Helium (Z=2): A Relic of the Big Bang and Stellar Actor

A Cosmological Birth

Helium (symbol He, atomic number 2) is the second chemical element in the periodic table. It is primarily formed in the first minutes after the Big Bang (primordial nucleosynthesis) and secondarily in stars. Inert and light, it plays a central role in stellar evolution and cryogenic technologies.

Contribution of helium to vital processes

Helium is a chemically inert noble gas under normal biological conditions. It does not participate in any vital biochemical processes and does not bind to biological molecules. However, its medical use (e.g., in breathing mixtures such as helium-oxygen or heliox) can assist breathing in patients with obstructive disorders, due to its low density which reduces respiratory effort. Consequence of a deficit: None, since helium is not an essential element for cellular metabolism or fundamental biological functions.

Discovery History

1868: Spectroscopic Discovery
During a solar eclipse, astronomers Pierre Janssen and Joseph Norman Lockyer detect an unknown yellow line in the Sun's spectrum, which they attribute to a new element: helium, from the Greek helios (sun).

1895: Isolation on Earth
Sir William Ramsay isolates helium from the mineral cleveite. He confirms that this gaseous element is identical to the one observed in the Sun.

Atomic Structure

Constitution: Two protons, two neutrons (for the stable isotope ⁴He), and two electrons.
Isotopes:

• ⁴He: most abundant stable isotope (≈99.9999% natural).
• ³He: rare stable isotope, used in cryogenics and neutron detection.
• ⁵He and more: radioactive isotopes, with very short lifetimes, observed in experimental nuclear physics.

Physical Properties

• Monoatomic gas, colorless, odorless, extremely light.
• Molar mass: ≈ 4.0026 g/mol
• Melting point: ≈ 0.95 K (at high pressure)
• Boiling point: 4.22 K (−268.93 °C)
• Density: ~0.1786 g/L
• Chemically inert, noble gas from group 18.
• Superfluidity observed at low temperature (⁴He below 2.17 K).

Chemical Reactivity

• Noble gas: very low chemical reactivity.
• No known stable chemical compounds in the normal state.
• Van der Waals complexes or molecular ions HeH⁺ have been observed in laboratories and astrophysics.

Industrial and Technological Applications

• Cooling of superconducting magnets (MRI, LHC, experimental nuclear fusion).
• As an inert atmosphere for welding or semiconductor crystal growth.
• Pressurization of cryogenic fuel tanks in astronautics.
• Breathing mixtures for deep diving (helium-oxygen, heliox).
• Leak detection and permeability testing (due to its small atomic size).

Cosmological and Astrophysical Role

• Primordial helium: ≈24% of the baryonic mass after the Big Bang.
• Produced by nuclear fusion in stars (proton-proton cycle and triple alpha).
• Present in planetary nebulae and the outer envelope of red giants.
• Ionized helium (He⁺, He²⁺) influences stellar spectra and atmospheric opacity.

Fundamental Physical Issues

• The ⁴He nucleus (alpha) is an extremely bound system, used in nuclear physics.
• Liquid ⁴He exhibits macroscopic quantum phases (superfluidity, quantum turbulence).
• Used as a tracer in cosmology to test the validity of the standard Big Bang model.
• ³He is studied for applications in neutron detection and low-radiation nuclear fusion.