Astronomy

Exploration of Mars: history, missions, and scientific goals Exploration of Mars Survey of Mars exploration covering mission types, key milestones, scientific findings, and future objectives in robotic and potential crewed exploration.

Exploration of Neptune History and future of Neptune exploration: Voyager 2 flyby, what we learned about the ice giant and Triton, scientific goals, challenges of missions, and proposed orbiter concepts.

Interstellar extinction (astronomy): causes, reddening, measurement and effects Extinction (astronomy) Interstellar extinction is the dimming and reddening of starlight by dust and gas. This article explains causes, wavelength dependence, measurement methods, corrections, and conseq…

Extragalactic Astronomy: study of objects beyond the Milky Way Extragalactic astronomy Overview of extragalactic astronomy: its scope, methods, history, major classes of objects, instruments and its role in understanding galaxy evolution and the large-scale universe.

Extrasolar planet (exoplanet) Extrasolar planet An extrasolar planet, or exoplanet, is a planet that orbits a star outside our Solar System. Overview of types, detection methods, formation, occurrence in the Milky Way, habitabil…

Extraterrestrial sky: appearance, causes, and observed examples Extraterrestrial sky An overview of skies as seen from worlds other than Earth: how atmospheres, light scattering, satellites and rings shape their appearance, with observed examples and methods for pr…

Extravehicular activity (EVA): spacewalks, suits, history and uses Extravehicular activity Extravehicular activity (EVA), or a spacewalk, is when a crew member leaves a spacecraft to operate in space or on another body's surface. EVAs enable construction, repairs, resear…

Farbauti (Saturn moon) Farbauti (moon) Farbauti (S/2004 S 9) is a small, irregular, retrograde moon of Saturn discovered in 2004; it orbits far from the planet and is named after a figure from Norse mythology.

Fenrir (moon) — an irregular outer satellite of Saturn Fenrir (moon) Fenrir (Saturn XL(I), S/2004 S 16) is a small, retrograde irregular moon of Saturn discovered in 2004–2005; named after the Norse wolf, it orbits far from Saturn with a multi-year…

Ferdinand (Uranian moon) Ferdinand (moon) Ferdinand is a distant irregular, retrograde moon of Uranus. Discovered in 2001, briefly lost, and recovered in 2003, it is the outermost known retrograde non-spherical satellite o…

Fermi paradox: the puzzle of the Great Silence Fermi paradox Why, given the vast age and size of the cosmos, we have not observed extraterrestrial civilizations; origins, proposed resolutions, search methods, and implications.

First light (astronomy) “First light” denotes a telescope’s inaugural observations and, separately, the cosmic epoch when the first stars lit the Universe, ending the Dark Ages.

Flame Nebula (NGC 2024) — emission and star-forming nebula in Orion Flame nebula The Flame Nebula (NGC 2024) is an emission nebula in Orion's Belt, illuminated by Alnitak and embedded in the Orion Molecular Cloud Complex; known for dark dust lanes and active st…

Flamsteed designation — numerical star names by constellation Flamsteed designation A system that identifies stars by number within each constellation, introduced by John Flamsteed; numbers are ordered roughly by right ascension and are widely used alongside Bayer…

Flare star: magnetic outbursts on cool, active stars Flare star A flare star is a variable star that produces sudden, short-lived increases in brightness from energetic magnetic flares. Most are red dwarfs, but flares occur across stellar types…

Fleming (lunar crater) Fleming is a large, heavily degraded impact crater on the Moon's far side, named for astronomer Williamina Fleming. It lies near Hertz and Lobachevskiy and shows a worn rim and a p…

Flora family (asteroid group) Flora family A large grouping of S-type asteroids in the inner main belt, important for studies of collisional evolution, orbital dynamics, and possible meteorite sources.

Fobos-Grunt: Russian Phobos sample-return mission (2011) Fobos-Grunt Fobos-Grunt was an uncrewed Russian mission launched in 2011 to return a soil sample from Phobos. The mission failed to leave Earth orbit and reentered in 2012, but influenced late…

Formation and evolution of the Solar System Summary of how the Solar System formed from a rotating cloud of gas and dust about 4.6 billion years ago, how planets and smaller bodies originated, and how the system has changed…

Former constellations: obsolete star patterns and their history Former constellations Overview of former constellations — star patterns no longer recognized by the IAU, why they were removed, notable examples, and their cultural and historical significance.

Fornax (constellation) Fornax Fornax is a faint southern constellation introduced in the 18th century. It contains several notable galaxies and a dwarf satellite of the Milky Way and is best known among astrono…

Fornjot (Saturn XLII) — an irregular outer moon of Saturn Fornjot (moon) Fornjot (Saturn XLII, S/2004 S 8) is a small, retrograde irregular moon of Saturn discovered in 2005. It orbits at ~24.5 million km and is named after a giant from Norse mythology.

Francisco (Uranian moon) Francisco (moon) Francisco is a small, irregular, non-spherical moon of Uranus discovered in images from 2001 and named for a minor lord in Shakespeare's The Tempest.

