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The stars are mostly made of hydrogen. Not entirely though. The sun, for example, is only 91.2 percent hydrogen atoms. It converts hydrogen to helium atoms, in a fusion reaction that gives off the energy that powers all life on earth.

Taken together, hydrogen and helium are 99.9 of all the atoms in the stars, and in the universe as well. Measured another way, hydrogen is only slightly more than 70% of the mass of our sun, because hydrogen is the lightest element, so the heavier elements account for more mass.

Hydrogen, symbol H, is the first member of the periodic table. The atomic number of hydrogen is 1, meaning it has 1 proton in its nucleus. On earth, hydrogen is an odorless colorless gas.

Hydrogen has three isotopes, protium, deuterium, and tritium. Protium is the most commonly found; it has one proton in its nucleus, and no neutrons. It is, so to speak, everyday hydrogen. Deuterium is hydrogen with one proton and one neutron. Its proportion to protium atoms in nature is about one in 5000. Deuterium is used in Nuclear Magnetic Resonance Spectroscopy and in nuclear fusion. When scientists and technicians talk about "heavy water," they usually mean water with deuterium in its molecule.

Tritium has one proton and two neutrons. It is even rarer than deuterium, because it is radioactive, and thus breaks down. Its half-life, the time it takes for half of a quantity of tritium to disappear, is 12.3 years. It is not found in nature on earth, hydrogen by cosmic radiation. It is produced here, and used as a marker in some reactions involving hydrogen, because it is radioactive. The presence of that radiation is easy to track through the reaction being studied.

The hydrogen in the stars is in a plasma state. It is not solid, liquid, or gas, but in a stripped state in which its electrons travel free. In our sun, and other stars, hydrogen is constantly being converted to helium in a process called fusion. In fusion, four hydrogen nuclei combine to form one helium. The byproducts of this reaction are two neutrinos and some gamma radiation. The sun's radiation streams to earth, reaching us in about 8.3 minutes, and we welcome it as sunlight.

Alchemist, astrologer, and physician Paracelsus was the first to isolate and describe hydrogen, which he described as: "An air which bursts forth like the wind." He isolated it by treating iron with sulfuric acid. Antoine Lavoisier, the great French polymath, actually named hydrogen, from the Greek words hydro (water) and gene

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