Nucleosynthesis Essay Research Paper The

Nucleosynthesis Essay, Research Paper

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.

FIGURE 1

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 positron (b+) and a neutrino (n). 8Be is itself radioactive, and almost immediately breaks into two 4He nuclei.

Protons repel each other. This repulsion becomes stronger as the protons get closer together (just like when you try to stick two magnets together north to north, or south to south. Try this! As you push the magnets closer together, it becomes harder to do). 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. (Magnets do not do this: two like poles will never stick together).

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. But, just as you can make two of the same magnetic poles touch each other by providing sufficient force, so too can protons “touch” when they have sufficient relative speed. This can take place in the center of the sun, where the temperature is extremely high. Temperature is related to atomic motion: the hotter something is, the faster its atoms are moving [] see demo “food coloring in water”[].

Table 1 shows the nuclear reactions that are taking place in our sun, as well as nuclear reactions that take place in stars that are either older than our sun, or hotter than our sun. The reactions in columns 2 and 3 occur after a star has entered the “red giant phase”. How fast a star evolves to this point depends on its mass: stars heavier than the sun can reach this phase in less than 5 billion years (the age of the sun) whereas stars with about our sun’s mass take about 10 billion years to get there. The particles you may be unfamiliar with are: n the neutrino, g a gamma ray (high energy light wave), and b+ the positron (the antimatter version of the electron).

TABLE 1. NUCLEAR REACTIONS IN STARS

OUR SUN NOW OLDER, OR HOTTER STARS

p + p . 2H + b+ + n 4He + 4He . 8Be + g 12C + p . 13N + g

2H + p . 3He + g 8Be + 4He . 12C + g 13N . 13C + b+ + n

3He + 3He . 4He + p + p 12C + 4He . 16O + g T1/2 = 10 min

16O + 4He . 20Ne + g 13C + p . 14N + g

3He + 4He . 7Be + g 20Ne + 4He . 24Mg + g 14N + p . 15O + g

7Be + p . 8B + g 15O . 15N + b+ + n

8B . 8Be + b+ + n T1/2 = 120 ms

8Be . 4He + 4He 15N + p . 12C + 4He

He burning (core) H burning shell

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.

As a star like the sun evolves, a vast amount of H is consumed, and a vast amount of 4He is produced. This 4He can not combine with other 4He because this reaction requires more energy than is available, i.e. the 4He nuclei are not moving fast enough. However, as the 4He accumulates in the star’s core, the pressure rises (which causes the temperature to rise since the core consists of gas). With increased temperature, more energy becomes available, and the star eventually reac

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