Frankfurt Universe (German American football team) Frankfurt Universe Frankfurt Universe is a German American football club from Frankfurt, Hesse. Founded in 2007, it rose through the German league system, plays at Frankfurter Volksbank Stadion and l…

Friedrich Georg Wilhelm von Struve — astronomer, cataloguer of double stars, and geodesist Friedrich Georg Wilhelm von Struve German-born astronomer (1793–1864) who worked in the Russian Empire; led Dorpat and Pulkovo observatories, compiled influential double-star catalogues, measured stellar parallaxes,…

From the Earth to the Moon (Jules Verne) From the Earth to the Moon An overview of Jules Verne’s 1865 science-fiction novel describing a 19th‑century attempt to reach the Moon by a gigantic cannon and the voyage’s characters, themes, and influence.

Full Moon The full moon is the lunar phase when the Moon's near side is fully illuminated as seen from Earth. It has scientific, cultural, and calendrical importance and shows variations suc…

G-type Main-Sequence Star (Yellow Dwarf) G-type main-sequence star Overview and characteristics of G-type main-sequence stars: physical traits, internal fusion, life cycle, habitability, and notable examples including the Sun.

Gaia (ESA space observatory) Gaia (spacecraft) Gaia is an ESA space observatory mapping about one billion stars to produce a precise three-dimensional survey of the Milky Way and other objects for studies of Galactic structure…

Galactic plane The galactic plane is the approximate midplane of a disk galaxy where most of its stars, gas, and dust are concentrated. This article explains its definition, observability, and im…

Galactic quadrant — astronomical sectors and fictional divisions Galactic quadrant A galactic quadrant is a quarter-sector of a galaxy used in astronomy for mapping the Milky Way and a separate fictional partition popularized by the Star Trek franchise.

Galatea (moon of Neptune) Galatea (moon) Galatea (Neptune VI) is a small, irregular inner moon of Neptune discovered in Voyager 2 images in 1989. It orbits close to Neptune and influences the planet's ring material while…

Galaxies: structure, classification, dark matter, and cosmic distribution Galaxy Overview of galaxy structure and morphology, the roles of stars, gas and dominant dark matter, central black holes, and how galaxies are organized from dwarf systems to clusters ac…

Galaxy cluster — structure, formation, and cosmological significance Galaxy cluster A galaxy cluster is a gravitationally bound system of hundreds to thousands of galaxies together with hot intracluster gas and dark matter; key tracers of large-scale structure and…

Galaxy group A galaxy group is a gravitationally bound collection of a few to a few dozen galaxies. This article summarizes their defining properties, formation, common examples such as the Loc…

Galaxy Science Fiction Prominent American science fiction magazine (1950–1980) noted for social satire, literary quality, and influential editors H. L. Gold and Frederik Pohl.

Galilean moons — the four largest satellites of Jupiter Galilean moons The Galilean moons are Jupiter's four largest satellites—Io, Europa, Ganymede and Callisto—discovered by Galileo. They differ widely in geology, history and potential for hosting s…

Galileo (spacecraft): NASA's exploration of Jupiter and its moons Galileo (spacecraft) Uncrewed NASA spacecraft launched in 1989 that studied Jupiter, its atmosphere, rings and Galilean moons; delivered an atmospheric probe in 1995 and was deliberately deorbited in 2…

Gamma Cygni (Sadr): a bright supergiant in the Cygnus constellation Gamma Cygni Gamma Cygni, commonly called Sadr, is a luminous white–yellow supergiant located at the heart of the Northern Cross in the constellation Cygnus, surrounded by rich starfields and n…

Gamma Tauri (Prima Hyadum) Gamma Tauri Gamma Tauri, also called Prima Hyadum, is a yellow giant star in Taurus and a member of the Hyades cluster, visible to the naked eye and important for studies of stellar evolution.

Gamma Velorum (Regor) — Massive binary in Vela Gamma Velorum Gamma Velorum, nicknamed Regor, is a nearby massive binary in Vela containing a Wolf–Rayet star and an O‑type blue supergiant. It is important for studying stellar winds, mass loss…

Gamma-ray burst A concise encyclopedia article on gamma‑ray bursts: brief, extremely energetic flashes of gamma rays from distant galaxies, their types, origins, detection history, afterglows, and…

Ganymede: Jupiter’s largest moon and the Solar System’s biggest natural satellite Ganymede (moon) Ganymede is the largest moon in the Solar System. Larger in diameter than Mercury, it has a unique magnetic field, complex surface, probable subsurface ocean, and a central role in…

Gas giant A gas giant is a very large planet dominated by a thick gaseous envelope over a dense core. This article describes structure, formation, Solar System examples, exoplanets and key d…

Gegenschein (Counterglow): Faint Antisolar Sky Glow Gegenschein The Gegenschein is a very faint sky glow seen at the point opposite the Sun, produced by sunlight scattering from interplanetary dust. It is visible only under very dark, moonless…

Gemini (astrology): The Twins — mutable Air sign ruled by Mercury Gemini (astrology) Gemini is the third sign of the Western zodiac, symbolized by the Twins. Associated with Mercury and the element Air, it spans late May to late June in common tropical practice.

Gemini (constellation) Gemini, the Twins, is a zodiac constellation notable for the stars Castor and Pollux, the Geminids meteor shower, and a long astronomical history from Ptolemy to modern stargazing.

Geocentrism Geocentrism is the historical model that places the Earth at the centre of the cosmos. This article explains its main features, development, decline, and lasting cultural significa…