Helium nucleosynthesis

First proposal of stellar nucleosynthesis by proton fusion to helium and heavier nuclides... 

Nucleosynthesis is the formation of elements. Hydrogen and helium were produced in the Big Bang all other elements are descended from these two, as a result of nuclear reactions taking place either in stars or in space. Some elements—among them technetium and promethium—are found in only trace amounts on Earth. Although these elements were made in stars, their short lifetimes did not allow them to survive long enough to contribute to the formation of our planet. However, nuclides that are too unstable to be found on Earth can be made by artificial techniques, and scientists have added about 2200 different nuclides to the 300 or so that occur naturally. 

The final outcome of these reactions, as a function of rj or equivalently Slboh2, is shown in Fig. 4.3. The primordial helium mass fraction TP, shown on a large scale, is not very sensitive to r), since this parameter only affects the time for neutron decay before nucleosynthesis sets in, and it can be fitted by the relation FP = 0.226 + 0.025log 0 + 0.0075(g - 10.75) + 0.014(r1/2( ) - 10.3 min). 

Helium is the second most abundant element in the visible Universe and accordingly there is a mass of data from optical and radio emission lines in nebulae, optical emission lines from the solar chromosphere and prominences and absorption lines in spectra of hot stars. Further estimates are derived more indirectly by applying theories of stellar structure, evolution and pulsation. However, because of the relative insensitivity of Tp to cosmological parameters, combined with the need to allow for additional helium from stellar nucleosynthesis in most objects, the requirements for accuracy are very severe better than 5 per cent to place cosmological limits on Nv and better still to place interesting constraints on t] or One can, however, assert with confidence that there is a universal floor to the helium abundance in observed objects corresponding to 0.23 < Fp < 0.25. 

The contributions of different stars to nucleosynthesis depend on their initial mass and chemical composition, their mass loss history in the course of evolution and effects of close binaries (especially SN la). When mass loss is small, as is believed to be the case for low metallicities, the distribution of primary elements (those synthesized directly from hydrogen and helium) in ejecta from massive stars is mainly the result of hydrostatic evolution with some modifications to deeper layers resulting from explosive nucleosynthesis in the final SN outburst, classically... 

The overall abundance of helium and heavy elements in the Universe today. This reflects the total effect of fuel consumption and nucleosynthesis by all the stars that ever existed. Roughly speaking, one may consider this in two... 

When the light is dominated by massive stars, e.g. in starburst galaxies, the luminosity is related in turn to the rate of metal production, since virtually all processed material is ejected in the form of metals (and some helium). Thus there is a relationship between the total co-moving luminosity density, the monochromatic luminosity density (deduced from star-forming galaxy redshift surveys with appropriate corrections for absorption) in a fixed frequency bandwidth (anywhere between 912 and about 2000 A in the rest frame) and the mass going into nucleosynthesis ... 

Hoyle successfully predicts existence of a 7.6 MeV resonance state of the carbon-12 nucleus on grounds that otherwise little carbon would survive further processing into oxygen during stellar nucleosynthesis by helium burning, whereas in fact the C/O ratio is about 0.5. Discovery of strange particles. 

Estimates of primordial helium and deuterium abundance with Big Bang nucleosynthesis theory limit number of light neutrino families to 4 or less (Schramm et al). 

Woosley SE, Langer N, Weaver TA (1995) The presupemova evolution and explosion of helium stars that experience mass loss. Fresenius Astrophys J 448 315-338 Woosley SE, Weaver TA (1995) The evolution and explosion of massive stars. II. Explosive hydrodynamics and nucleosynthesis. Astrophys J Suppl 101 181-235... 

The beginning of nuclear evolution, launched in the Big Bang, the creation of matter, the emergence of nucleons and the construction of the first nuclei hydrogen, deuterium, helium and lithium that took place in its immediate wake, followed by formation of the first stars in the early Universe and the establishment of stellar nucleosynthesis leading to production of carbon and all the other elements. 

