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Helene (moon)
Helene is a small Trojan moon of Saturn that shares Dione's orbit, staying 60 degrees ahead of it at a stable Lagrange point. It was discovered in 1980.
Hélène Langevin-Joliot
Hélène Langevin-Joliot (born 1927) is a French nuclear physicist, director of research at CNRS, known for her work at Orsay and as the granddaughter of Marie and Pierre Curie.
Helge von Koch
Helge von Koch, full name Nils Fabian Helge von Koch, was a Swedish mathematician whose 1904 curve without a tangent became the early fractal known as the Koch snowflake.
Helicene
Helicene is an ortho-fused polycyclic aromatic or heteroaromatic compound whose angularly arranged rings form a helix, making it chiral; hexahelicene is the prototype, and applications include enantioselective catalysis and spintronics.
Helicity (chemistry)
Helicity in chemistry is the chirality of a helical or screw-shaped molecular entity, assigned the stereodescriptors P (right-handed) or M (left-handed), formalized by Cahn–Ingold–Prelog rules.
Helicity injection
Helicity injection is a method of driving current in magnetized plasmas by adding magnetic helicity, used to sustain spheromaks and spherical tokamaks without a central solenoid.
Helicon current drive
Helicon current drive is a proposed off-axis current drive method for tokamak plasmas using helicon waves near 0.5 GHz, pursued on the DIII-D tokamak for its predicted high efficiency.
Helicon plasma
A helicon plasma is a dense, low-temperature plasma produced efficiently by radio-frequency helicon waves in a magnetized gas, used in materials processing, electrodeless space propulsion, and fusion current drive.
Helielectric nematic phase
The helielectric nematic phase (HN) is a liquid-crystal state in which a fluid's spontaneous polarization winds into a helix, first reported in 2021 at South China University of Technology.
Helike (moon)
Helike, also designated Jupiter XLV and provisionally S/2003 J 6, is a small retrograde irregular moon of Jupiter in the Ananke group, discovered in 2003.
Heliocentrism
Heliocentrism, also known as the Copernican heliocentric system, is the astronomical model placing the Sun rather than Earth at the center, advanced by Aristarchus and published by Nicolaus Copernicus in 1543.
Helion Energy
Helion Energy is an American fusion company in Redmond, Washington, developing pulsed field-reversed configuration reactors and signing the first fusion power agreement with Microsoft in 2023.
Heliopause
The heliopause is the outer boundary of the heliosphere, the sharp surface where the Sun's solar wind is stopped by the interstellar medium about 14 billion kilometers away.
Helioseismology
Helioseismology, also known as solar seismology, is the study of the Sun's interior through its natural modes of oscillation, showing that the solar core rotates about four times faster than the surface.
Heliosheath
The heliosheath is the outer region of the heliosphere where the solar wind slows to subsonic speed against the interstellar medium, first explored by Voyager 1 in 2004.
Heliosphere
The heliosphere is the bubble of magnetized plasma created by the solar wind that encloses the solar system; Voyager 1 and 2 crossed its outer boundary, the heliopause, in 2012 and 2018.
Heliospheric current sheet
The heliospheric current sheet (HCS) is the wavy surface separating oppositely directed solar magnetic field regions, spiraling outward from the Sun with the solar wind.
Heliotail
The heliotail is the trailing region of the heliosphere, extending opposite the Sun's motion through the galaxy like a wake, first imaged by NASA's IBEX mission in 2013.
Heliotrope (mineral)
Heliotrope, also called bloodstone, is a dark green variety of chalcedony spotted with red iron-oxide markings resembling drops of blood, long used for seals and signet rings.
Helium
Helium is a chemical element with symbol He and atomic number 2, a colorless, unreactive noble gas and the second most abundant element in the universe.
Helium compounds
Helium compounds are chemical species in which helium binds to other atoms as ions, matrix-trapped species, or high-pressure solids, since neutral helium compounds cannot be formed.
Helium flash
The helium flash, or core helium flash, is the thermonuclear runaway that ignites helium burning in the degenerate core of a low-mass red giant star, fusing helium into carbon at about 10^8 K.
Helium hydride ion
The helium hydride ion (HeH+), or protonated helium, is a positively charged molecular ion of helium and a proton that formed the Universe's first molecular bond.
Helium liquefaction
Helium liquefaction is the process of cooling helium gas below its boiling point of 4.2 K into liquid helium, used to cool superconducting magnets and medical imaging equipment.
Helium-3
Helium-3 (He-3) is a stable, non-radioactive helium isotope with two protons and one neutron, extraordinarily rare on Earth and prized for cryogenics, neutron detection, and fusion research.
Helium-4
Helium-4 (⁴He) is the stable isotope of helium whose nucleus is the alpha particle, making up almost all naturally occurring helium and about 25% of the universe's baryon mass.
Helium-cadmium laser
The helium-cadmium laser (HeCd laser) is a continuous-wave gas laser using excited helium to vaporize cadmium, emitting ultraviolet and blue-violet light for microscopy and lithography.
Helium-core white dwarf
A helium-core white dwarf is a low-mass white dwarf, below about 0.48 solar masses, made of helium, formed when a binary companion strips a red giant's envelope.
Helium-ion microscopy
Helium-ion microscopy (HIM) is an imaging technique that scans a focused beam of helium ions to form sub-nanometer surface images. It is highly surface-sensitive, offering better contrast and a larger depth of field than an SEM.
Helium–neon laser
The helium–neon laser, or HeNe laser, is a continuously operating gas laser first demonstrated at Bell Labs in December 1960, the first laser to produce a continuous beam.