Astronomy

Dorado (constellation) Dorado is a southern-hemisphere constellation best known for containing a large portion of the Large Magellanic Cloud, the Tarantula Nebula, and several important variable stars.

Double Cluster (NGC 869 and NGC 884) Double Cluster A pair of bright open star clusters in the constellation Perseus — NGC 869 and NGC 884 — visible to the naked eye and located roughly 7,500 light‑years from Earth within the Milky…

Draco (constellation) Draco is a large northern constellation shaped like a winding dragon. It is circumpolar for many observers, contains notable stars and deep-sky objects, and figures in classical my…

Drake Equation: estimating communicative extraterrestrial civilizations Drake equation A concise account of the Drake Equation: its terms, origin, uses, limitations, and role in debates about extraterrestrial intelligence and the search for technosignatures.

Dumbbell Nebula (Messier 27, NGC 6853) Dumbbell nebula A bright planetary nebula in Vulpecula, M27 (the Dumbbell Nebula) is a nearby example of a star's late-life envelope being ionized by its hot remnant, visible with small telescopes…

Dune universe The Dune universe is the fictional setting created by Frank Herbert for his Dune novels: a vast, layered future of politics, ecology, religion and technology centered on the desert…

Dwarf galaxy A dwarf galaxy is a small galaxy with millions to a few billion stars. Often satellites of larger systems, dwarf galaxies illuminate galaxy formation, dark matter, and the hierarch…

Dwarf planet (astronomical classification) Dwarf planet A dwarf planet is a Sun-orbiting object massive enough to be rounded by gravity but too small to clear its orbital zone; includes Ceres, Pluto and other bodies in the asteroid belt…

Dwarf star Stars significantly smaller or less luminous than giants — includes main-sequence low-mass stars, white dwarfs, brown dwarfs and related classes; common, varied, and astrophysicall…

Dysnomia — moon of the dwarf planet Eris Dysnomia (moon) Dysnomia is the only known moon of the dwarf planet Eris, discovered in 2005. Its size and composition remain uncertain; its orbit has been used to measure Eris's mass and study ou…

Dyson sphere (stellar megastructure) Dyson sphere A Dyson sphere is a hypothetical megastructure around a star intended to capture most or all of its energy output. The concept appears in scientific thought experiments, SETI, and…

Eagle Nebula (Messier 16, NGC 6611) Eagle nebula A young open star cluster and emission nebula in Serpens, famous for the Hubble image of the 'Pillars of Creation' and for active regions of star formation within its gas and dust.

Earth-crosser asteroid Asteroids whose orbits intersect Earth's path. Covers definitions, orbital types, origins, impact risk and monitoring, and notable examples and distinctions among near-Earth astero…

Earth's orbit: path, characteristics, seasons and long-term variation Earth's orbit Overview of Earth's path around the Sun, its shape and parameters, causes of seasons, long-term cycles that affect climate, and why the orbit matters for life and observation.

Eclipse (astronomy) Eclipse An eclipse occurs when one celestial body passes into the shadow of another, producing solar, lunar, and related events; important for observation, culture, and scientific measurem…

Ecliptic — the plane of Earth's orbit and the Sun's apparent path Ecliptic The ecliptic is the plane of Earth's orbit and the Sun's apparent annual path among the stars. It defines the zodiac, underlies the ecliptic coordinate system, and tilts relative t…

Eddington limit (Eddington luminosity) Eddington limit The Eddington limit is the maximum steady luminosity where outward radiation pressure balances inward gravity. It sets a key scale for stellar winds, accretion onto compact objects…

Edmond Halley — English astronomer and scientist Edmond Halley Edmond Halley (1656–1742/43) was an English astronomer, mathematician, and physicist best known for identifying the periodic nature of Halley's Comet and for contributions across a…

Edwin Hubble American astronomer whose observations established that many nebulae are galaxies beyond the Milky Way, provided evidence for an expanding universe, and introduced a galaxy classif…

Einstein's Cross (QSO 2237+0305): the fourfold image of a distant quasar Einstein's Cross Einstein's Cross is a well-known gravitational lens in which a single distant quasar appears as four images around a foreground galaxy, used to study lensing, quasar structure and…

Ejnar Hertzsprung (1873–1967): astronomer and co‑creator of the HR diagram Ejnar Hertzsprung Danish chemist and astronomer who helped develop the Hertzsprung–Russell diagram, a foundational tool for classifying stars and understanding stellar evolution.

Elara (moon) Elara is a small, irregular, prograde moon of Jupiter, a member of the Himalia group. Discovered in 1905, it has a non-spherical shape and an orbit strongly affected by solar and p…

Elliptical galaxy A smooth, ellipsoidal galaxy type with old stars, little gas, and low star formation. Common in cluster centers and thought to form mainly through mergers and accretion.

Elliptical orbit A closed orbital path in the shape of an ellipse followed by planets, moons, and many spacecraft; characterized by eccentricity between 0 and 1 and governed by Kepler's laws and Ne…

Elongation (astronomy): angle, observation, and significance Elongation Elongation is the apparent angular separation between the Sun and another celestial body as seen from Earth. It governs when planets are visible and reaches characteristic maxima f…

Emission nebula: glowing interstellar clouds ionized by nearby stars Emission nebula An emission nebula is an interstellar cloud of ionized gas that radiates light, often driven by ultraviolet radiation from nearby stars; types include H II regions and planetary ne…

Enceladus — Saturn’s bright, icy moon with active plumes Enceladus (moon) Enceladus is a small, highly reflective moon of Saturn known for its subsurface ocean, south-pole geysers that feed the E ring, and active geology revealed by spacecraft observatio…

Ephemeris A tabulated set of computed positions for celestial objects over time, used in astronomy, navigation and spaceflight to predict locations of planets, moons, comets and satellites.

