burning commences. Hydrogen burning is the conversion of hydrogen nuclei into helium nuclei and thereby the generation of nuclear energy. It is this nuclear energy which will fuel the star for roughly 90 percent of its life. As part of this generation of nuclear energy comes the emission of high-energy photons in the core. These photons will generate an outward pressure which will balance the inward pressure of gravity. This balance or equilibrium causes the collapse of the star to cease and the star becomes stable. This hydrogen burning signifies the end of the protostar stage and the beginning of the star's life. This stage is referred to as zero age main sequence. "Zero Age" indicates it is just born.
While a main sequence star, energy production by nuclear fusion in the core causes a transformation from hydrogen nuclei to helium nuclei. At some point in time the hydrogen nuclei in the core will run out. When this occurs there will no longer be a balance between gravitational pressure inward and photon pressure outward. This is due to the fact that the photon production ceases when the fusion process runs out of fuel. At this point the core (almost pure helium) will begin to collapse again. As the core collapses its temperature will rise. Surrounding the core is a shell of hydrogen. This hydrogen shell will increase in temperature due to its proximity to the core. At some point the hydrogen shell will reach the temperature to burn hydrogen. When this happens the radiation pressure outward will be greater than the gravitational pressure inward and the star will expand. (Recall that gravitational forces will decrease with distance from the center.) As the outer layers of the star expand the temperature of the gas cools causing it to turn red in color. In addition, the increase in radius will cause an increasing luminosity. The star is now considered to be a red giant.
Meanwhile, the helium core is continuing to compress and get hot. At some point it will reach 100 million Kelvin and helium will begin to fuse. This fusion converts helium nuclei into carbon and oxygen nuclei. And once again photons generate radiation pressure outward balancing the inward pressure of gravity. In the case of a star like the sun, helium burning will not occur until the core has collapsed to a point called degeneracy. Degeneracy occurs when electrons, generally ignored up to this point, are pushed too close to each other. When this happens the gas is more metallic in nature than gaseous. The effect is that energy generated by helium fusion increases the temperature of the core but the core has no mechanism to release that energy as a gas would. Therefore, the core continues to increase in temperature until it explodes in what is called a helium flash. This flash does not destroy the core but forces it to re-contract until stable helium burning can take place.
As was the case with hydrogen burning, the fuel for helium burning will eventually run out in the core. Once again the core, now carbon and oxygen, will collapse in an attempt to increase its temperature to the point of carbon burning. However, in a star like the sun that core temperature will never be attained. The collapse will cause the temperature of a helium shell surrounding the core to increase to the point of helium burning. When shell helium burning commences the outward pressure from radiation exceeds the inward pressure from gravity and the outer star expands once again toward the red giant stage of the HR diagram. In the case of a star like the sun this expansion will continue until the outer portions of the star float away from the central portion. This outer portion is referred to as a planetary nebula and it leaves behind the core which is referred to as a white dwarf.
The Ring Nebula (M57), a planetary nebula
This page was last updated on 6/28/01.