Astronomy

Geometric albedo: definition, measurement, and significance Geometric albedo Geometric albedo measures how bright a body appears at zero phase angle compared with an ideal diffuse reflecting disk; it helps infer surface properties and size and varies with w…

George's Secret Key to the Universe A 2007 children's science-adventure by Stephen Hawking, Lucy Hawking and Christophe Galfard that combines fiction with clear explanations of cosmology and basic physics for middle-…

Georges Lemaître: Belgian priest and pioneer of modern cosmology Georges Lemaître Georges Lemaître (1894–1966), Belgian priest, mathematician and physicist who first proposed an expanding universe and the 'primeval atom' hypothesis, laying groundwork for the Big…

Geostationary orbit A circular, equatorial orbit where a satellite matches Earth's rotation and appears fixed over one longitude. Used widely for communications, weather monitoring, and broadcast serv…

Geosynchronous orbit An Earth-centered orbit with an orbital period equal to the Earth's rotation, including the special equatorial geostationary case used for communications, weather monitoring, and b…

Gerard Kuiper: Dutch‑American Astronomer and Pioneer of Planetary Science Gerard Kuiper Gerard P. Kuiper (1905–1973) was a Dutch-born American astronomer who advanced planetary spectroscopy, discovered moons, founded a major lunar and planetary laboratory, and lent hi…

Gertrude (crater) — largest known impact feature on Titania Gertrude (crater) Gertrude is the largest confirmed impact crater on Uranus’s moon Titania, roughly 326 km across (about 20% of the moon’s diameter). It informs studies of impact processes and icy-m…

Giant impact hypothesis: origin of the Moon Giant impact hypothesis Scientific model that explains the Moon's origin as the result of a massive collision between early Earth and a Mars-sized body, summarizing evidence, problems, and research direct…

Giant Magellan Telescope The Giant Magellan Telescope is a next-generation ground-based optical/infrared observatory in Chile. It uses seven 8.4 m mirror segments to achieve the resolving power of a 24.5 m…

Giant star: definition, structure, evolutionary stages, and importance Giant star A giant star is a post–main-sequence star with substantially larger radius and luminosity than a similar-temperature main-sequence star. This article explains types, evolution, and…

Gihō calendar An early Japanese lunisolar calendar adapted from Chinese astronomical models and used in the Asuka period.

Ginga (Astro‑C): Japanese X‑ray Astronomy Satellite Ginga (satellite) Ginga (Astro‑C) was a Japanese X‑ray astronomy satellite launched in 1987 by ISAS. It carried large‑area detectors and an all‑sky monitor to study X‑ray binaries, transients, and c…

Gliese 581 — nearby red dwarf and its planetary system Gliese 581 Gliese 581 is a nearby M‑class red dwarf star in Libra notable for a multi‑planet system that sparked early debate about potentially habitable exoplanets and the challenges of dete…

Gliese 581 c — characteristics, discovery, and habitability debate Gliese 581 c Gliese 581 c is a super-Earth exoplanet orbiting the red dwarf Gliese 581 about 20 light‑years away. Discovered in 2007, it sparked early hopes for habitability but later studies c…

Gliese 667 Cc — potentially habitable super-Earth in the Gliese 667 system Gliese 667 Cc Gliese 667 Cc is a super-Earth orbiting the red dwarf Gliese 667 C, about 22.2 light years away. It lies near the inner edge of the star's habitable zone and was detected by radial…

Gliese 876 — nearby red dwarf with a resonant multiplanet system Gliese 876 Gliese 876 is a nearby red dwarf star in Aquarius that hosts a compact, dynamically interacting system of four known planets, notable for orbital resonances and interest in habitab…

Globular cluster Dense, roughly spherical assemblies of old stars that orbit galaxies’ halos and bulges; important tracers of galactic formation, stellar evolution, and dynamics.

GN-z11 — an early, distant galaxy GN-z11 GN-z11 is a high-redshift galaxy observed as it was in the very early universe. It was the most distant confirmed galaxy until 2022 and remains important for studies of cosmic dawn…

Gould Belt: nearby partial ring of young stars and star-forming regions Gould Belt A tilted, partial ring of young, massive stars and clouds near the Sun in the Milky Way. The Gould Belt is ~3,000 ly across, 30–50 Myr old, and traces nearby star formation and OB…

Gravitational lensing Bending of light and other radiation by mass according to general relativity, producing multiple images, rings, arcs and magnification; used to study dark matter, distant galaxies,…

Gravity assist (gravitational slingshot) Gravity assist Gravity assist is a spacecraft maneuver that uses a moving body's gravity and motion to change a spacecraft's speed and trajectory, conserving fuel and enabling complex interplanet…

GRB 970228 — first gamma‑ray burst with a detected afterglow GRB 970228 GRB 970228, observed 28 February 1997, produced the first detected X-ray and optical afterglow. Its localization and host identification established that many GRBs are extragalacti…

Great Attractor: a nearby gravitational concentration in the local universe Great Attractor A large mass concentration in the nearby cosmos whose gravity produces coherent peculiar motions of galaxies; partly hidden by the Milky Way and studied with redshifts, X-rays and…

Great Comet of 1811 (C/1811 F1) Great Comet of 1811 C/1811 F1 was a bright long-period comet visible to the naked eye for about 260 days in 1811–1812, reaching roughly magnitude 0 and producing a prominent tail; it held a longevity…

Great Dark Spot (Neptune) Great Dark Spot A transient dark vortex in Neptune's atmosphere discovered by Voyager 2 in 1989; unlike Jupiter's long-lived anticyclone, Neptune's dark spots form, change and vanish over years.

