Astronomy

Barnard's Loop — the large emission arc in Orion Barnard's Loop Barnard's Loop is a vast emission nebula in Orion: a faint red arc of ionized hydrogen about 1,600 light-years away and roughly 300 light-years across, shaped by stellar winds and…

Barnard's Star: a nearby high–proper-motion red dwarf Barnard's star Barnard's Star is a nearby, ancient red dwarf about 1.8 pc (≈6 ly) away, notable for the largest known stellar proper motion and for searches for planetary companions.

Barycenter: the center of mass in astronomy and geometry Barycenter The barycenter is the common center of mass about which two or more bodies orbit or balance. This article explains its definition, examples (Earth–Moon, solar system), geometry cou…

Bayer designation A star naming convention introduced by Johann Bayer that pairs Greek letters with constellation names to identify stars; commonly used in star charts and popular astronomy.

Beagle 2 — British Mars lander and astrobiology mission Beagle 2 Beagle 2 was the United Kingdom's small Mars lander carried by ESA's Mars Express in 2003 to search for signs of past life. It failed to communicate at touchdown and was located on…

Bebhionn (moon of Saturn) Bebhionn (moon) Bebhionn (Saturn XXXVII) is a small prograde irregular moon of Saturn, about 6 km across, discovered in 2004–2005 and named for a figure in Irish mythology; it orbits at ~16.9 mill…

Belinda — an inner moon of Uranus Belinda (moon) Belinda is a small, elongated inner moon of Uranus discovered in Voyager 2 images in 1986. It belongs to the Portia group and is known mainly by its orbit, size, low albedo and str…

BepiColombo: ESA and JAXA mission to Mercury BepiColombo BepiColombo is a joint European Space Agency and Japan Aerospace Exploration Agency mission to study Mercury’s composition, interior, exosphere and magnetosphere; launched 2018, ar…

Bergelmir — a small irregular moon of Saturn Bergelmir (moon) Bergelmir (Saturn XXXVIII) is a small, retrograde irregular moon of Saturn discovered in 2005. It orbits far from the planet with a long period and was named after the giant Bergel…

Berlin Observatory (Berliner Sternwarte) Berlin Observatory Historic astronomical institution in Berlin, founded from early-18th scientific societies; developed observational, timekeeping and calendrical work and evolved through several sit…

Sir Bernard Lovell — pioneering English physicist and radio astronomer Bernard Lovell English physicist and radio astronomer; first director of Jodrell Bank Observatory, creator of the large steerable radio telescope there, and a leading figure in early radio astron…

Bestla (moon of Saturn) Bestla (moon) Bestla (Saturn XXXIX) is a small, irregular, retrograde moon of Saturn, notable for its highly eccentric orbit. Discovered in 2005, it belongs to the Norse group and is thought to…

Beta Centauri (Hadar, Agena) — bright triple star in Centaurus Beta Centauri Beta Centauri (β Cen, Hadar, Agena) is a bright triple-star system in Centaurus, visible from the Southern Hemisphere and used as a pointer to the Southern Cross.

Beta Columbae (β Col) — a star of the Columba constellation Beta Columbae An accessible overview of Beta Columbae (β Col), a moderately bright star in the southern constellation Columba, covering its identity, visibility, historical context and astronomi…

Betelgeuse — the red supergiant in Orion Betelgeuse An overview of Betelgeuse: its nature as a red supergiant, observable properties, evolutionary status, notable features such as variability and bow shock, and why it matters to ast…

Bianca (Uranian moon) Bianca (moon) Bianca is a small inner moon of Uranus discovered by Voyager 2 in 1986. A member of the Portia group, it is elongated, dark (low albedo), about 27 km in radius, and known mainly fr…

Big Bang (cosmology) Big Bang Overview of the Big Bang: the scientific model for the universe’s origin, its evidence, historical development, main concepts, and remaining questions.

Binary pulsar: pulsars in gravitationally bound stellar systems Binary pulsar A binary pulsar is a pulsar orbiting a companion star. Precise pulse timing reveals orbital motion and relativistic effects, enabling tests of gravity, mass measurements, and studi…

Binary star Two stars bound by gravity that orbit a common centre; important for measuring stellar masses, studying stellar evolution, and producing diverse phenomena from eclipses to mass tra…

Black dwarf (stellar remnant) Black dwarf A black dwarf is a theoretical cooled remnant of a white dwarf that no longer emits significant heat or light. Formation would require far longer than the current age of the Univer…

Black Eye Galaxy (Messier 64) Black Eye galaxy Nearby spiral galaxy famous for a dark dust lane that looks like a 'black eye'. Contains counter-rotating gas, roughly 17–25 million light-years away, and is visible in small teles…

Black hole: formation, structure, detection, and significance Black hole An overview of black holes: their structure, formation channels, observational methods, theoretical properties like Hawking radiation, and their role in astrophysics and open quest…

Blazar: a jet-dominated active galactic nucleus Blazar A blazar is an active galactic nucleus with a relativistic jet aimed toward Earth, producing extreme broadband emission, rapid variability, and strong polarization across the elect…

Blueshift: the Doppler shift toward shorter wavelengths Blue shift Blueshift is the decrease in electromagnetic wavelength (or increase in frequency) produced when a source moves toward an observer. It appears in astronomy, radar, and other Dopple…

