Posted by: T. Boyd | March 25, 2009

What kind of fuel is burning in stars?

Astrophysicists think the energy that fuels the huge output of heat and light from a star is nuclear fusion. All of the measurements of the radiation from stars is consistent with fusion. But what is nuclear fusion? And for comparison, what is nuclear fission?

Both of the processes involve producing final products that are more stable than the initial ingredients. Fission happens when a heavy nucleus breaks up into two or more lighter nuclei, releasing lots of energy, because the lighter nuclei are more stable. Fusion goes the other way. Two very light nuclei are combined to give a heavier, more stable nucleus, releasing even more energy per gram, because the difference in stability between the “parents” and the “child” is even greater.

The most stable nuclei have 50-60 neutrons plus protons in them (iron and nickle, are typical in this range). So the final product at the end of the life of a star, for example, is a large chunk of this material. The lighter elements (nuclei) have fused together into these medium-heavy nuclei.

For comparison, nuclear fission starts with very heavy nuclei, Uranium, for example, with 238 neutrons and protons, and splits up into products frequently in this same range of medium-heavy nuclei.

So, to review, a star spends something like 99% of its life converting Hydrogen into Helium in the fusion process, then it changes character, spending the rest of its life (much shorter time scale) combining Helium into heavier nuclei, and those products into heavier ones still, until the stability maximum is reached, then it “dies”.

Since fusion is more efficient than fission, then why haven’t fusion reactors been made successfully for man’s use? The answer is, because it is very difficult to get the Hydrogen nuclei close enough to combine. They naturally repel each other because the protons are positively charged (remember, like charges repel).

Because a star is so massive, the strong gravitational force overcomes the repulsion between like charges, forcing them close enough to fuse. This kind of force has been very difficult to produce in the laboratories on earth. Only in the Hydrogen bomb has fusion been consistently possible. The atomic bomb is a fission process.

So that is a brief account of how a star “burns”. Another article will talk about the sequence of the life of a star including the time scale involved.

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