Astronomy

Nova: sudden stellar brightening from thermonuclear surface explosions Nova A nova is a sudden, dramatic brightening of a white dwarf in a close binary caused by a surface thermonuclear runaway. This article explains mechanisms, observable properties, exam…

Nozomi — Japan’s early Mars probe (PLANET-B) Nozomi (spacecraft) Nozomi (のぞみ), also known as PLANET-B, was Japan’s first dedicated spacecraft mission to Mars. Launched in 1998 by ISAS, it failed to achieve Mars orbit but provided experience and…

OB star Massive, hot O- and B-type stars that are luminous, short-lived, and dominant sources of ultraviolet radiation and stellar feedback in star-forming regions.

Oberon (moon) Oberon is the outermost major moon of Uranus, about half ice and half rock. Discovered in 1787, it is heavily cratered, tidally locked, and was imaged by Voyager 2 during its 1986…

Observable universe: definition, size, history, and scientific importance Observable universe The observable universe is the spherical region from which light or other causal signals have had time to reach an observer since the universe began; this article explains its limi…

Observatory A facility for systematic observations of natural phenomena, most often astronomical sites with telescopes; sited for clear skies, low humidity and minimal light pollution.

Occultation (astronomy and general): definition, types, and significance Occultation An occultation occurs when one object passes in front of another from an observer's viewpoint. This article explains types, geometry, scientific uses, observing methods, and notabl…

Octans — Southern Constellation Marking the South Celestial Pole Octans Octans is a faint southern circumpolar constellation named for the octant instrument; it contains the south celestial pole and the dim star Sigma Octantis, the southern pole star.

Ohzora — Japanese scientific satellite (launched 14 February 1984) Ohzora Ohzora was a Japanese artificial satellite launched by the Institute of Space and Astronautical Science (University of Tokyo) on 14 February 1984 for scientific study of the near‑E…

Olympus Mons Olympus Mons is a vast shield volcano on Mars and the highest known mountain and volcano in the solar system, famed for its enormous size, low slopes, and layered summit caldera.

Omega Centauri (ω Centauri, NGC 5139) — largest globular cluster of the Milky Way Omega Centauri Omega Centauri is the Milky Way's largest globular cluster, visible to the naked eye in Centaurus. It contains millions of old stars, multiple stellar populations, and may be the r…

Omega Nebula (M17) — the Swan Nebula, a bright H II region Omega Nebula M17 (the Omega or Swan Nebula) is a luminous H II star-forming region in Sagittarius about 5,500 light-years away, home to the young cluster NGC 6618 and a frequent target for mult…

Omniverse The Omniverse is the hypothetical totality of all possible universes, realities and multiverses. It appears in cosmology, philosophy and fiction as an all-encompassing concept beyo…

Oort cloud — the distant, icy reservoir of long‑period comets Oort cloud The Oort cloud is a vast, distant shell of icy bodies surrounding the Solar System, thought to supply long‑period comets and to mark the outer limit of the Sun's gravitational doma…

Open star cluster A loose grouping of stars born from the same molecular cloud. Open clusters are young, share composition and age, and are important tools for studying stellar evolution, distances…

Ophelia (moon) Ophelia is a small inner moon of Uranus, discovered in Voyager 2 images in 1986. It is irregular, about 23 km in radius, faint (low albedo) and serves as an outer shepherd to Uranu…

Ophiuchus: the Serpent-Bearer constellation Ophiuchus Ophiuchus is a large constellation near the celestial equator, associated with the serpent-bearer (Asclepius). It contains bright stars like Rasalhague, several globular clusters,…

Ophiuchus Supercluster Explosion: the largest known cosmic eruption Ophiuchus Supercluster explosion A vast energetic outburst in the Ophiuchus region produced cavities and shock features in intracluster gas, attributed to a powerful active galaxy nucleus; observed across X-ray, r…

Opposition (astronomy and astrology) Opposition describes when two celestial bodies appear opposite each other in Earth's sky (180° apart). It is important for observing planets and used as an astrological aspect mean…

Orbit (astronomy and mechanics) Orbit An orbit is the regularly repeating path of one body around another under gravity. This article explains definitions, orbital shapes and elements, physical laws, history, and every…

Orbital (relating to orbits, orbitals, and orbital regions) Orbital Relating to an orbit: used in astronomy, physics, chemistry and anatomy to describe paths, probability regions for electrons, or the eye socket area; also used in engineering and e…

Orbital eccentricity Orbital eccentricity measures how much an orbit deviates from a perfect circle, classifying conic-section trajectories and influencing apses, energy, and orbital behavior.