Bolometric magnitude Bolometric magnitude quantifies an object's total electromagnetic brightness across all wavelengths, extending the magnitude system to represent true radiative output for stars and…

Bond albedo: definition, measurement, and significance in planetary energy balance Bond albedo Bond albedo is the fraction of incident electromagnetic energy a body reflects in all directions and wavelengths. It determines how much radiant energy a planet or other body absor…

Boötes (constellation) Boötes Boötes is a prominent northern constellation traditionally portrayed as the herdsman. It contains the bright star Arcturus and the large, sparsely populated Boötes Void, and is bes…

Brian P. Schmidt — astrophysicist and Nobel laureate Brian Schmidt Brian P. Schmidt is an Australian-based astrophysicist who shared the 2011 Nobel Prize for evidence that the universe's expansion is accelerating. He is a senior researcher and pro…

Brown dwarf (substellar object) Brown dwarf A brown dwarf is a substellar object too small to sustain hydrogen fusion yet larger than planets. They bridge the gap between stars and giant planets and are studied via infrared…

Bya (unit) — “billion years ago” in geology and astronomy Bya (unit) Bya is an informal abbreviation meaning “billion years ago,” used to express times before the present in geology, paleontology and astronomy. Modern scientific usage prefers Ga (gi…

C-type asteroid Dark, carbon-rich asteroids that dominate the outer main belt; primitive bodies containing hydrated minerals, organics, and clues to early Solar System history.

Caelum (constellation) Caelum Caelum is a small, faint southern constellation introduced by Nicolas-Louis de Lacaille in the 18th century; its name means “the chisel” and it contains only dim stars visible to d…

Caesar's Comet (Comet C/-43 K1) Caesar's Comet Bright non‑periodic comet observed in 44 BC and widely read by Romans as a sign of Julius Caesar's deification; notable for exceptional brightness and a parabolic orbit.

Caldwell catalogue: a complementary list of bright deep‑sky objects for observers Caldwell catalogue A concise guide to the Caldwell catalogue (C1–C109): its purpose, composition, history, examples, and its role alongside the Messier and NGC/IC listings for amateur observing.

Caliban (moon) — irregular retrograde satellite of Uranus Caliban (moon) Caliban is an irregular, retrograde moon of Uranus discovered in 1997. It is a non-spherical satellite of roughly 72 km diameter and likely part of a small dynamical cluster with s…

Callirrhoe (Jupiter XVII) — irregular moon of Jupiter Callirrhoe (moon) Callirrhoe is a small, retrograde irregular moon of Jupiter (Jupiter XVII). Discovered in 1999, it orbits far from Jupiter and belongs to the Pasiphaë group of captured, non-spheri…

Callisto — outer Galilean moon of Jupiter Callisto (moon) Callisto is a heavily cratered Jovian satellite, nearly Mercury-sized, with a mixed ice–rock composition, a very old surface, a tenuous atmosphere, and possible subsurface liquid w…

Calypso — small Trojan moon of Saturn Calypso (moon) Calypso is a small, irregular moon of Saturn that shares Tethys's orbit as a trailing Trojan. It was found in 1980, imaged by spacecraft, and helps scientists study co-orbital dyna…

Camelopardalis — the faint northern constellation representing the giraffe Camelopardalis Camelopardalis is a large but faint northern constellation created in the early 17th century; it contains no Messier objects and is best seen from dark skies where its dim stars an…

Cancer (constellation) Cancer is a faint zodiac constellation whose name means “crab.” Noted for the Beehive Cluster (M44), it lies between Gemini and Leo and has a long history in astronomy and mytholog…

Canes Venatici — the Hunting Dogs constellation Canes Venatici Canes Venatici is a small, faint northern constellation introduced by Johannes Hevelius. It lies near Ursa Major and contains the star Cor Caroli and several notable galaxies such…

Canis Major: Southern Constellation Containing Sirius and VY Canis Majoris Canis Major Canis Major, a southern constellation anchored by Sirius—the night sky's brightest star—and home to red hypergiant VY Canis Majoris. Explore its notable stars, history, deep-sky ob…

Canis Major Dwarf Galaxy A debated stellar overdensity near Canis Major, proposed as a nearby dwarf galaxy or as a Milky Way disk/halo structure; its distance, stellar content and origin remain under study…

Canis Minor — the Lesser Dog constellation Canis Minor Canis Minor is a small northern constellation dominated by the bright star Procyon. It represents the 'lesser dog' that follows Orion and is notable for its two principal stars and…

Canopus — Alpha Carinae, the bright star of the southern sky Canopus Canopus (Alpha Carinae) is the second-brightest night star and a luminous evolved giant roughly 310 light-years away. This article summarizes its properties, measurement history, u…

Capricornus — the Sea‑Goat constellation Capricornus Capricornus is a zodiac constellation traditionally depicted as a sea‑goat. It is a faint ecliptic constellation with several notable stars and a long history in Babylonian, Greek…

Carina (constellation) Carina is a southern-hemisphere constellation representing a ship’s keel. It contains Canopus and the Carina Nebula and was once part of the larger Argo Navis before being divided…

Carina Nebula (NGC 3372): overview, structure, and scientific importance Carina nebula A bright southern H II emission nebula (NGC 3372) in the constellation Carina, home to Eta Carinae, multiple star clusters and active star formation; prominent in optical and infra…