Astronomy

Umbriel — the dark moon of Uranus Umbriel (moon) Umbriel is a dark, heavily cratered moon of Uranus discovered in 1851. It is a mid-sized icy satellite with an old surface, low reflectivity, and limited geological activity; Voyag…

Unidentified flying object (UFO) Unidentified flying object An unidentified flying object (UFO) is any airborne observation that remains unexplained after reasonable investigation; the term covers many natural and human-made phenomena and a…

Universe An accessible overview of the Universe: its contents, large-scale structure, origin, observation methods, and major unresolved questions in modern cosmology.

Uranus — the Ice Giant of the Solar System Uranus Uranus is the seventh planet from the Sun, an ice giant with a tilted axis, faint rings, many moons and a cold methane-rich atmosphere. Voyager 2 remains the only spacecraft to vis…

Uranus in fiction: portrayals, themes, and literary history Uranus in fiction How Uranus and its moons have been imagined in literature and other media — common tropes, historical shifts in portrayal, uses of its name, and why it attracts particular kinds of…

Ursa Major: The Great Bear constellation and the Big Dipper Ursa Major Overview of Ursa Major, a prominent northern constellation. Covers its asterism (Big Dipper), chief stars and deep-sky objects, cultural myths, navigation role, visibility and obse…

Ursa Minor: the Little Bear and the Little Dipper Ursa Minor Ursa Minor is a small northern constellation containing the north celestial pole and Polaris. Known as the Little Bear or Little Dipper, it has navigational, cultural, and astronom…

UY Scuti — one of the largest known red supergiants UY Scuti UY Scuti is a pulsating red supergiant (or hypergiant) in Scutum, notable for an extremely large and uncertain radius, great luminosity, and its use in studies of late-stage massiv…

V-type asteroid (Vestoid) V-type asteroid V-type asteroids, or Vestoids, are basaltic bodies with spectral signatures like 4 Vesta. They record planetary differentiation, supply HED meteorites, and include fragments in the…

V382 Carinae (x Carinae) — a yellow hypergiant in Carina V382 Carinae V382 Carinae (x Carinae) is a naked-eye yellow hypergiant in the constellation Carina. It is extremely large and luminous, important for studying the late evolution of very massive…

V509 Cassiopeiae V509 Cassiopeiae is a rare, luminous yellow hypergiant in Cassiopeia, a variable star with an extended, unstable atmosphere and an estimated radius near 900 times that of the Sun.

V838 Monocerotis V838 Monocerotis is an eruptive variable star in Monoceros notable for a dramatic 2002 outburst that produced a striking light echo and an extreme, temporary expansion of its outer…

Valles Marineris Valles Marineris is a vast canyon system on Mars, stretching several thousand kilometers. It is one of the Solar System's largest rifts, revealing tectonic, erosional and sedimenta…

Van Allen radiation belts Van Allen radiation belt Zones of energetic charged particles trapped by Earth's magnetic field that influence space weather, satellite operations, and the near‑Earth radiation environment.

Variable nebula A variable nebula is a reflection nebula whose brightness and shape change over time due to variations in its illuminating star or moving dust. They offer clues about young stars a…

Variable star A star whose brightness as seen from Earth changes over time. Variability arises from internal processes, geometry (binaries, rotation), eruptions or transits and is central to ste…

Vega (Alpha Lyrae): a nearby bright star, rapid rotator and debris-disk system Vega Vega (α Lyrae) is a nearby A-type star, the brightest in Lyra and a historic photometric standard. Noted for rapid rotation, an infrared excess from a debris disk, and extensive ob…

Vela (constellation) Vela is a southern-hemisphere constellation representing the sails of the mythical ship Argo; notable for bright stars, the Vela supernova remnant and the Vela Pulsar, and many ope…

Venera: Venus and the Soviet Venera spacecraft program Venera Venera is the Russian name for the planet Venus and the name of the Soviet program that sent multiple spacecraft to study Venus, achieving the first probe landings and early surfac…

Venus — the second planet and Earth's often-contrasted sibling Venus An overview of Venus: its position, physical properties, atmosphere, rotation and orbit, exploration history, and why it differs from Earth despite similarities in size.

Verona Rupes Verona Rupes is an immense cliff on Miranda, a moon of Uranus, notable for being the tallest known escarpment in the Solar System and for the clues it offers about Miranda's violen…

Very Large Telescope (VLT) — ESO's flagship optical observatory Very Large Telescope The Very Large Telescope (VLT) is a ground-based optical/infrared observatory run by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile.

Viking program (NASA Mars missions) Viking program NASA's Viking program sent two orbiter-lander pairs to Mars in 1975–76. The mission returned the first high-resolution surface images, conducted in situ experiments, and shaped dec…

Virgo (constellation) Virgo is a large zodiac constellation lying on the celestial equator. It contains the bright star Spica and the Virgo Cluster of galaxies and has a rich mythological and observatio…

Virgo Cluster A nearby rich cluster of galaxies in the constellation Virgo; central to the Virgo Supercluster and home to many well-known galaxies such as M87 and M49.

