Astronomy

Abell 2142 — a massive, X‑ray bright merging galaxy cluster Abell 2142 Abell 2142 is an extremely massive, X‑ray luminous galaxy cluster in Corona Borealis formed by the merger of two clusters; it contains hundreds of galaxies and a vast reservoir of…

Accretion disk A rotating disk of gas, dust or plasma that spirals inward under gravity around a central mass; it converts gravitational potential into heat and radiation and appears across stell…

Achernar (Alpha Eridani): the rapid-rotating blue star at the end of Eridanus Achernar Achernar, Alpha Eridani, is a hot, blue, rapidly rotating B-type star in the constellation Eridanus. It is unusually oblate, part of a binary system, and notable in astronomy and c…

Active galactic nucleus (AGN) Active galactic nucleus A compact, extremely luminous region at a galaxy's center powered by accretion onto a supermassive black hole. AGN emit across the electromagnetic spectrum and can produce powerful…

Ad astra (Latin phrase) Ad astra (phrase) Latin phrase meaning "to the stars". Explains origins, grammatical form, historic uses, common variations such as "per aspera ad astra", and its role as motto and cultural referenc…

Adler Planetarium & Astronomy Museum Historic Chicago planetarium founded in 1930 offering exhibitions, sky shows, public observing, education and outreach. First planetarium in the Western Hemisphere and a National H…

Adrastea (Jupiter moon) Adrastea (moon) Adrastea is a small, irregular moon orbiting very close to Jupiter. Discovered in 1979 in Voyager images, it lies within Jupiter's ring system and helps supply ring material throug…

Adriaan Blaauw: Dutch astronomer and leader in stellar kinematics Adriaan Blaauw Biography and overview of Adriaan Blaauw (1914–2010), his research on young star groups and runaway stars, and his leadership roles at the European Southern Observatory and the IAU…

Aegaeon (moon) Aegaeon is a tiny, irregular moon of Saturn embedded in the planet's faint G ring. Discovered in 2008 images and initially designated S/2008 S1, it supplies dust to a ring arc and…

Aegir (Saturn XXXVI) Aegir (moon) Aegir is a small, irregular, retrograde moon of Saturn discovered in 2005. It orbits far from the planet, belongs to the Norse group, and is named after the sea giant Ægir from Nor…

Age of the Universe The age of the universe is the time elapsed since the Big Bang. Modern observations—especially the cosmic microwave background measured by Planck—give a best estimate of about 13.8…

Aitne (Jupiter moon) Aitne (moon) Aitne (Jupiter XXXI) is a small, irregular retrograde satellite of Jupiter discovered in 2001; part of the Carme group and named after the divine personification of Mount Etna.

Akari (satellite): Japan’s infrared space observatory Akari (satellite) Akari (あかり, “light”) is a Japanese infrared space telescope launched in 2006 to perform an all-sky survey in near-, mid- and far-infrared, producing catalogs and maps used across i…

Akebono — Japanese satellite for auroral and magnetospheric research Akebono (satellite) Akebono (あけぼの) was a Japanese scientific satellite launched in 1989 to investigate auroras and Earth's magnetosphere, operated by ISAS/University of Tokyo and providing decades of…

Albiorix — an irregular moon of Saturn Albiorix (moon) Albiorix is a prograde irregular satellite of Saturn, largest member of the Gallic group. Discovered in 2000, it is a small, non-spherical body thought to be a remnant of early col…

Albireo (Beta Cygni): the striking double star in Cygnus Albireo Albireo (Beta Cygni) is a famous double star in the constellation Cygnus noted for its contrasting golden and blue components; popular with amateur astronomers and useful as a visu…

Aldebaran — the orange giant star in Taurus Aldebaran Aldebaran is a prominent orange giant (α Tauri) about 65 light-years away in Taurus. Bright and historically notable, it is a nearby evolved star with reports of a possible substel…

Alderaan: fictional pacifist world in the Star Wars universe Alderaan Alderaan is a fictional peaceful planet in the Star Wars saga, known as the home of Princess Leia and Bail Organa and for its cinematic destruction by the Death Star, which shaped…

Algol — the eclipsing triple star of Perseus Algol Algol (Beta Persei) is a well-known eclipsing binary and triple star system in Perseus, historically called the Demon Star. Its periodic dimming made it a key target for stellar as…

Allen Telescope Array A radio interferometer of small dishes at Hat Creek, California, built for combined radio astronomy and SETI work; notable for commensal observing, wideband receivers, and its larg…

Alnitak (Zeta Orionis): a bright multiple star in Orion's Belt Alnitak Alnitak (Zeta Orionis) is a luminous multiple-star system in Orion's Belt. Its hot O-type primary and close companions illuminate nearby nebulae and serve as a laboratory for massi…

Alpha Centauri — the nearest stellar system to the Sun Alpha Centauri Alpha Centauri is a nearby triple star system in Centaurus: a Sun‑like binary plus Proxima, the closest individual star, notable for brightness, exoplanet discoveries, and interest…

Alpha Cephei (Alderamin) Alpha Cephei Alderamin (Alpha Cephei) is the brightest star in Cepheus: an A-type white star, a rapid rotator with an oblate shape, visible to the naked eye and notable for its role in stellar…

Alpha Crucis (Acrux) — the bright multiple star of the Southern Cross Alpha Crucis Alpha Crucis (Acrux) is a blue, hot multiple star system in the Crux constellation, noted as the brightest member of the Southern Cross and important in navigation and cultural sym…

Alpha Herculis (Rasalgethi) Alpha Herculis Alpha Herculis, or Rasalgethi, is a red giant multiple star system and the brightest star in Hercules, known for its orange-red color and traditional Arabic name.

