Encyclopedia of Astrobiology

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


  • Henderson James (Jim) CleavesII
Living reference work entry
DOI: https://doi.org/10.1007/978-3-642-27833-4_750-4


Hydrogen (chemical symbol H) is a chemical element with atomic number 1. It has an atomic weight of 1.00794 u (or Da) (1.007825 u for the isotope 1H containing no neutrons). It is the lightest and most abundant element, making up ∼75 % of the Universe’s detectable mass and ∼90 % of its detectable atoms.


Main sequence stars are mostly composed of ionized hydrogen in plasmas, which plays a vital role in powering stars through proton-proton reactions and the CNO nuclear fusion cycle. Hydrogen has three naturally occurring isotopes: 1H, 2H, and 3H, which all contain 1 proton and 0, 1, or 2 neutrons, respectively. While hydrogen gas is rare in the Earth’s atmosphere (∼1 ppm by volume) because it escapes from the Earth’s gravity field more easily than heavier gases, it is the third most abundant element on the Earth’s surface, mainly as a component of water.

Its most common isotope is 1H (protium), which contains a single proton. In ionic compounds, hydrogen can take a...


Molecular Hydrogen Interstellar Medium Hydrogen Isotope Ionize Hydrogen Molecular Compound 
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Copyright information

© Springer-Verlag Berlin Heidelberg 2014

Authors and Affiliations

  1. 1.Earth-Life Science Institute (ELSI)Tokyo Institute of Technology, Meguro-kuTokyoJapan
  2. 2.Institute for Advanced StudyPrincetonUSA
  3. 3.Blue Marble Space Institute of ScienceWashingtonUSA
  4. 4.Center for Chemical EvolutionGeorgia Institute of TechnologyAtlantaUSA