Virgo Supercluster (Local Supercluster) Virgo Supercluster A nearby collection of galaxy groups and clusters that includes the Virgo Cluster and the Local Group; spans roughly 33 Mpc (~110 million ly) and sits within the larger Laniakea co…

Void (astronomy) A cosmic void is a large, underdense region in the Universe’s large-scale structure with few galaxies, forming the empty spaces between filaments and superclusters.

Volans (constellation) Volans Volans is a small southern constellation named for the flying fish, introduced by Petrus Plancius from Dutch navigators’ star observations in the late 16th century.

Vostok 1 — first human spaceflight led by Yuri Gagarin Vostok 1 Vostok 1 was the Soviet mission that carried Yuri Gagarin into orbit on 12 April 1961, the first human journey into outer space. The flight demonstrated crewed orbital operations a…

Voyager 1: NASA's Deep-Space Probe and the First Interstellar Voyager Voyager 1 Voyager 1 is a NASA robotic probe launched in 1977 to study Jupiter and Saturn; it later crossed the heliosphere into interstellar space and continues returning scientific data whi…

Voyager 2: NASA's Deep‑Space Probe to the Ice Giants and Beyond Voyager 2 Voyager 2 is NASA's long‑lived robotic probe that performed the only close flybys of Uranus and Neptune, crossed into interstellar space in 2018, and carries the Voyager Golden Rec…

Voyager program: NASA's long-range interstellar probe missions Voyager program Two robotic probes launched in 1977 that explored the outer planets and are now traveling through interstellar space, carrying scientific instruments and the Golden Record.

Vulcan (hypothetical planet): history, searches, and resolution Vulcan (hypothetical planet) Vulcan was a proposed intra‑Mercurial planet posited in the 19th century to explain Mercury's anomalous perihelion advance. Searches failed and Einstein's General Relativity later…

Vulcanoid asteroids: hypothetical small bodies orbiting inside Mercury's path Vulcanoid asteroid Vulcanoids are a proposed population of small asteroids that would orbit between the Sun and Mercury. None have been confirmed; their existence informs models of inner Solar System…

Vulpecula — the Little Fox (northern constellation) Vulpecula Vulpecula is a faint northern constellation, best known for the Dumbbell Nebula (M27) and the Coathanger asterism. It lies within the Summer Triangle and has no stars brighter than…

VV Cephei — a massive eclipsing binary in Cepheus VV Cephei VV Cephei is a long-period eclipsing binary in the constellation Cepheus, notable for a red supergiant primary with an extremely large radius and a hot, smaller companion star.

VY Canis Majoris — a red hypergiant star VY Canis Majoris VY Canis Majoris is a red hypergiant in Canis Major. One of the largest known stars, it is notable for extreme size estimates, heavy mass loss, irregular variability, and an extend…

Westerlund 1 — Massive Young Star Cluster in the Milky Way Westerlund 1 Westerlund 1 is a highly obscured, massive young super star cluster in the Milky Way, notable for its large population of evolved massive stars, an unusual X-ray pulsar, and import…

Westerlund 2: A Young Massive Star Cluster in the Milky Way Westerlund 2 Westerlund 2 is a very young, compact and massive star cluster embedded in the RCW 49 nebula. It hosts numerous O-type stars, pre-main-sequence stars and at least two Wolf–Rayet bi…

Whirlpool Galaxy (M51, NGC 5194) Whirlpool galaxy M51, the Whirlpool Galaxy (NGC 5194), is a nearby grand-design spiral interacting with companion NGC 5195, notable for its prominent arms, active nucleus and role in studies of gal…

White dwarf — compact stellar remnant White dwarf A white dwarf is a dense stellar remnant left after low- and intermediate-mass stars exhaust nuclear fuel. Supported by electron degeneracy pressure, it cools slowly and plays key…

Winter solstice: astronomical cause and cultural significance Winter solstice The winter solstice marks the year’s shortest day in one hemisphere and longest in the other; it reflects Earth’s axial tilt, defines the sun’s extreme declination, and has long ri…

Wolf 359: a nearby red dwarf star Wolf 359 Wolf 359 is a faint red dwarf about 7.8 light-years away in Leo. Noted for strong magnetic activity and flares, it is one of the nearest stars to the Sun and a frequent subject of…

Wolf–Rayet star Wolf–Rayet stars are very hot, massive, evolved stars with powerful, fast winds and emission-line spectra; they are stripped stellar cores that often precede certain supernovae and…

The World: meanings, senses, and cultural uses World Summary of how the term “world” is used to mean Earth, other planets, the universe, cultural or political spheres, and metaphorical senses in science, philosophy, and everyday lang…

Wormhole (Einstein–Rosen Bridge) Wormhole A wormhole is a hypothetical tunnel-like connection through spacetime that could link distant points in space and time. It is a theoretical concept from general relativity with no…

WR 102ka (Peony Nebula star) — a very luminous massive star near the Galactic Center WR 102ka WR 102ka, often called the Peony Nebula star, is a highly luminous, massive star in Sagittarius near the Milky Way's center. It is notable for extreme brightness, a large radius, a…

Year: astronomical basis, calendar definitions, and practical uses Year A year is the interval in which Earth completes one orbit of the Sun. This article explains different scientific definitions, calendar practices, historical adjustments, and practi…