Encyclopedia of Astrobiology

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

Hydrogen

Living reference work entry
DOI: https://doi.org/10.1007/978-3-642-27833-4_750-4

Definition

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.

Overview

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...

Keywords

Molecular Hydrogen Interstellar Medium Hydrogen Isotope Ionize Hydrogen Molecular Compound 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
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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