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Organotrifluoroborate
Organotrifluoroborates, organoboron anions of the form RBF₃⁻, are air- and moisture-stable crystalline salts typically isolated as potassium salts and used in Suzuki–Miyaura coupling and photoredox radical chemistry.
Orgueil meteorite
The Orgueil meteorite is a carbonaceous chondrite that fell as an observed fall near Orgueil, France, in 1864, and its composition is the reference standard for the solar system.
Orichalcum
Orichalcum, Greek oreichalkos, is the classical name for a copper-zinc alloy like brass. In Roman times it was the metal of sestertius and dupondius coins, and Plato linked it to Atlantis.
Orientability
Orientability is a property of a surface or manifold stating whether a consistent orientation, such as clockwise, can be assigned continuously over the entire space; spheres are orientable, Möbius bands are not.
Orientation distribution function
The orientation distribution function (ODF) is a probability density describing how crystallites in a polycrystalline material are oriented, the central quantity of texture analysis, typically recovered from diffraction pole figures.
Orientation mapping (crystallography)
Orientation mapping in crystallography measures and images the local crystallographic orientation of grains across a material's surface, most often using electron backscatter diffraction in a scanning electron microscope.
Oriented core
An oriented core is a cylindrical rock sample recovered by drilling whose original in situ position has been determined, typically by marking the core and measuring the mark's azimuth against geographic north.
Oriented matroid
An oriented matroid is a combinatorial structure recording the plus, minus, and zero sign patterns of finite point, vector, or hyperplane configurations, generalizing graphs, polytopes, and linear programming.
Orifice metering
Orifice metering measures fluid flow by placing a plate with a precise hole in a pipe and inferring flow rate from the pressure drop across it.
Orifice plate
An orifice plate, or orifice meter, is a flat plate with a sharp-edged bore clamped in a pipe to measure flow rate from the pressure drop it creates.
Origin of the cosmic-ray knee and ankle
The origin of the cosmic-ray knee and ankle concerns two bends in the cosmic-ray energy spectrum, at 3 to 4 PeV and about 5 EeV.
Origin of the Moon
The origin of the Moon is explained by the giant impact hypothesis, the consensus since 1984: a Mars-sized body struck Earth about 4.5 billion years ago.
Origin of water on Earth
The origin of water on Earth, also called the origin of the oceans, is the unsolved question of how Earth acquired its water, with asteroids the leading source.
Orion
Orion, known as "the Hunter", is a constellation straddling the celestial equator, one of 88 recognized by the International Astronomical Union and containing the Orion Nebula.
Orion (constellation)
Orion is one of the 88 officially recognized constellations, straddling the celestial equator and containing Betelgeuse, Rigel, the Belt, and the Orion Nebula, the nearest large star-forming region.
Orion (Ὠρίων) (mythology)
Orion (Ὠρίων) was a giant hunter of Greek mythology whom Zeus placed among the stars after his death, forming the constellation that still bears his name.
Orion A
Orion A is a giant molecular cloud in Orion about 1,300 light-years away, the nearest site of massive star formation and host of the Orion Nebula.
Orion Arm
The Orion Arm, also called the Local Arm or Orion spur, is the spiral arm of the Milky Way containing the Sun, lying between the Sagittarius and Perseus arms.
Orion B
Orion B is a giant molecular cloud in Orion, about 400 parsecs away, forming the northern half of the Orion Molecular Cloud Complex and hosting the Horsehead and Flame Nebulae.
Orion Bar
The Orion Bar is a bright edge-on ionization front and photodissociation region in the Orion Nebula, where ultraviolet light from the star θ¹ Orionis C strikes the molecular cloud.
Orion correlation theory
The Orion correlation theory, proposed by Robert Bauval in 1989 and popularized in The Orion Mystery (1994), claims the Giza pyramids reproduce Orion's Belt stars; Egyptologists reject it.
Orion I
Orion I, also called Orion Source I or source I, is a compact radio source in the Orion Kleinmann-Low nebula, likely a massive protostar surrounded by an edge-on ionized disk.
Orion KL
Orion KL, the Kleinmann–Low nebula, is the brightest far-infrared region of the Orion Molecular Cloud 1 and the nearest high-mass star-forming environment, discovered in 1967 by Kleinmann and Low.
Orion molecular cloud complex
The Orion molecular cloud complex, also called the Orion complex, is a vast cloud of gas, dust, and young stars in Orion, the nearest region of ongoing star formation.
Orion Nebula
The Orion Nebula, also known as Messier 42, is a diffuse nebula south of Orion's Belt where new stars form, the nearest region of massive star formation to Earth.
Orion Nebula Cluster
The Orion Nebula Cluster (ONC) is a young star cluster in the Orion A molecular cloud about 400 parsecs from the Sun, containing roughly 2000 stars within 2 parsecs.
Orion's Belt
Orion's Belt is an asterism of three blue supergiant stars, Alnitak, Alnilam, and Mintaka, arranged in an almost straight line at the center of the constellation Orion.
Orionids
The Orionids are an annual meteor shower produced when Earth crosses debris shed by Halley's Comet, active each October with a peak around October 21–22 and roughly 10 to 20 meteors per hour.
Ornithine
Ornithine is a non-proteinogenic amino acid that serves as the central intermediate of the urea cycle, which the liver uses to convert toxic ammonia into urea.
Ornstein–Uhlenbeck operator
The Ornstein–Uhlenbeck operator is the infinitesimal generator of the Ornstein–Uhlenbeck semigroup, acting as Δf − ⟨x, ∇f⟩; it is the Gaussian analog of the Laplacian, named for its 1930 paper.