Orbital inclination: definition, measurement, and significance Orbital inclination Orbital inclination is the angle between an orbiting body's orbital plane and a chosen reference plane. It determines prograde/retrograde motion, transit visibility, and orbital dy…

Orbital period Time required for an object to complete one orbit around another body; includes sidereal and synodic measures, examples like years and months, and applications in astronomy and nav…

Orbital Plane (Astronomy): Definition, Properties, and Significance Orbital plane (astronomy) The orbital plane is the flat geometric plane in which an orbit lies, defined relative to a chosen reference plane (e.g., the ecliptic or the invariable plane); it controls inclina…

Orbital resonance: dynamics, examples, and significance Orbital resonance Orbital resonance occurs when orbiting bodies exert regular gravitational influences because their orbital periods form ratios of small integers. It shapes planetary systems, rings…

Orbital revolution: motion of planets, moons and satellites Orbital revolution Orbital revolution describes the motion of one body around another under gravity. This article explains the mechanics, types of orbits, historical laws, examples, and distinctions…

Orbital speed Rate at which an object moves along its orbit around a common center of mass; depends on distance, masses, and orbit shape. Covers instantaneous and mean speeds, basic formulas, an…

Orion (constellation) Orion Orion is a prominent equatorial constellation containing some of the brightest stars, a nearby active star-forming region (the Orion Nebula), and a rich history in global myth and…

Orion (constellation): stars, nebulae, mythology and visibility Orion (constellation) Orion is a prominent constellation straddling the celestial equator, known for its Belt, bright stars (Rigel, Betelgeuse), the Orion Nebula, and a major star‑forming complex visibl…

Orion (spacecraft) Orion is NASA's crew-capable deep-space spacecraft, designed to carry astronauts beyond low Earth orbit to lunar orbit, support Gateway operations, and enable future missions to Ma…

Orion Arm (Local Spur) — the Solar System's region of the Milky Way Orion Arm The Orion Arm, or Local Spur, is a minor spiral arm of the Milky Way that contains the Solar System. Overview, structure, notable objects, how it is mapped, and its significance ar…

Orion Molecular Cloud Complex Orion complex A vast, nearby star-forming region in the constellation Orion comprising emission, reflection and dark nebulae, young clusters and H II regions visible from Earth in optical and in…

Orion Nebula (Messier 42) — nearest bright region of massive star formation Orion nebula The Orion Nebula (M42, NGC 1976) is a bright emission nebula in Orion visible to the naked eye. The H II region hosts young O-type stars and a dense star cluster, and it is part of…

Orion's Belt A prominent asterism of three bright stars in the constellation Orion—Alnitak, Alnilam and Mintaka—used for navigation, cultural symbolism, and locating nearby deep-sky objects.

Orrery: Mechanical models of the Solar System Orrery An orrery is a geared, usually heliocentric mechanical model that illustrates the positions and motions of planets and moons for education, demonstration, and historical study.

Orthosie (moon of Jupiter) Orthosie (moon) Orthosie (Jupiter XXXV) is a small retrograde irregular satellite of Jupiter discovered in 2001; it belongs to the Ananke group and orbits at about 20.6 million km with a period of…

Ōsumi (satellite) Ōsumi was Japan’s first artificial satellite, launched on 11 February 1970 by the University of Tokyo’s ISAS. Its successful orbital insertion made Japan the fourth country to plac…

Outer Planets of the Solar System Outer planet The outer planets are the four large planets beyond the asteroid belt: Jupiter, Saturn, Uranus and Neptune. This article explains their characteristics, origins, rings, moons, and…

Outer space: overview, structure, history, and importance Outer space Outer space is the near-vacuum region beyond Earth's atmosphere where planets, stars, galaxies and other celestial objects exist; it has distinct physical properties and broad scie…

Owl Nebula (Messier 97) Owl nebula The Owl Nebula (M97) is a nearby planetary nebula in Ursa Major. Recognized by two dark 'eyes', it marks a late stage of stellar evolution and is a common target for amateur and pr…

P Cygni: a luminous blue variable star in Cygnus P Cygni P Cygni is a luminous blue variable (LBV) star in Cygnus, noted for its strong stellar wind, variable brightness, and the characteristic spectral 'P Cygni profile'.

Paaliaq — an irregular moon of Saturn Paaliaq (moon) Paaliaq is a small, irregular prograde moon of Saturn, discovered in 2000. It belongs to the Inuit group, has a light-red surface, and likely formed in a collisional breakup of a l…

Pallene (moon of Saturn) Pallene (moon) Pallene is a small inner moon of Saturn, about 4 km across, discovered by the Cassini Imaging Team in 2004. It orbits between Mimas and Enceladus and supplies material to a faint l…

Palomar Observatory Caltech-owned optical observatory on Palomar Mountain, California, home to the 200-inch Hale telescope, the Samuel Oschin survey telescope and facilities for wide-field surveys, in…

Pan (moon) Pan is a small inner moon of Saturn that orbits within the Encke gap of the A ring. Discovered in Voyager images, it is notable for its distinctive equatorial ridge and role cleari…

Pandora (moon of Saturn) Pandora (moon) Pandora is a small, irregular moon of Saturn that shepherds the outer edge of the planet's F ring. It is heavily cratered, affects ring structure through gravitational interactions…

Parabolic Orbit: The Eccentricity-1 Boundary Between Bound and Unbound Motion Parabolic trajectory Overview of parabolic orbits: the eccentricity‑1 boundary between bound and unbound motion, its physics and math under inverse-square gravity, observational challenges, and relevan…