640 articles
Age of the universe
The age of the universe is the time elapsed since the Big Bang, estimated at 13.8 billion years from the cosmic microwave background and confirmed by the oldest stars.
AGN feedback
AGN feedback is the process by which energy and jets from an active supermassive black hole heat or eject gas in its host galaxy, quenching star formation.
AKARI
AKARI (あかり, "light"), known before launch as ASTRO-F, was Japan's first dedicated infrared astronomy satellite, launched by ISAS in February 2006 to carry out an all-sky infrared survey far more sensitive than IRAS.
Aladin Sky Atlas
Aladin Sky Atlas is an interactive sky atlas developed at the Strasbourg Astronomical Data Center in France in 1999, letting users view surveys and overlay astronomical catalogs.
Aligned, Multiple-transient Events in the First Palomar Sky Survey
A 2025 research paper by Beatriz Villarroel and colleagues reporting aligned groups of point-like optical transients on 1950s Palomar Observatory Sky Survey plates and a deficit of transients in Earth's shadow.
Allen Telescope Array
The Allen Telescope Array is a radio telescope in Northern California with 42 dishes, dedicated in 2007 as the first built for the search for extraterrestrial intelligence.
Alpha-attractors
Alpha-attractors are a class of cosmological inflation models introduced in 2013 by Kallosh, Linde, and Roest, generalizing the Starobinsky model, where a single parameter alpha controls predictions that converge to universal values.
Amaterasu particle
The Amaterasu particle is an ultra-high-energy cosmic ray detected in 2021 in Utah with an energy of 244 EeV, second only to the 1991 Oh-My-God particle.
Anglo-Australian Observatory
The Anglo-Australian Observatory is an Australian astronomical organization operating the 3.9 m Anglo-Australian Telescope at Siding Spring Observatory, commissioned in 1974 and known for the 2dF galaxy redshift surveys.
Angular diameter
Angular diameter, also called angular size or apparent diameter, is the angle an object subtends at the observer; Betelgeuse became the first star so measured, in 1920.
Angular diameter distance
The angular diameter distance is a cosmological distance measure defined as an object's physical transverse size divided by its angular size, which decreases beyond a redshift of about 1.
Antennae Galaxies
The Antennae Galaxies, NGC 4038 and NGC 4039, are a pair of colliding spiral galaxies in Corvus, discovered by William Herschel in 1785 and named for their insect-like tidal tails.
Anthropic principle
The anthropic principle is a rule in cosmology stating that what we observe must be compatible with our existence as observers, introduced by Brandon Carter in 1973.
AO 0235+164
AO 0235+164, also written 0235+164, is a highly variable blazar at redshift 0.94, classified as a BL Lac object, known for major flares including an October 2008 optical peak.
Apache Point Observatory
Apache Point Observatory is a research astronomical observatory in the Sacramento Mountains of southern New Mexico, operated by the Astrophysical Research Consortium and home of the Sloan Digital Sky Survey.
Aperture photometry
Aperture photometry is a standard technique of astronomical photometry that measures brightness by summing light within a circle and subtracting sky background, working best in uncrowded fields.
Aperture synthesis
Aperture synthesis is an interferometric technique combining signals from widely separated telescopes to achieve the resolution of one aperture as large as the whole array, underlying radio interferometry.
Apparent magnitude
Apparent magnitude, also known as visual or stellar magnitude, measures how bright a celestial object appears from Earth on a scale where lower numbers mean brighter.
Arecibo Observatory
The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center, was a 305-meter radio telescope in Puerto Rico that operated from 1963 until its collapse in 2020.
ARGO-YBJ
ARGO-YBJ was a Sino-Italian extensive air shower detector at the Yangbajing Cosmic Ray Observatory in Tibet, China, operating from 2007 to 2013 for gamma-ray and cosmic-ray physics.
Armillary sphere
An armillary sphere, also known as a spherical astrolabe, is a model of the heavens built from rings representing the great circles of the sky.
ASTRI-Horn
ASTRI-Horn is an Italian prototype Cherenkov telescope on Mount Etna, Sicily, the first to use a dual-mirror design, built by INAF for the Cherenkov Telescope Array.
Astrometric calibration
Astrometric calibration ties measured positions of astronomical sources to a standard celestial reference frame, producing World Coordinate System solutions for images and global solutions for Gaia.
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars, and galaxies, investigating the universe's origin and evolution; about 7,000 work professionally in North America.
Astronomia Nova
Astronomia Nova is a treatise by Johannes Kepler published in Prague in 1609, formulating the first two laws of planetary motion, based on Tycho Brahe's observations of Mars.
Astronomical spectroscopy
Astronomical spectroscopy disperses the light of stars and galaxies into spectra, revealing chemical composition, temperature, and motion; it measures dark matter, stellar masses, and the Universe's expansion.
Astronomy
Astronomy is the natural science that studies celestial objects, including stars, planets, and galaxies, through observation of their light; practiced since ancient Babylonia, it now includes astrophysics and cosmology.
Astrophysics
Astrophysics is the branch of astronomy that applies physics and chemistry to explain the physical nature, composition, and evolution of celestial bodies, rather than merely measuring their positions and motions.
Asymmetric dark matter
Asymmetric dark matter is a class of dark matter models in which dark matter's abundance shares its origin with visible matter's, through an asymmetry between dark particles and dark antiparticles.
Atacama Cosmology Telescope
The Atacama Cosmology Telescope (ACT) was a 6-meter millimeter-wave telescope at 5190 meters on Cerro Toco in Chile, built to map the cosmic microwave background from 2007 to 2022.