Astronomy

Carl Sagan: Astronomer, Science Communicator, and Advocate for Exploration Carl Sagan Overview of Carl Sagan’s life and work: planetary science, public communication (Cosmos), SETI advocacy, books such as Contact and Pale Blue Dot, founding of the Planetary Society,…

Carme — irregular retrograde moon of Jupiter Carme (moon) Carme is a small, non-spherical irregular moon of Jupiter discovered in 1938. It gives its name to the Carme group of retrograde satellites and is thought to be a fragment of a cap…

Carme group — irregular retrograde moons of Jupiter Carme group A concise overview of the Carme group: orbital properties, likely origin, physical traits, principal members and naming conventions for this family of Jupiter’s irregular moons.

Carpo (Jupiter XLVI) — small prograde irregular moon of Jupiter Carpo (moon) Carpo is a small (~3 km) prograde irregular moon of Jupiter discovered in 2003. It follows a high-inclination, eccentric orbit, is dynamically isolated from major groups, and was n…

Cartwheel Galaxy (ESO 350-40) Cartwheel galaxy A striking ring galaxy in Sculptor, created by a galactic collision; notable for its luminous ring, radial spokes and active star formation about 500 million light-years away.

Cassini–Huygens: NASA/ESA Mission to Saturn and Titan Cassini–Huygens Cassini–Huygens was a multinational robotic mission (1997–2017) that orbited Saturn, studied its rings and moons, and delivered the Huygens probe to Titan's surface.

Cassiopeia Cassiopeia: mythological queen and a prominent northern constellation, noted for its distinctive W shape, bright stars (Schedar, Caph, Gamma Cassiopeiae), deep-sky objects and role…

Cassiopeia (constellation) Cassiopeia is a prominent northern constellation shaped like a W, home to bright stars (Schedar, Caph), several star clusters and nebulae, and notable historical supernovae includi…

Cave Nebula (Sh2-155) — emission nebula in Cepheus Cave nebula The Cave Nebula (Sh2-155) is an emission, H II region in Cepheus. At about 2,400 light-years away it shows a dust-carved cavity lit by young stars and is popular with astrophotogra…

Celestia — 3D real-time space simulation software Celestia Celestia is a free, open-source 3D astronomy program that lets users travel through a realistic virtual universe, view stars and planets in three dimensions, and create scripted to…

Celestial equator: definition, geometry, and astronomical role Celestial equator The celestial equator is the projection of Earth's equator onto the celestial sphere. This article explains its geometry, observational appearance, role in coordinates, and importa…

Celestial navigation: locating position by the Sun, Moon, stars and planets Celestial navigation Celestial navigation is the practice of finding position and direction at sea or on land by observing celestial bodies with instruments such as the sextant, chronometer and almanac…

Celestial sphere — concept and role in astronomy and navigation Celestial sphere An explanatory overview of the celestial sphere: an abstract, concentric coordinate surface used to locate objects on the sky, its parts, history, applications, and limitations.

Centaur (planetoid) A Centaur is a small Solar System body with an orbit typically between Jupiter and Neptune, often showing characteristics of both asteroids and comets and originating from the oute…

Centaurus: Southern Constellation with Nearby Stars and Deep-Sky Objects Centaurus Centaurus is a large southern constellation noted for bright stars including Alpha Centauri and rich deep-sky objects such as Omega Centauri, Centaurus A and the Shapley Superclust…

Centaurus A (NGC 5128) — a nearby active radio galaxy Centaurus A Centaurus A (NGC 5128) is a nearby active galaxy with a prominent dust lane, a supermassive black hole and enormous radio jets; it is a key object for studying galactic mergers and…

Cepheid variable star Cepheid variable Cepheid variables are luminous pulsating stars with a well-defined period–luminosity relation, used as standard candles to measure distances across the Milky Way and to nearby gala…

Cepheus (constellation) Cepheus is a northern constellation, one of the 48 classical constellations. It contains notable stars such as Alderamin, Delta Cephei and Mu Cephei and several nebulae and cluster…

Ceres: the dwarf planet of the main asteroid belt Ceres (dwarf planet) Ceres is the largest object in the main asteroid belt and the Solar System's smallest recognized dwarf planet; an ice‑rich, nearly spherical world explored by NASA's Dawn mission.

Cetus (constellation) Cetus Cetus is a large northern constellation traditionally depicted as a sea monster. It contains the famous variable star Mira, bright stars such as Deneb Kaitos, and deep-sky objects…

CfA2 Great Wall — large galaxy filament discovered in the CfA redshift survey CfA2 Great Wall Overview of the CfA2 Great Wall: its discovery, approximate dimensions and composition, role in the cosmic web, observational limitations, and its significance in studies of large-…

Chaldene — an irregular moon of Jupiter Chaldene (moon) Overview of Chaldene (Jupiter XXI), an irregular, retrograde Jovian satellite: discovery, orbit, physical traits, group association, and observational notes.

Chamaeleon (constellation) Chamaeleon Chamaeleon is a faint southern constellation named for the lizard, notable for its dark clouds and star-forming regions.