Stellar nucleosynthesis thereby leap-frogs over three fragile elements, lithium, beryllium and boron, moving more or less directly from helium to... 

Whatever caused this stunning blue sheen, it represented a unique opportunity to test the theory of how massive stars explode and how nucleosynthesis takes place within the explosion. This theory predicted that isotopes of mass 44, 56 and 57 would be produced by the sudden, explosive grafting of alpha particles (helium nuclei) and protons onto silicon nuclei (see Appendix 3). They would be synthesised in their radioactive forms, nickel-56, nickel-57 and titanium-44, in that order of importance (see Table 7.1). After a suitable series of decays, these sparsely scattered nuclei in the supernova debris would arrive at their stable forms, iron-56, iron-57 and calcium-44. 

Explosive events like the Big Bang and supernovas are the professionals in the nucleosynthesis game. They are the great dispensers and generous donators of atomic nuclei in the Universe. The quantity and simplicity of nuclear species created by the Big Bang - hydrogen and helium - can only be balanced by the quality, diversity and refinement of species produced in supernovas, including 90 atomic types from carbon to uranium. 

Primordial nucleosynthesis really puts the Big Bang cosmology to the test. One might call it a baptism of fire. From these brief but brilliant and fertile beginnings arose a series of light nuclei that are today found everywhere in nature above all hydrogen, followed by helium, which between them amount to 98% of the total mass of atomic matter in the Universe. 

The crucial species in primordial nucleosynthesis of the light elements is helium. There are two reasons ... 

Fig. A1.3. Comparison between observed abundances and abundances predicted by the theory of primordial nucleosynthesis. The horizontal axis shows the ratio r between the number of baryons and the number of photons. The vertical axis shows the mass fraction of helium and the numerical ratios D/H, He/H and li/H. Observational data are represented by boxes with height equal to the error bar. In the case of helium and lithium, there are two boxes, indicating the divergence between different observers. Deuterium holds the key to the mystery, but it is difficult to measure. The region of agreement is shown as a shaded vertical ribbon (after Buries Tytler 1997). A higher level of deuterium would lead to a lower baryonic density, of the order of 2%. This would agree better with the lithium data, which have been remarkably finely established. This idea is supported by E. Vangioni-Flam and shared by myself. (From Tytler 1997.)...

So Norman Loclg er and William Crookes (see pages 74, 86) were right in a way, if not in the details there is an evolution of elements in stars. The creation of elements in stars is called nucleosynthesis, and it is responsible for the Earth and almost everything we see on it. Only hydrogen, plus some helium and a mere smattering of... 

The creation of carbon, like that of the other elements, is part of the history of the universe. All but the lightest of the natural elements were created in the cores of stars at extreme temperatures. In this process, called nucleosynthesis, protons are smashed together to form nuclei with more and more protons (heavier and heavier elements). Carbon was formed by collisions of three helium nuclei, each... 

There are several lines of evidence that nucleosynthesis takes place in stars. The compositions of the outer envelopes of evolved low- and intermediate-mass stars show enhancements of the products of nuclear reactions (hydrogen and helium burning and s-process nucleosynthesis, as defined below). The ejecta of supemovae (stellar explosions) are highly enriched in short-lived radioactive nuclides that can only have been produced either just before or during the explosion. At the other extreme, low-mass stars in globular clusters, which apparently formed shortly after the universe formed, are deficient in metals (elements heavier than hydrogen and helium) because they formed before heavy elements were synthesized. 

In this chapter, we reviewed the broad outlines of the Big Bang model for the origin of the universe and discussed some of the supporting observations. We showed that the Big Bang gave rise to hydrogen, helium, and some lithium, beryllium, and boron, but that other elements were produced primarily in stars. The rest of the elements were synthesized in stars via the nuclear reactions that cause the stars to shine. To understand stellar nucleosynthesis, it is necessary to understand the characteristics of stars. Astronomers use... 

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