Liquid helium

Helium cooled below its boiling point is a cryogenic fluid that remains liquid at atmospheric pressure down to absolute zero. Its low-temperature behavior is dominated by quantum effects.

Helium-4: The common helium isotope, with two protons and two neutrons, whose liquid form becomes superfluid below 2.17 kelvin. Its atoms form the familiar superfluid phase of liquid helium.
Helium: The chemical element with atomic number 2, a colorless noble gas at ordinary conditions. Liquid helium is the low-temperature liquid phase of this element.
Cryostat: An apparatus that maintains a sample or device at a very low, controlled temperature. Helium-cooled cryostats provide stable environments for low-temperature experiments.
Heike Kamerlingh Onnes: A Dutch physicist who liquefied helium in 1908 and discovered superconductivity in 1911. His liquefaction of helium opened experimental access to temperatures near absolute zero.
Liquid nitrogen: Nitrogen cooled below its boiling point, commonly used as a cryogenic refrigerant near 77 kelvin at atmospheric pressure. It is cheaper and warmer than liquid helium, but cannot reach helium's temperature range.
Helium-3: A rare helium isotope with two protons and one neutron that becomes superfluid only at millikelvin temperatures. Its distinct quantum statistics produce a very different superfluid phase.
Boiling point: The temperature at which a liquid's vapor pressure equals the surrounding pressure. At atmospheric pressure, helium boils at about 4.2 kelvin.
Magnetic resonance imaging: A medical imaging technique that uses nuclear magnetic resonance to form images of the body. Many MRI scanners use helium to cool superconducting magnet coils.
Willem Hendrik Keesom: A Dutch physicist who studied helium at low temperatures and identified its lambda transition. His measurements established the transition between helium-4's normal and superfluid phases.
Solid helium: The crystalline solid phase of helium, formed under pressure because cooling alone does not freeze it at atmospheric pressure. Unlike most substances, helium remains liquid at atmospheric pressure even near absolute zero.

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