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

The solid line shows the progression of the s-process starting from the seed nucleus 56Fe. We can get a rough idea of the neutron flux (the number of neutrons hitting a given location each second) by comparing the half-lives of 'branching isotopes' with non-branching isotopes. For instance, 69Zn has a half-life of 13.8 hours, and 75Ge has a half-life of 82.8 minutes, whereas 63Ni has a half-life of 100 years and 85Kr of 10.7 years. Thus neutrons are only absorbed very infrequently (probably on the order of weeks between absorptions).

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. This accumulates in the core (recall that we can't go beyond 56Fe with fusion). As always, there is a battle between gravity (which tries to compact the core) and heat (which tries to expand the core). Eventually, after enough 56Fe is produced, gravity wins. When the Fe core collapses, it does so dramatically, and generates pressures which are truly incredible. 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 (1.4 solar masses worth!). An implosion shock wave reaches the core's center and rebounds. As it does so, it sweeps vast numbers of neutrons out with it, and they smash into the matter above them. Now the neutron absorptions occur rapidly enough to bridge the gap out to nuclides like 70Zn. In fact, they happen rapidly enough to bridge the gap from Bi to the heavier elements (we know that 244Pu existed in the early solar system. This means that a 209Bi nucleus would have to absorb at least 35 neutrons before any a decay could occur!). Thus in one brief event, lasting at most only a few seconds, we produce all known elements heavier than Fe.

In our sun, the first three nuclear reactions {shaded} are the major source of energy. The second group (of four) reactions also occur in the sun, but much less frequently than the first group (which is called the p-p chain). In both cases, the fuel (hydrogen) is converted into the product (helium), and energy (in the form of heat and light) is produced. The third column of reactions is called the CNO cycle, because carbon (C), nitrogen (N) and oxygen (O) are produced and C is recycled. The CNO reaction cycle is now occurring in the sun (the energy required for these reactions is roughly the same as that required for the p-p chain) because the sun had carbon to begin with (it hasn't made any C yet!). Since the sun had carbon present when it formed, it is referred to as a 'later generation' star. The 'first generation' of stars contained only H, He and Li from the big bang. Later generation stars contain material that has been processed in other stars.

Our bodies, at a temperature of about 40 °C (~100 °F) give off infrared radiation which can be seen with special cameras. A log in a fire, at a temperature of about 600 °C (~1100 °F) glows red. Molten metal in a furnace, at a temperature of about 1500 °C (~2700 °F) shines with intense white light. Thus as temperature increases, the radiation (light) emitted becomes more energetic (changes color to shorter wavelengths) as well as more intense (more photons emitted per second). This is basically a result of the increased energy of the atomic collisions in the hot material []see "Blackbody Radiation" module[]. 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. 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. This is the first step in making a laser beam

Some common words found in the essay are:
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, 12c + 13n, glance periodic table, radioactive decay module, produce elements heavier,
Approximate Word count = 2780
Approximate Pages = 11 (250 words per page double spaced)