Encyclopedia of Astrobiology

Living Edition
| Editors: Muriel Gargaud, William M. Irvine, Ricardo Amils, Henderson James Cleaves, Daniele Pinti, José Cernicharo Quintanilla, Michel Viso

Abundances of Elements

  • Nikos PrantzosEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-642-27833-4_10-4


Chemical composition Nucleosynthesis Nuclide 


The relative amount (or fraction) of a given nuclide in a sample of matter is called the abundance of that nuclide. It can be expressed either in absolute terms (i.e., with respect to the total amount of matter in the sample) or in relative terms (with respect to the amount of some key element, e.g., the most abundant one, in the sample). Similarities and differences in the elemental and isotopic composition of stars and galaxies are key ingredients for understanding their origin and evolution.


The composition of remote objects (the Sun, stars, interstellar gas, and galaxies) is determined through spectroscopy, which usually allows the determination of elemental abundances; in rare cases, particularly for interstellar clouds, some isotopic abundances may be determined in those objects. For Earth, lunar, and meteoritic samples, nuclear mass spectroscopy allows precise determination of most isotopic abundances; this is also the case for cosmic rays, albeit only for the most abundant nuclides at present. Hydrogen (H) being the most abundant element in the Universe, spectroscopists express the abundance of element i as the number ratio of its nuclei with respect to those of H: n i = N i /N H , and they use a scale where log(N H ) = 12. In the meteoritics community, the silicon scale of log(N Si ) = 6 is used. Theoreticians use the mass fraction X i = N i A i /N j A j , where Aj is the mass number of nuclide j; obviously, ∑X i  = 1. Conversion of mass fractions to abundances by number requires use of the quantity Y i = X i /A i called the mole fraction (notice that ∑Y i  ≠ 1).

According to our current understanding, the material of the proto-solar nebula had a remarkably homogeneous composition, as a result of high temperatures (which caused the melting of nearly all the dust grains) and thorough mixing. This composition characterizes the present-day surface layers of the Sun, which remain unaffected by nuclear reactions occurring in the solar interior (with a few exceptions, e.g., the fragile D and Li). Furthermore, after various physicochemical effects are taken into account, it appears that the elemental composition of the Earth and meteorites matches extremely well with the solar photospheric composition. The composition of stars in the Milky Way presents both striking similarities and considerable differences with the solar composition. The universal predominance of H (90 % by number, but ∼70 % by mass) and He (9 % by number, but ∼25 % by mass) and the relative abundances of “metals” (to astronomers, elements heavier than He) is the most important similarity. On the other hand, the fraction of metals (metallicity, about 1.5 % in the Sun) appears to vary considerably within the solar neighborhood (where the oldest stars have a metallicity of 0.1 solar), across the Milky Way disk (with young stars in the inner Galaxy having three times more metals than the Sun), or in the galactic halo (with stellar metallicities ranging from 0.1 to 0.00001 solar). These variations in composition reflect the history of “chemical evolution” of the Milky Way (Fig. 1).
Fig. 1

Solar system abundances (by number) of the 92 chemical elements, in a logarithmic scale where log(N) = 6 for Silicon (from a compilation in Lodders 2003)

See Also

References and Further Reading

  1. Asplund M, Grevesse N, Sauval AJ, Scott P (2009) The chemical composition of the sun. Ann Rev Astron Astrophys 47:481–522CrossRefADSGoogle Scholar
  2. Lodders K (2003) Solar system abundances and condensation temperatures of the elements. Astrophys J 591:1220–1247CrossRefADSGoogle Scholar

Copyright information

© Springer-Verlag Berlin Heidelberg 2014

Authors and Affiliations

  1. 1.Institut d’Astrophysique de ParisParisFrance