Nucleosynthesis
The "big bang" which created the universe, only created the elements Hydrogen (H) and Helium (He) and possibly a very small amount of Lithium (Li). However, a glance at the periodic table of the elements shows that today (some 15 billion years after the big bang) there are at least 108 known elements. Every atom of every element heavier than Li has been produced since the big bang! The "factories" which make these elements are stars. "Nucleosynthesis" or the synthesis of nuclei, is the process by which stars (which start out consisting mostly of H and He) produce all other elements. The key is nuclear fusion, in which small nuclei are joined together to form a larger nucleus. (This contrasts with nuclear fission, in which a large nucleus breaks apart to form two smaller nuclei). Fusion requires an extremely large amount of energy (see fig. 1), and can typically only take place in the centers of stars. a) Low energy proton is strongly repelled by the 7Be nucleus.b) High energy proton moves so fast that it can strike the 7Be nucleus. Once the proton touches the nucleus, it has a chance to stick. If the proton sticks, the 7Be becomes a 8B nucleus.c) 8B is radioactive and changes into 8Be plus a pos
In both cases, the fuel (hydrogen) is converted into the product (helium), and energy (in the form of heat and light) is produced. However, if the protons can actually touch each other, they have a chance to stick together! This is because of the "strong nuclear force" which attracts nucleons (protons or neutrons) together, and is much stronger (at close range) than the "electromagnetic force" repulsion that makes protons repel other protons. In order to get a proton to strike another proton (or a nucleus that contains several protons) they must be traveling at high relative speeds; if their "closing velocity" is not great enough, they will never get close enough to stick together, because they strongly repel each other. These isotopes, and the elements above Bi are produced in the 'r-process'. These dust grains survive to the present day, preserved in primitive meteorites []see "Interstellar Grains" module[]. ) But from figure 2 of the "Radioactive Decay" module, we see that this excited state has a much shorter half-life than the ground state (4. Thus the star eventually has a distribution of nuclides between 56Fe and 209Bi (above 209Bi, a decays happen rapidly. The word "rapid" is actually an understatement; it could be called "explosive"; the r-process occurs in supernova explosions! Here's how it works: Before a supernova, a star has produced an excessive amount of 56Fe. Later generation stars contain material that has been processed in other stars. However, it also changes the chemical element (because the number of nuclear protons increases). For temperatures characteristic of star cores (hundreds of millions of C) the collisions produce nuclear reactions as well as an abundant supply of high energy gamma rays. The pressure is so great that the orbital electrons are pushed into their nucleus! Thus in one incredible electron capture reaction, all of the Fe in the core is converted to neutrons (1. When these gammas are absorbed by a nucleus, they can make the nucleus transition to an excited energy state (just as visible or ultraviolet light can make an atomic electron transition to a higher orbital.
Common topics in this essay:
HOTTER STARS,
Lithium Li,
Pb Bi,
Radioactive Decay,
Interstellar Grains,
Fe Appendix,
Blackbody Radiation,
+ 4he,
,
figure 2,
s-process path,
elements heavier,
nuclear reactions,
12c +,
produce elements,
absorb neutron,
heavier fe,
+ 13n,
+ b+ +,
Hydrogen Helium,
produce elements heavier,
+ 12c +,
+ 14n +,
elements heavier fe,
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Acceptance Essays