NASA Great Observatories program Great Observatories program The Great Observatories were four NASA space telescopes operating across the electromagnetic spectrum—gamma rays to infrared—designed as a coordinated fleet to study the universe i…

Great Red Spot (Jupiter's long‑lived storm) Great Red Spot A giant anticyclonic storm on Jupiter, visible for centuries. Notable for its size, longevity, changing color and shape, and as a laboratory for studying atmospheric dynamics on gi…

Great Wall (cosmic large-scale structures) Great Wall A survey of the term “Great Wall” as applied to the largest known arrangements of galaxies and gamma‑ray burst concentrations, their discovery methods, notable examples, and the sc…

Great White Spot (Saturn): periodic planet‑encircling storms Great White Spot Large, bright storm systems on Saturn that recur roughly once per Saturnian year; visible to telescopes and studied by spacecraft for their chemistry, dynamics, and role in planeta…

Greip (Saturn moon) Greip (moon) Greip (Saturn LI, S/2006 S 4) is a small, irregular, retrograde moon of Saturn discovered in 2006; it belongs to the distant Norse group and is notable for its eccentric, high-incl…

Grus (constellation) Grus, Latin for 'the Crane', is a southern-hemisphere constellation notable for its bright star Alpha Gruis, several red giants and faint galaxies. It was established during Europe…

H I region (neutral atomic hydrogen cloud) H I region A region of the interstellar medium dominated by neutral atomic hydrogen. H I regions are observed mainly by their 21‑cm radio emission and shape our view of galaxies and high‑ener…

H II region A cloud of ionized hydrogen surrounding young massive stars; a principal site of recent star formation and a tracer of chemical composition and distance in galaxies.

Habitable zone (Goldilocks zone) Habitable zone Region around a star where conditions may allow liquid water on a planet’s surface; central to searches for life but limited by atmosphere and stellar effects.

Halimede — irregular outer moon of Neptune Halimede (moon) Halimede (Neptune IX) is an irregular, non-spherical outer moon discovered in 2002. It has a highly inclined, eccentric retrograde orbit, an estimated diameter ~62 km, and may be r…

Halley's Comet Short-period comet visible from Earth every ~75–76 years; first recognized as periodic and visited by spacecraft in 1986. Returns next in 2061, notable for its dark, dusty nucleus…

Harpalyke (moon) Harpalyke is a small irregular retrograde moon of Jupiter, about 4 km across. Discovered in 2000, it belongs to the Ananke group and orbits at ~21 million km with a ~624-day period…

Hati (moon) — Saturn XLIII, a small retrograde irregular satellite Hati (moon) Hati is a small irregular, retrograde moon of Saturn (Saturn XLIII), about 6 km across. Discovered in 2004–2005, it orbits far from Saturn and belongs to the Norse group of satelli…

Haumea (dwarf planet) — a fast-rotating trans-Neptunian world Haumea (dwarf planet) Haumea is a rapidly spinning dwarf planet in the Kuiper belt, notable for its elongated shape, two moons (Hiʻiaka and Namaka), a surface dominated by crystalline water ice, and a l…

Hayabusa (Japanese asteroid sample-return spacecraft) Hayabusa Hayabusa was a Japanese robotic mission that collected and returned samples from near-Earth asteroid 25143 Itokawa. Launched in 2003, it overcame failures and returned material to…

Hegemone (Jupiter XXXIX): an irregular retrograde moon of Jupiter Hegemone (moon) Hegemone is a small irregular, retrograde moon of Jupiter (Jupiter XXXIX), discovered in 2003. It belongs to the Pasiphaë group and orbits far from Jupiter with a high inclination…

Helene (moon of Saturn) Helene (moon) Helene is a small icy moon that shares Dione’s orbit at a stable Lagrange point, discovered from Earth in 1980 and later imaged by spacecraft; notable as a Trojan companion of a la…

Helike (Jupiter XLV): small retrograde moon of Jupiter Helike (moon) Helike (Jupiter XLV) is a small, retrograde irregular moon of Jupiter discovered in 2003. About 4 km across, it belongs to the Ananke group and orbits at roughly 20.5 million km wi…

Heliocentrism: The Sun-centered model of the Solar System Heliocentrism Heliocentrism is the model that places the Sun near the center of the Solar System. This article outlines its principles, historical development, evidence, and how it differs from…

Heliosphere The heliosphere is the vast bubble of charged particles and magnetic field created by the Sun that separates the solar environment from the local interstellar medium.

Helix Nebula (NGC 7293): a nearby planetary nebula in Aquarius Helix nebula The Helix Nebula (NGC 7293) is a large, nearby planetary nebula in Aquarius, about 700 light-years away. It is noted for its ring-like structure, cometary knots and role in studyin…

Henry Norris Russell American astronomer (1877–1957) known for co-creating the Hertzsprung–Russell diagram and for Russell–Saunders (LS) coupling; a central figure in early 20th-century stellar astroph…

Hercules–Corona Borealis Great Wall A proposed colossal concentration of galaxies identified by a cluster of gamma‑ray bursts, notable for its extreme size and the debates it has provoked about cosmic homogeneity.