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Gaia Data Release 3
Gaia Data Release 3, or Gaia DR3, is a 2022 catalog from the European Space Agency's Gaia mission covering about 1.8 billion stars and other objects.
Galactic Center GeV excess
The Galactic Center GeV excess, also called GCE, is an unexplained gamma-ray glow at the Milky Way's center, first seen in 2009 and debated as dark matter or pulsars.
Galactic plane
The galactic plane is the plane containing most of the disk of the Milky Way Galaxy, appearing in the sky as the luminous band of the Milky Way.
Galaxy And Mass Assembly survey
The Galaxy And Mass Assembly (GAMA) survey is a spectroscopic survey of about 290,000 nearby galaxies, made with the AAOmega spectrograph on the Anglo-Australian Telescope from 2008 to 2014.
Galaxy cluster
A galaxy cluster is a gravitationally bound system of galaxies, hot X-ray-emitting plasma, and dark matter, about 80% dark matter, and the site of the most energetic events since the Big Bang.
Galaxy formation and evolution
Galaxy formation and evolution is the branch of astrophysics that studies how galaxies arose from primordial density fluctuations in dark matter and gas and changed across 13.8 billion years.
Galaxy spectroscopy
Galaxy spectroscopy is the technique of dispersing the light of whole galaxies into spectra to measure redshifts, internal velocities, and the physical properties of their stars and gas.
Gamma-ray imaging
Gamma-ray imaging is a set of techniques for mapping the direction, energy, and intensity of gamma rays, using coded masks, Compton telescopes, and pair-conversion telescopes.
GECAM
GECAM (Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor; Chinese name 极目) is a pair of Chinese microsatellites launched in 2020 to monitor the whole sky in X-rays and soft gamma rays for gamma-ray bursts.
Gemini Observatory
The Gemini Observatory is a pair of identical 8.1-meter optical and infrared telescopes, Gemini North on Maunakea in Hawai'i and Gemini South on Cerro Pachón in Chile.
Geocentric model
The geocentric model, also known as the Ptolemaic theory or Ptolemaic cosmology, places Earth at the center of the universe and dominated astronomy for over a thousand years.
Georges Lemaître
Georges Lemaître was a Belgian Catholic priest, mathematician, and astronomer at the Université catholique de Louvain who first proposed the expanding universe and the primeval atom, the basis of Big Bang theory.
Geostationary orbit
A geostationary orbit, also called the Clarke Belt or Clarke orbit, is a circular equatorial orbit at 35,786 km where a satellite stays fixed above one spot on Earth.
Geosynchronous orbit
A geosynchronous orbit (GEO) is an Earth orbit with a period of one sidereal day, at about 35,786 km altitude, so satellites stay over a fixed longitude.
Giant Arc (large-scale structure)
The Giant Arc is a proposed large-scale structure of galaxies, clusters, and gas spanning about 1 gigaparsec, reported in 2021 and seen at redshift 0.8.
Giant Magellan Telescope
The Giant Magellan Telescope is a 25.4-meter optical and infrared telescope under construction at Las Campanas Observatory in Chile's Atacama Desert, with seven 8.4-meter mirrors.
Giant Metrewave Radio Telescope
The Giant Metrewave Radio Telescope (GMRT) is a radio interferometric array of thirty 45-meter dishes near Pune, India, proposed by Govind Swarup and operational since 1995.
Goode Solar Telescope
The Goode Solar Telescope is a 1.6-meter off-axis solar telescope at Big Bear Solar Observatory in California, operated by NJIT and second in size only to DKIST.
Gran Telescopio Canarias
The Gran Telescopio Canarias (GranTeCan) is a 10.4 m optical and infrared telescope at the Roque de los Muchachos Observatory on La Palma, Spain, the world's largest since 2007.
Graveyard orbit
A graveyard orbit is an orbit above geostationary altitude where retired satellites are parked at end of mission so they no longer interfere with operating spacecraft.
Gravitational collapse
Gravitational collapse is the contraction of a region of matter under its own gravity when supporting pressure is insufficient, first analyzed by James Jeans in 1902.
Gravitational lensing of the cosmic microwave background
CMB weak lensing is the gravitational distortion of the cosmic microwave background by intervening matter, which traces the universe's projected mass distribution and constrains dark energy and neutrino mass.
Gravitational slip
Gravitational slip is the difference between the two scalar metric potentials in a perturbed expanding universe. A nonzero slip signals a departure from general relativity or anisotropic stress, altering structure growth and lensing.
Gravitational wave background
A gravitational wave background is a stochastic, noise-like signal from many overlapping, unresolved gravitational waves; in 2023 pulsar timing arrays reported evidence for a nanohertz background, likely from supermassive black hole binaries.
Gravitational-wave astronomy
Gravitational-wave astronomy studies cosmic sources through gravitational waves, ripples in spacetime predicted by Einstein and first detected by the LIGO detectors in 2015, when they recorded the binary black hole merger GW150914.
Gravitational-wave early warning alert
A gravitational-wave early warning alert is a notice from the LIGO–Virgo–KAGRA detectors sent before a compact binary merger, giving telescopes a head start on the flash that follows.
Great Debate (astronomy)
The Great Debate was a 1920 discussion between astronomers Harlow Shapley and Heber Curtis at the National Academy of Sciences over whether spiral nebulae were within the Milky Way.
Great Observatories Origins Deep Survey
The Great Observatories Origins Deep Survey (GOODS) is a multiwavelength astronomical survey combining Hubble, Spitzer, and Chandra data on two deep fields to study galaxy formation and evolution.
Green Bank Telescope
The Green Bank Telescope is a 100-meter radio telescope in Green Bank, West Virginia, and the largest fully steerable radio telescope on Earth, dedicated in 2000.
Griffith Observatory
Griffith Observatory is a public astronomy observatory and science education center on Mount Hollywood in Griffith Park, Los Angeles, opened in 1935 and run by the city with free admission.