The Astrophysics Spectator

Issue 2.32, September 21, 2005

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September 21, 2005

This issue of The Astrophysics Spectator improves the discussions on the “Stars” survey path of the thermonuclear fusion of helium, carbon, and oxygen. Up to this time, these three stages of nuclear fusion were discussed on a single page. Now this discussion is spread over three pages. One page is devoted to the thermonuclear fusion of helium; it discusses a broader range of fusion processes than did the earlier version. The second page illustrates with a live figure the dependence of the helium fusion rate on temperature. The third page is devoted to carbon and oxygen fusion.

The fusion of helium, carbon, and oxygen are part of the red giant stage of stellar evolution. As the name implies, stars undergoing these later stages of nuclear fusion are physically larger, with cooler surface temperatures, than when they are in their hydrogen-burning main-sequence stage. The transition to each of these stages follows a pattern that is dictated by the size, and therefore the temperature, of the star's core.

A star spends most of its life as a main sequence star. While on the main sequence, the star converts the hydrogen in its core into helium. When the hydrogen at the core of a main sequence star is exhausted, the core starts to contract as the heat in the core diffuses out of the star. This contraction releases gravitational energy, so while the total amount of energy in the core, the thermal energy plus the gravitational potential energy, falls, the temperature actually rises from the release of gravitational potential energy. Eventually the core of the star becomes compact and hot enough for helium to burn to carbon and oxygen.

The onset of helium fusion is sudden, and its sends a shock wave through the star that drives away the star's outer layer to create a planetary nebula around the star. The star is not disrupted, but its outer layers do expand outward, making the star a giant. The helium fusion stage lasts until all of the helium is converted into carbon and oxygen.

Again, the core of the star collapse once all of the helium is converted into carbon and oxygen, and its temperature rising until the thermonuclear fusion of carbon commences. Carbon is converted to heavier elements, such as magnesium and neon.

Once the carbon is gone, the star collapses until oxygen begins to burn. This stage converts oxygen into elements such as sulfur and silicon.

For the smallest stars, the helium stage is the final stages of their lives. Once fusion of helium ceases, the star becomes a white dwarf, stabilized by the degeneracy pressure of the electrons in the star. The more massive stars continue their collapse to smaller core sizes after the oxygen burning stage. These stars burn the heavier elements created during the carbon and oxygen burning stages, until the core is composed of iron. This is the endpoint of nuclear fusion, because the creation of elements heavier than iron do not release energy, but remove energy from a gas. Once the iron stage is reached, the star collapsed to either a neutron star or a black hole.

The helium-, carbon-, and oxygen-burning stages release much less energy than the hydrogen-burning stage. Measured per unit mass, helium fusion releases 9% of the energy of hydrogen fusion, carbon fusion releases less than 3% of the energy of hydrogen fusion, and oxygen fusion releases less than 2% of the energy of hydrogen fusion. In a red giant, the hydrogen fusion in the outer layers of the star releases energy at a rate that is comparable to the rate of energy release through helium fusion in the star's core.

Publication Notice. The next issue of The Astrophysics Spectator is slated for October 4.


Helium Fusion Rates. The helium nuclear fusion rates are presented and discussed. These rates are displayed by a live figure as two plots. The plots give the fusion rates as a function of temperature. The reader can set the temperature units to either degrees Kelvin or keV. The figure shows that the nuclear fusion of helium-4 is dominated by the triple-alpha process. (continue)

Carbon and Oxygen Fusion. The third and fourth stages of nuclear fusion convert carbon and oxygen into heavier element. These burning stages are short-lived stages of a red giant's life. This page outlines the principal carbon-carbon and oxygen-oxygen fusion processes. (continue)


Helium Fusion. The scope of this page has been narrowed: while the previous version discussed helium fusion, carbon fusion, and oxygen fusion, the current version discusses only helium fusion, with the discussion of carbon fusion and oxygen fusion moved to a new page. The discussion of helium fusion has been expanded to include secondary processes that convert the carbon-13 and nitrogen-14 left from the CNO hydrogen fusion cycle into heavier elements. These secondary processes produce neutrons that can be captured by other nuclei, creating isotopes not on the helium fusion paths. (continue)


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