Epimetheus — co-orbital moon of Saturn Epimetheus (moon) Epimetheus is a small, irregular, icy moon of Saturn that shares an orbital path with Janus. Discovered in 1966 and clarified by Voyager 1 in 1980, it performs a rare horseshoe orb…

Epoch (astronomy) A defined moment in time used as the reference for positions, coordinates, or orbital elements of celestial objects; essential for comparing, propagating, and cataloguing astronomi…

Epsilon Aurigae — a long-period eclipsing binary in Auriga Epsilon Aurigae Epsilon Aurigae (Almaaz) is a bright, long-period eclipsing binary whose companion is hidden in a large dusty disk; it has been studied for its unusual, multi-year eclipses.

Epsilon Eridani: nearby K-type star with planets and debris Epsilon Eridani Epsilon Eridani is a nearby K2 main-sequence star in Eridanus, visible to the naked eye. It hosts at least one confirmed planet, a complex debris system, and shows youthful magneti…

Epsilon Pegasi (Enif) — orange supergiant star in Pegasus Epsilon Pegasi Epsilon Pegasi (Enif) is a prominent orange K‑type supergiant in the constellation Pegasus. Overview, physical properties, name and cultural history, evolutionary status, and obser…

Equinox: Astronomical Definition, Seasonal Role, and Cultural Importance Equinox An equinox occurs when the Sun crosses the celestial equator, producing nearly equal day and night; it marks the start of spring and autumn and has astronomical, calendrical, and c…

Equuleus: the Little Horse constellation Equuleus Equuleus is a small, faint northern constellation traditionally depicted as a foal. It is one of the smallest constellations and has a long classical heritage in celestial cartogra…

Eridanus (constellation) Eridanus is a long, river-shaped constellation in the southern sky. It contains the bright star Achernar, nearby Epsilon Eridani, numerous galaxies and groups, and has roots in cla…

Erinome (Jupiter XXV): an irregular, retrograde moon of Jupiter Erinome (moon) Erinome is a small, non-spherical irregular satellite of Jupiter discovered in 2000. It belongs to the retrograde Carme group and orbits far from Jupiter with a period of about 712…

Eris — distant trans-Neptunian dwarf planet Eris (dwarf planet) Eris is a large trans-Neptunian dwarf planet in the scattered disc beyond Pluto. Discovered in 2005, it helped prompt the modern definition of dwarf planets and has one known moon,…

Erriapus (Saturnian moon) Erriapus (moon) Erriapus is a small irregular prograde moon of Saturn (~10 km) discovered in 2000. It orbits at about 17.3 million km with an ~871-day period and belongs to the Gallic group, likel…

ESTCube-1: Estonia's first satellite (1U CubeSat) ESTCube-1 ESTCube-1 was Estonia's first satellite, a 1U CubeSat developed by students to demonstrate a tether-based electric sail concept and provide hands-on training; launched 7 May 2013 o…

Eta Carinae — a massive luminous blue variable in the Carina Nebula Eta Carinae Eta Carinae is a highly luminous, massive binary system in the southern Carina Nebula, famous for its 19th‑century Great Eruption and the bipolar Homunculus Nebula it expelled.

Euanthe — a small irregular moon of Jupiter Euanthe (moon) Euanthe (Jupiter XXXIII) is a small, irregular, retrograde moon of Jupiter discovered in 2001. It belongs to the Ananke group and offers clues to capture and collisional processes…

Eukelade (Jupiter XLVII) Eukelade Eukelade is a small irregular, retrograde moon of Jupiter discovered in 2003. It belongs to the Carme group and follows a distant, inclined orbit; it is studied as part of Jupiter'…

Euporie (moon) — small retrograde satellite of Jupiter Euporie (moon) Euporie is a tiny, retrograde irregular moon of Jupiter discovered in 2001. This article summarizes its discovery, orbit, physical traits, name origin, and relationship to the Anan…

Europa — Jupiter's Icy Moon with a Hidden Ocean Europa (moon) Europa is a large, icy moon of Jupiter notable for a smooth surface, probable subsurface ocean kept warm by tidal heating, and strong interest for astrobiology and future explorati…

European Southern Observatory (ESO) European Southern Observatory Intergovernmental astronomical organisation founded in 1962 that builds and operates major observatories in the southern hemisphere, including the VLT and the ELT, and supports div…

Eurydome (Jupiter moon) Eurydome (moon) Eurydome (Jupiter XXXII, S/2001 J 4) is a small, retrograde irregular moon of Jupiter discovered in 2001; it belongs to the Pasiphaë group and orbits at ~23.2 million km.

Exosphere (outermost layer of a planetary atmosphere) Exosphere The exosphere is the outermost, extremely rarified layer of a planet's atmosphere where particles move ballistically and may escape into space; composition and extent vary with sol…