• The second most abundant element in the universe after hydrogen, and the lightest element apart from hydrogen. Helium was discovered in the laboratory after being suspected (by Norman Lockyer) from the existence of unexplained lines in the spectrum of the Sun. Most stars shine with the energy produced by fusing hydrogen into helium (carbon cycle). This means that the abundance of helium in the universe is constantly increasing. But this process is so slow that most of the helium we observe is that created in the big bang itself. This may amount to 20–30 per cent of the total mass of the universe, although the exact figure is difficult to determine. Within the solar system, helium is present in the Sun and in the gas giants but is only a trace element in the Earth’s atmosphere. The helium used by astronomers to hoist telescopes into the atmosphere in balloons is extracted from natural gas.
  • A chemical element whose atomic number is 2. It is a colorless non-combustible noble gas, and its boiling point and melting point are the lowest of any known substance, the latter occurring at less than one degree of absolute zero. It is the second most abundant element in the known universe, and has 8 known isotopes, of which 2 are stable. Its applications include its use in lasers, chromatography, luminous signs, fuel pressurization, as an inert atmosphere for growing crystals such as those utilized in semiconductors, as a coolant in nuclear reactors, as an inert gas in welding, and in many cryogenic and superconductivity applications and investigations. Its chemical symbol is He.
  • chemical symbol He

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