Copy of `Berkeley Laboratory - Astrophysics Glossary`

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Berkeley Laboratory - Astrophysics Glossary
Category: Meteorology and astronomy > Astrophysics
Date & country: 11/09/2007, USA
Words: 16

Alpha-Carbon Reaction
After three helium nuclei have combined to form 12C in a red giant, another alpha particle is captured by the 12C to form 16O and gamma rays. These particles, 12C and 16O, are the ashes from a dying star. They are also the basis of life on Earth.

Carbon-Nitrogen-Oxygen Cycle
In stars more massive than the sun (>1.1 Solar masses), this cycle is the primary process which converts hydrogen into helium. 12C serves as a catalyst, an ingredient which is necessary for the reaction but is not consumed.

Electron Capture
The nuclei formed early in the birth of the universe still had not enough electrons to form a neutral atom. But, the free electrons had plenty of energy. After 700,000 years of cooling, the nuclei started to capture electrons. These electron clouds are important. They stopped radiation from passing through the atoms.

Helium Nucelosynthesis
When proton/neutron collisions produced deuterium, this sets up the way for the synthesis of the helium-4 nucleus. 4He was the largest nuclei produced in this stage of the universe. The energy density was too low to allow heavier nuclei to stick. At the start of nucelosynthesis, the relative abundance of protons and neutrons was 87% and 13% respectively. All of the neutrons were incorporated by the 4He. At the end of this wave, the universe consisted of roughly 25% helium and 75% hydrogen.

Hydrogen Burning
Hydrogen burning is the fusion of four hydrogen nuclei (protons) into a single helium nucleus (two protons and neutrons.) The process is a series of reactions. The type of reactions depend on the mass of a star and its core temperature and density. In our Sun, the process is a proton-proton chain. In more massive stars, the C-N-O cycle (Carbon-Nitrogen-Oxygen) serves to fuse hydrogen into helium.

Nucleosynthesis of Heavy Isotopes
The processes described of stars only produce nuclei with mass up to 70. Gold and uranium came out of three additiional processes:s-process r-process rp-process

Pair Production
A collision process for gamma rays with energies greater than 1022-keV (two electron masses) where an electron /positron pair is produced. A heavy nucleus must be present for pair production. For high-energy gamma rays the pair production process is proportional to Z2 and ln(gamma).

Positron Annihilation
Positron decay in matter by annihilation with an electron. Usually and 'atom' of positronium (e+e-) forms which annihilates to produce two 511-keV photons. Occasionally, the positron will annihilate in flight to produce on or more photons sharing the total rest mass and kinetic energy of the positron and electron.

Proton Separation Energy
The energy required to remove a proton from a nucleus.

Proton-Neutron collision
After about 14 seconds after 'the beginning' of the universe, the temperature dropped to 3,000,000,000 K. When neutrons and protons collided, it was to hot for them to stick together. Since deuterium (a proton and a neutron) could not be formed, there were no large molecules. As the universe cooled down, (see Universe Expansion) the nucleons (protons and neutrons) were able to stick together. Then, deuterium was able to form into helium.

Proton-Neutron conversion
Because of the high energy density at the start of the universe, the collisions which produced neutrons and protons balanced each other. As the universe cooled, the balance was skewed towards the protons. Neutrons have slightly more mass than protons. Because of the extra mass, they need more energy to form. When the energy available decreased, the reaction with more entropy was favored.

Proton-Proton Chain
In the Sun and other less massive stars, this chain is the primary source of heat and radiation. The proton-proton chain converts hydrogen into helium releasing energy in the form of particles and gamma-rays. Hydrogen is converted into helium in a chain of reactions. The first reaction takes an average of 1 billion years to occur while the others are much shorter. One step is only 1 second long. In the Sun, there are so many hydrogen nuclei that the 1 billion year waiting period does not stop it…

The r-process is a rapid sequence of neutron absorptions. It starts with a seed nuclide Z,A from the iron region, and one neutron is absorbed to give A+1. However, in the very rapid r-process, it is assumed that A+1 may not have time to decay before it absorbs another neutron and goes to A+2. This sequence continues moving toward the so-called 'neutron drip line' until the probability for absorbing a new neutron is overwhelmed by the probability that a neutron will be knocked off by photodisinte…

The rp-process is very similar to the r-process, except it goes by successive proton absorption and + decay; thus, it tracks somewhere between the valley of stability and the 'proton drip line.'

In the s-process, one starts with existing iron-group nuclei. Therefore, it would only be expected to take place in second-generation stars that collapsed out of the residue of a previous supernova explosion. A flux of neutrons is required, and it is most likely that these neutrons come from various (,n) reactions in the helium-burning region of a red giant star. The seed isotope Z,A from the iron region absorbs a neutron, changing from A to A+1. If the new isotope is stable, it can absorb anoth…

Universe Expansion
In 'the beginning', the universe was densely packed with radiation in the forms of photons and neutrinos. This energy collided with itself to create a dynamic wall. After each collision, the universe would get bigger. As the universe grew, the total energy stayed the same. Thus, the average energy or the temperature dropped to allow several important reactions proceed: Proton/Neutron collisions, Helium Nucleosynthesis.