What is being built during nucleosynthesis

The s-process works as long as the decay time for unstable isotopes is longer than the capture time. After aboutyears the process of recombination occurred.

Nuclear reactions in stars

Fortunately for astronomers, there are indicators of how much chemical evolution particular objects have undergone, most importantly the presence of elements such as oxygen and nitrogen. In order to infer the primordial helium-4 abundance, astronomers turn to certain dwarf galaxies. Any light that reaches us from those quasars shows us the universe as it was about ten billion years ago. Low abundances indicate that the star is rather old. If we trust the current models to be basically correct in their prediction that on average, more helium-3 is produced in stars than is destroyed, this gives as an upper limit for the original abundance of helium The problem here again is that deuterium is very unlikely due to nuclear processes, and that collisions between atomic nuclei are likely to result either in the fusion of the nuclei, or in the release of free neutrons or alpha particles. If the reaction rates are high, then a net flux of energy is produced. After recombination, photons are free to travel through all of space. Light elements namely deuterium, helium, and lithium were produced in the first few minutes of the Big Bang, while elements heavier than helium are thought to have their origins in the interiors of stars which formed much later in the history of the Universe. Lithium 7 could also arise form the coalescence of one tritium and two deuterium nuclei. Putting it all together Armed with the results described above, we can confront the predictions of Big Bang Nucleosynthesis with astronomical observation. This would bring all the mass of the Universe to a single point, a "primeval atom", to a state before which time and space did not exist. In the very early Universe the temperature was so great that all matter was fully ionized and dissociated. Oxygen plus helium produces neon.

Neutron capture can happen by two methods, the s and r-processes, where s and r stand for slow and rapid. Subsequently, Hoyle's picture was expanded during the s by contributions from William A. In stellar nuclear fusion processes, any deuterium that might be present- whether primordial or more recent - is quickly converted to helium Gravity took over and eventually these atoms were pulled together into massive clouds gas in the vastness of space.

The present measurement of helium-4 indicates good agreement, and yet better agreement for helium The presence of elements such as oxygen or nitrogen see above or, in this particular case, the presence of iron serve as indicators of chemical evolution: Substantial amounts of these elements suggest that a star is young, produced from the debris of other stars.

Helium fusion

In the years immediately before World War II, Hans Bethe first elucidated those nuclear mechanisms by which hydrogen is fused into helium. Largely, it is fused into carbon via the triple-alpha process in which three helium-4 nuclei alpha particles are transformed. The second reason for researching non-standard BBN, and largely the focus of non-standard BBN in the early 21st century, is to use BBN to place limits on unknown or speculative physics. Larger quantities of these lighter elements in the present universe are therefore thought to have been restored through billions of years of cosmic ray mostly high-energy proton mediated breakup of heavier elements in interstellar gas and dust. The next diagram shows the results for helium-3 and deuterium: [Adapted from an image by E. Stars have a layered structure - nuclear fusion reactions take place in the inner, hotter regions, but not in the outermost layers. While there is no direct way to ascertain how much of the helium detected is helium-4, as opposed to helium-3 the latter has only a single neutron in each nucleus , measurements in our own galaxy show helium-3 to be exceedingly rare, accounting for a mere thousandth of a per cent of total helium. It is now known that the elements observed in the Universe were created in either of two ways. Hence, in the interior of stars, light elements are constantly created from even lighter precursors, and destroyed as subsequent fusion produces heavier atomic nuclei.

Hence observations about deuterium abundance suggest that the universe is not infinitely old, which is in accordance with the Big Bang theory. Density Fluctuations: The time of neutral atom construction is called recombination, this is also the first epoch we can observe in the Universe. The next diagram shows the results for helium-3 and deuterium: [Adapted from an image by E.

These are elements with nuclei that are produced by nuclear fusion reactions in stars, but that definitely could not have been produced during Big Bang Nucleosynthesis.

Analysing the spectrum of the quasars' radiation, in particular special features "absorption lines" due to deuterium and to ordinary hydrogen, the deuterium abundance can be determined. The Universe becomes transparent at this point.

primordial nucleosynthesis
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Big Bang Nucleosynthesis