Helium: abundance 103 formation

The majority of the Universe is made from hydrogen and helium produced during the Big Bang, although some He has been made subsequently. The relative cosmic abundance of some of the elements relevant to the formation of life is given in Table 1.2, with all elements heavier than H, He and Li made as a result of fusion processes within stars, as we shall see later. The cosmic abundance is assumed to be the same as the composition of the Sun. 

The solar system abundances of the elements are the result of the Big Bang, which produced hydrogen and helium, 7.5 billion years of stellar nucleosynthesis, which produced most of the rest of the elements, and the physical processes that mixed the materials together to form the Sun s parent molecular cloud. The unique features of the solar system composition may also reflect the stochastic events that occurred in the region where the Sun formed just prior to solar system formation. 

Up to now, the sdB/sdOB stars, the classical sdOs and the extremely helium-rich luminous sdOs have been analyzed for the most important (and accessible) metal abundances. The analyses usually require extensive non-LTE line formation calculations to solve the statistical equilibrium in detailed model atoms simultaneously with the radiative transfer equations for all relevant frequencies. With the advent of computer codes based on modern powerful solution algorithms (Auer and Heasley, 1976 Werner and Husfeld, 1985) it has now become possible to test (and eventually remove) approximations necessary in older computations. This and the availability of improved atomic data make the non-LTE predictions more reliable, and obstacles in obtaining accurate abundance determinations come now mainly from the observational side where high-quality spectra are needed to identify and to measure weak... 

Helium, the second most abundant element in the universe after hydrogen, is rare on Earth because its atoms are so light that a large proportion of them reach high speeds and escape from the atmosphere. However, it is found as a component of natural gases trapped under rock formations (notably in Texas), where it has collected as a result of the emission of a particles by radioactive elements. An a particle is a helium nucleus (4He2+), and an atom of the element forms when the particle picks up two electrons from its surroundings. 

It is important here to call attention to the revised determinations of the oxygen and carbon abundances in the Sun. Allende Prieto et al. (2001) derived an accurate oxygen abundance for the Sun of log s(0) = 8.69 0.05 dex, a value approximately a factor of 2 below that quoted by Anders and Grevesse (1989). Subsequently, Allende Prieto et al. (2002) determined the solar carbon abundance to be log s(C) = 8.39 0.04 dex, and the ratio C/O = 0.5 0.07. The bottom line here is a reduction in the abundances of the two most abundant heavy elements in the Sun, relative to hydrogen and helium, by a factor 2. The implications of these results for stellar evolution, nucleosynthesis, the formation of carbon stars, and galactic chemical evolution remain to be explored. 

Guided by early compilations of the cosmic abundances as reflected in solar system material (e.g., Suess and Urey, 1956), Burbidge cr a/. (1957) and Cameron (1957) identified the nuclear processes by which element formation occurs in stellar and supernova environments (i) hydrogen burning, which powers stars for —90% of their lifetimes (ii) helium burning, which is responsible for the production of and the two most abundant elements heavier than helium (iii) the a-process, which we now understand as a combination of... 

Thus, the formation of elementary particles and elements follows a logical order, which in turn can be used by scientists to delineate cosmic events and time tags. It is easier to form light elements by fusion of hydrogen, deuterium and helium, as oxygen and carbon are the two most abundant elements after hydrogen and helium. 

A transparent Universe. After 300,000 yr temperatures dropped to 4,500 K and gave rise to the formation of atomic matter, and atoms of hydrogen, helium, and deuterium were formed. Because electrons were removed from the plasma through the formation of atoms, radiation streamed out and the Universe became transparent. Initially the Universe contained abundant ultraviolet-and X-rays, now cooled down to microwave wavelengths. This is what is recorded as the Cosmic Background radiation. 

The processes of cosmochemistry constrain planetary materials to be one of three types -gas (hydrogen and helium - the most abundant elements of the Universe), ice (water, methane, ammonia, nitrogen - the next most abundant elements in nucleosynthesis), and rock - principally made up of Mg-Fe silicates (Stevenson, 2004). If the standard model of planetary formation is applied to the formation of the Earth and the terrestrial planets, then temperatures in the solar nebula in the vicinity of the Earth s orbit would have been between 500 and 800 K. At these temperatures rock - Fe-Mg silicates, and Fe-Ni metal would condense but not water ice. Micron-sized particles of these minerals would have grown in series of stages into the present configuration of planetary bodies as outlined below and summarized in Table 2.3. 

One of the ways that nuclides with more than 83 protons change to reach the band of stability is to release two protons and two neutrons in the form of a helium nucleus, which in this context is called an alpha particle. Natural uranium, which is found in many rock formations on earth, has three isotopes that all experience alpha emission, the release of alpha particles. The isotope composition of natural uranium is 99.27% uranium-238, 0.72% uranium-235, and a trace of uranium-234. The nuclear equation for the alpha emission of uranium-238, the most abundant isotope, is... 

The principal products of helium burning are thus 0 and C. That these are the third and fourfti most abundant isotopes in the Solar System (after H and He) shows that helium burning is a significant player in the formation of the elements. Other key products of helium burning are 0 and Ne produced fi om left over from hydrogen burning ... 

Third, the element must be taken up into the cell and yield a sufficiently high cytoplasmic concentration. This requires the presence of uptake systems for an element and a sufficient concentration of the element in the environment. Finally, an element must be chemically active to be of any biological importance. This excludes the rare or noble gases from any biological importance because they are chemically inactive. Helium - the second most abundant element in the universe - is strongly impoverished in sea water compared to the universe and reaches only nanomolar concentrations, probably due to diffusion losses during formation of the Earth. 

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