Alpha Persei Cluster (Melotte 20 / Collinder 39) Alpha Persei Cluster A nearby young open cluster in Perseus, dominated by blue B-type stars and the bright supergiant Mirfak; important for distance calibration and studies of early stellar evolution.

Altair (α Aquilae) — Bright A-type Star in the Constellation Aquila Altair Altair (α Aquilae) is a nearby, rapidly rotating A-type main-sequence star in Aquila. It is the twelfth brightest night-sky star, notable for its oblate shape, role in the Summer T…

Amalthea — Jupiter’s irregular inner moon Amalthea (moon) Amalthea is a small, irregular inner moon of Jupiter discovered in 1892. Reddish and non‑spherical, it is a source of dust for Jupiter’s gossamer rings and was imaged by Voyager an…

Amor asteroids Amor asteroid Amor asteroids are near-Earth asteroids that approach Earth from outside its orbit but usually do not cross it. The class is named for 1221 Amor and includes 433 Eros.

Ananke (moon) Irregular, retrograde moon of Jupiter discovered in 1951. Namesake of the Ananke group of small, captured satellites; originally designated Jupiter XII and renamed in 1975 after th…

Ananke group — Jupiter's retrograde irregular moons Ananke group A cluster of small, retrograde irregular moons orbiting Jupiter with similar distant, tilted orbits. Thought to be fragments of a captured body, they share orbital and physical tra…

Andromeda (constellation) Northern constellation named for the mythical princess; contains the Andromeda Galaxy (M31), several bright stars and deep‑sky objects, and figures prominently in history, mytholog…

Andromeda XIX — an unusually extended dwarf satellite of M31 Andromeda XIX Andromeda XIX is a very diffuse dwarf satellite of the Andromeda Galaxy (M31) in the Local Group, notable for an exceptionally large half-light radius and extremely low surface bri…

Angular size (angular diameter) Angular size Angular size describes how large an object appears from a given vantage point, expressed as an angle. It relates physical size and distance and is widely used in astronomy, optics…

Antares — the red supergiant at the heart of Scorpius Antares Antares (Alpha Scorpii) is a prominent red supergiant in the constellation Scorpius, one of the brightest stars in the night sky and a nearby example of a massive star nearing the…

ANTARES Neutrino Telescope Antares Telescope ANTARES is a deep‑sea observatory that detects neutrinos using arrays of light sensors on lines anchored to the Mediterranean seabed, serving astrophysics and environmental researc…

Antennae galaxies (NGC 4038 and NGC 4039): interacting starburst galaxies Antennae galaxies The Antennae galaxies (NGC 4038/4039) are a nearby pair of colliding spiral galaxies in Corvus whose tidal interaction has produced long stellar tails and an intense starburst phas…

Anthe (moon of Saturn) Anthe (moon) Anthe is a tiny, irregular natural satellite of Saturn discovered in Cassini images in 2007. It orbits between Mimas and Enceladus and is one of several small moons associated with…

Antlia — the southern constellation representing a pump Antlia Antlia is a faint southern constellation created in the 18th century to represent an air pump. It lies near Hydra, Pyxis, Vela and Centaurus and contains few bright stars but sever…

Aoede (Jupiter XL I): a small irregular moon of Jupiter Aoede (moon) Aoede (Jupiter XLI) is a small, retrograde irregular moon of Jupiter discovered in 2003. It is about 4 km wide, orbits at ~23 million km with a highly eccentric, retrograde path, a…

Aphelion — the point in an orbit farthest from the Sun Aphelion Aphelion is the point in an object's solar orbit where it is farthest from the Sun. The term, its orbital effects, Earth's aphelion timing, and related terminology are explained an…

Apollo 6 — Second Saturn V test and last unmanned Apollo flight Apollo 6 Apollo 6, launched April 4, 1968, was the second Saturn V test and final unmanned Apollo mission; it validated reentry performance despite second- and third-stage engine problems t…

Apollo 7: First Crewed Flight of the Block II Apollo Command Module Apollo 7 Apollo 7 (launched Oct 11, 1968) was NASA's first crewed Apollo mission. It tested the Block II Command/Service Module in Earth orbit, validated life‑support and control systems, a…

Apollo asteroids: Earth‑crossing near‑Earth objects Apollo asteroids A group of near‑Earth asteroids with orbits that cross Earth’s path. Named after 1862 Apollo, they include many well-studied objects and a subset classified as potentially hazardou…

Apollo Command and Service Module: design, history and missions Apollo command and service module Comprehensive overview of the Apollo Command and Service Module (CSM): design, systems, operational role in lunar missions, development history, major missions and legacy in human…

Apollo program: United States crewed effort to land humans on the Moon Apollo program Apollo was NASA's 1960s–70s program to land humans on the Moon and return them safely. It produced the first lunar landings, important scientific return, hardware innovations, and…

Apsis (astronomy): periapsis, apoapsis, and the line of apsides Apsis An apsis is the extreme point in an orbit — nearest (periapsis) or farthest (apoapsis) — with named variants for different central bodies and importance in orbital mechanics and ce…

Apus — the southern constellation of the Bird‑of‑Paradise Apus Apus is a small, faint constellation in the far southern sky. Named from a Greek word meaning “without feet,” it represents the bird‑of‑paradise and is best observed from southern…