Nucleosynthesis.docx

Nucleosynthesis The Proton-Proton Chain (produces 90% of Sun's energy). T = 10 million K. Hydrogen burns to form Helium, with about 0.6% of the mass converted to energy. That is, when four hydrogen nuclei fuse to make one helium nucleus, a small amount of matter seems to disappear: That mass is converted to energy according to Einstein's equation, Note that one joule (J) is about equal to the energy of an apple falling from a table to the floor, and is enough energy to light a 10-watt light bulb for almost one-half a trillionth of a second. In order to generate the energy output of the sun, the solar luminosity, L(sun), the sun converts 600 million metric tons of hydrogen into helium each second. But the sun is so massive, there's enough fuel to continue burning at this rate for another 5 billion years. These reactions release energy in the form of gamma rays, positrons, and neutrinos. The neutrinos, carrying about 2% of the energy, leave immediately. The positrons combine with electrons, annihilating one another and creating another gamma ray. The gamma rays can travel only a few centimeters before they are absorbed by the surrounding gas. Thus they heat the star's interior as they make their way to the surface, reaching it in about 10,000 years, and giving us STARSHINE. The CNO Cycle (produces 10% of the Sun's energy). This process, and the ones to follow, require a higher temperature because of the larger charge in the nucleus. This charge produces electrostatic repulsion, which must be overcome before a nuclear reaction can take place. T = 16 million degrees K. The CNO cycle is also a hydrogen-burning process. Triple Alpha Process T = 100 million degrees K. Helium burns to form Be (which disintegrates in only 10-12 seconds), C, O, Ne, Mg, etc. Carbon burning T = 600 million degrees K. Carbon burns to form the heavier elements.  STEP 1:  Two hydrogen collide to produce a deuteron, a positron, and a neutrino. The positron subsequently collides with an electron --- they annihilate one another, and create a gamma ray.  STEP 2:  The deuteron collides with another hydrogen, yielding a helium-3 nucleus and another gamma ray.  STEP 3:  Two helium-3 collide, yielding one helium-4 and two hydrogen. 4 hydrogen nuclei = 6.693 x 10-27 kg - 1 helium nucleus = 6.645 x 10-27 kg difference in mass = 0.048 x 10-27 kg E = m c2   = (0.048 x 10-27) x (3 x 108 m/s)2   = 0.43 x 10-11 J The triple-alpha process, which takes place only at temperatures above about 108. Beryllium, 4Be8, is but an intermediate step. These processes produce heavier and heavier elements while also producing energy. The sequence stops after making IRON since processes involving iron TAKE energy, rather than PRODUCE it!