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Hydrogen (1 H) has three naturally occurring isotopes: 1 H, 2 H, and 3 H. 1 H and 2 H are stable, while 3 H has a half-life of 12.32(2) years. [3] [nb 1] Heavier isotopes also exist; all are synthetic and have a half-life of less than 1 zeptosecond (10 −21 s). [4] [5] Of these, 5 H is the least stable, while 7 H is the most.
Deuterium (hydrogen-2, symbol 2 H or D, also known as heavy hydrogen) is one of two stable isotopes of hydrogen; the other is protium, or hydrogen-1, 1 H. The deuterium nucleus (deuteron) contains one proton and one neutron, whereas the far more common 1 H has no neutrons. The name deuterium comes from Greek deuteros, meaning "second".
Nuclear fusion is a reaction in which two or more atomic nuclei (for example, nuclei of hydrogen isotopes deuterium and tritium), combine to form one or more atomic nuclei and neutrons. The difference in mass between the reactants and products is manifested as either the release or absorption of energy .
Neutron stars have a radius on the order of 10 kilometers (6 mi) and a mass of about 1.4 M ☉. [2] Stars that collapse into neutron stars have a total mass of between 10 and 25 solar masses (M ☉), or possibly more for those that are especially rich in elements heavier than hydrogen and helium. [3]
Stars fuse hydrogen and helium into heavier and heavier elements (up to iron), producing energy. For example, the observed light curves of supernova stars such as SN 1987A show them blasting large amounts (comparable to the mass of Earth) of radioactive nickel and cobalt into space. However, little of this material reaches Earth.
Fe 2 SiO 4 + H 2 O → 2 Fe 3 O 4 + SiO 2 +H 2. Closely related to this geological process is the Schikorr reaction: 3 Fe(OH) 2 → Fe 3 O 4 + 2 H 2 O + H 2. This process also is relevant to the corrosion of iron and steel in oxygen-free groundwater and in reducing soils below the water table. [citation needed]
Hydrogen fusion (nuclear fusion of four protons to form a helium-4 nucleus [20]) is the dominant process that generates energy in the cores of main-sequence stars. It is also called "hydrogen burning", which should not be confused with the chemical combustion of hydrogen in an oxidizing atmosphere.
Once temperatures are lowered, out of every 16 nucleons (2 neutrons and 14 protons), 4 of these (25% of the total particles and total mass) combine quickly into one helium-4 nucleus. This produces one helium for every 12 hydrogens, resulting in a universe that is a little over 8% helium by number of atoms, and 25% helium by mass.