Chandra Deep Field South Deep Chandra X‑ray survey of a low‑absorption patch in Fornax. The CDF‑S reveals faint active galaxies and obscured black holes and anchors multiwavelength studies of the distant u…

Chandrasekhar limit The Chandrasekhar limit is the approximate maximum mass (~1.4 solar masses) a white dwarf can have while supported by electron degeneracy pressure. It determines white dwarf stabil…

Chandrayaan-1: India's first mission to the Moon Chandrayaan-1 Chandrayaan-1 was India's inaugural lunar orbiter, launched by ISRO in October 2008. It mapped most of the lunar surface, carried international instruments and provided key evidenc…

Chandrayaan-2 — India's second lunar mission by ISRO Chandrayaan-2 Chandrayaan-2 was India’s 2019 lunar mission featuring an orbiter, lander and rover to study the Moon’s south polar region; the orbiter remains active while the lander failed durin…

Charles Messier 18th‑century French astronomer who compiled the Messier Catalog of bright deep‑sky objects to help comet hunters distinguish fixed nebulae and clusters from moving comets.

Charon — moon of Pluto Charon (moon) Charon is the largest moon of the dwarf planet Pluto. Roughly half Pluto's diameter, it forms a binary system with Pluto and was explored in detail by NASA's New Horizons in 2015.

Chi Cygni — a Mira-type pulsating red giant in Cygnus Chi Cygni Chi Cygni is a Mira-class pulsating red giant in Cygnus, notable for very large visual brightness changes and an extended atmosphere hundreds of times larger than the Sun.

Chiron (hypothetical moon of Saturn) Chiron (hypothetical moon) A reported but unconfirmed moon of Saturn first claimed in 1861 by Hermann Goldschmidt; later observations failed to reproduce the sighting and it is regarded as nonexistent.

Chriet Titulaer — Dutch astronomer and popularizer of science and technology Chriet Titulaer Chriet Titulaer (1943–2017) was a Dutch astronomer, TV presenter and writer who popularized science and technology in the Netherlands through television series, books and a product…

Circinus (constellation) Circinus Circinus is a small southern constellation named for the drafting compass. It contains faint stars, several variable stars, and the nearby active Circinus Galaxy. Best seen from so…

Circinus galaxy — a nearby Seyfert galaxy behind the Milky Way Circinus galaxy The Circinus galaxy is a nearby Type 2 Seyfert galaxy about 13 million light‑years away, lying close to the plane of the Milky Way. It hosts an obscured active nucleus, circumnucle…

Circular orbit A circular orbit is the special case of an orbital path with zero eccentricity: an object travels at constant distance and speed around a central mass. Physics, stability, and prac…

Cloverleaf quasar (H1413+117) — a four-image gravitationally lensed quasar Cloverleaf quasar The Cloverleaf quasar (H1413+117) is a high-redshift (z ≈ 2.56) quasar in the constellation Boötes whose light is split into four similar images by gravitational lensing and studie…

Coalsack Nebula A large, conspicuous dark nebula visible from the Southern Hemisphere that appears as a dark patch against the Milky Way; noted for its cultural importance and proximity to Earth.

Colonization of Mars: prospects, challenges, and approaches Colonization of Mars A balanced overview of human colonization of Mars: why Mars is considered, what technical and biological challenges exist, historical proposals, likely infrastructure and ethical/l…

Columba (constellation) Columba is a small southern constellation representing a dove. Introduced in the late 16th century, it contains the bright star Alpha Columbae (Phact) and several faint star cluste…

Coma Berenices Coma Berenices is a small northern constellation whose name means ‘Berenice’s Hair’. It contains a nearby open cluster, a rich galaxy cluster and many notable deep-sky objects.

Comet Comets are small icy bodies that orbit the Sun, developing glowing comas and tails when heated. They originate in distant reservoirs and reveal information about the early solar sy…

Comet Borrelly (19P/Borrelly) Comet Borrelly 19P/Borrelly is a short-period, Jupiter-family comet discovered in 1904 and imaged up close by NASA’s Deep Space 1 in 2001; it has a dark, elongated nucleus with localized jets pro…

Comet Hale–Bopp (C/1995 O1) — the Great Comet of 1997 Comet Hale–Bopp Comet Hale–Bopp was a spectacular long‑period comet discovered in 1995 that became one of the brightest and most widely observed comets of the 20th century in 1997.

Comet Shoemaker–Levy 9 and the 1994 Impacts on Jupiter Comet Shoemaker–Levy 9 Comet Shoemaker–Levy 9, discovered in 1993, fragmented and struck Jupiter in July 1994. The event was the first directly observed collision between Solar System bodies and advanced…

Comet Swift–Tuttle (109P/Swift–Tuttle) — parent of the Perseid meteor shower Comet Swift–Tuttle Periodic Halley-type comet with a 133-year orbit and ~26 km nucleus, discovered in 1862; recorded in antiquity, linked to the Perseid meteor shower, and periodically returns to pro…

Comet West (1975) — discovery, appearance and legacy Comet West Comet West was a bright long‑period comet discovered in 1975 by Richard M. West. It became visible in daylight, fragmented near perihelion, and has an estimated orbital period of r…

Comoving distance in cosmology Comoving distance Comoving distance is the coordinate separation between objects in an expanding universe; it is preserved for objects following the Hubble flow and is used to compare positions, vol…

Compact star Compact stars are dense stellar remnants—white dwarfs, neutron stars, exotic dense stars, and black holes—formed at the end of stellar evolution and observed through radiation, pul…