An interesting case of spontaneous nuclear fission is californium²⁵⁴ (charge 98), with a half-life of 55 days. This isotope is formed in large quantities in certain stellar explosions called super-novae. Once in a millennium one of a collection of a billion stars flares into incredible brilliance. For a few weeks this single star shines with the combined energy and luster of a billion ordinary stars—then it fades away gradually. Such a “new” star (nova), with the greatest power of radiation, is called a “super-nova.”
We believe that many nuclear reactions take place in a super-nova. It has been observed that a few weeks after the initial outburst of light, the intensity of light is reduced almost exactly by a factor of two every 55 days for a year or so. This is precisely what would be expected if the energy generated in the star during this time were due to the spontaneous fission of californium²⁵⁴. Here we see a model of what happens to naturally radioactive elements. Of these we have retained on earth only the ones with the longest half-lives, like uranium, thorium, and potassium.
CHAPTER VI
Reactions Between Nuclei
The alchemists tried to transform one element into another artificially. They used heat, they used chemicals; they even used witchcraft. They failed. Their simplest method—to heat the substance in order to transform it—was really correct. The trouble was that their temperatures were too low by a factor of more than 10,000. What is needed, is a temperature of the order of tens of millions of degrees.
At such high temperatures two nuclei may occasionally approach each other in spite of the electrical repulsion between them. Sometimes they may even get close enough to each other to undergo a nuclear reaction. This, of course, happens with least difficulty if the nuclear charge is small. Hydrogen nuclei, which carry charge 1, participate in such reactions most easily.
In the interior of stars temperatures range from about 10 to 100 million degrees, and nuclear reactions do occur. The reaction responsible for the production of energy in the stars is:
4H¹ → He⁴ + energy
Four protons combine to make an alpha particle with a release of energy. Actually this reaction does not take place all at once but several steps are required. That energy should be released, one expects from the fact that the alpha particle is very stable. Any process in which light nuclei combine to form a heavier nucleus with a release of energy is known as “fusion.”
The particular fusion process that goes on in the stars releases its energy in many forms: as positrons, neutrinos, electromagnetic radiation, and motion of the reacting particles. The positrons also carry off the excess charge of the reaction.
The neutrinos fly through the star without interacting, carrying their energy away into outer space, probably never again to make contact with the material universe. The remainder of the fusion energy is deposited within the star’s interior, which is thus kept hot enough so that the fusion reaction can keep going. The name “thermonuclear” is appropriately applied to this type of reaction.