Astronomy

Serpens: the Serpent Constellation Serpens Serpens is a unique northern constellation split into two parts by Ophiuchus; it represents a serpent and contains bright stars, clusters and nebulae visible from mid-northern lati…

Setebos (Uranian moon) Setebos (moon) Setebos is a distant, retrograde irregular moon of Uranus discovered in 1999. It is small and non-spherical, possibly related dynamically to other outer moons but differing in surf…

Sextans (constellation) Sextans Sextans is a faint equatorial constellation introduced in the 17th century and named for the sextant instrument. It lies south of Leo and contains several dwarf galaxies and the le…

Seyfert galaxy — nearby active galactic nuclei Seyfert galaxy A Seyfert galaxy is a nearby active galaxy with a bright, compact nucleus powered by a supermassive black hole. It shows strong emission lines, multiwavelength variability, and cle…

SGR 1806-20: a magnetar in Sagittarius and the 2004 giant flare SGR 1806-20 SGR 1806-20 is a highly magnetized neutron star (magnetar) in Sagittarius, notable for a December 2004 giant gamma-ray flare that became the brightest extrasolar burst recorded and…

Shangri-La (Titan) Shangri‑La is a broad dark region on Saturn’s moon Titan, notable for extensive dune fields of organic material, bright highland 'islands', and proximity to the Huygens landing sit…

Shani (Saturn): Hindu Deity, Astrological Influence, and Cultural Role Shani Shani, the Hindu deity identified with the planet Saturn, is a prominent graha in Jyotiṣa associated with karma, discipline, and Saturday; worship and astrological beliefs surround…

Shape of the Universe: curvature, topology, and observational status Shape of the universe An accessible account of what scientists mean by the 'shape' of the universe — local curvature, global topology, observational tests, leading models, and why the question matters.

Shapley 1 (Sp 1, PLN 329+2.1) — a planetary nebula in Norma Shapley 1 Shapley 1 is a faint planetary nebula in the southern constellation Norma, discovered by Harlow Shapley in 1936. It lies roughly 1,000 light‑years away and exemplifies late stages…

Shapley Supercluster (Shapley Concentration) Shapley Supercluster A very large, nearby overdensity of galaxy clusters in the direction of Centaurus, about 650 million light-years away (z≈0.046), notable for its mass and influence on local cosmic…

Shinsei — early Japanese scientific satellite (launched 1971) Shinsei Shinsei (しんせい), often rendered “New Star”, was Japan's third artificial satellite. Launched on 28 September 1971, it was developed by the Institute of Space and Astronautical Scien…

Shukrayaan-1 — Indian proposed Venus orbiter Shukrayaan-1 Shukrayaan-1 is a proposed Indian Space Research Organisation orbiter to study Venus's atmosphere and surface. The mission has been under study with international cooperation but h…

Siarnaq: Saturn's prograde irregular moon and largest member of the Inuit group Siarnaq (moon) Siarnaq (Saturn XXIX) is a small, non-spherical prograde irregular moon of Saturn, discovered in 2000. It is the largest member of the Inuit group and is notable for its reddish co…

Sidereal time — astronomical timekeeping and practical uses Sidereal time Sidereal time is a time scale tied to Earth's rotation relative to the fixed stars. Astronomers use it to locate celestial objects and to schedule observations because it aligns ri…

Siding Spring Observatory Major optical observatory near Coonabarabran, New South Wales, run by the Australian National University; notable southern-hemisphere telescopes, surveys, and public programs.

Sinope (moon of Jupiter) Sinope (moon) Sinope is an irregular, retrograde satellite of Jupiter discovered in 1914. Likely a captured object, it orbits far from Jupiter and is often associated with the Pasiphae group; it…

Sirius (Dog Star): Brightest star in the night sky Sirius Sirius, the Dog Star (Alpha Canis Majoris), is a nearby binary system whose bright primary and white-dwarf companion have long astronomical and cultural significance.

Skathi (Saturn XXVII): an irregular, retrograde moon of Saturn Skathi (moon) Skathi (Saturn XXVII) is a small, irregular retrograde moon of Saturn. Discovered in 2000, it orbits at ~15.6 million km with a high inclination and eccentricity and likely formed…

Skoll (Saturn XLVII) — irregular moon of Saturn Skoll (moon) Skoll (Saturn XLVII, S/2006 S 8) is a small non-spherical irregular moon of Saturn, roughly 6 km across. Discovered in 2006, it follows a distant, eccentric, highly inclined orbit…

Sky: appearance, causes of color, visible features, and planetary differences Sky The sky is the atmosphere seen from the ground; its colors and contents vary with scattering, particles, and light. This article explains why the sky looks as it does and what we c…

Skylab: America's First Space Station Skylab Skylab was the United States' first space station (1973–1979), hosting three crewed missions that advanced solar research, life sciences, and long-duration spaceflight techniques.

Small Dark Spot (Neptune) Small Dark Spot A prominent dark vortex observed on Neptune in 1989 by Voyager 2; a transient atmospheric storm that developed companion clouds and faded by the mid-1990s.

Small Solar System Body Small Solar System Bodies are objects orbiting the Sun that are neither planets nor dwarf planets; they include most asteroids, comets, many trans-Neptunian objects, Centaurs and o…

SN 1987A — Nearby core‑collapse supernova in the Large Magellanic Cloud SN 1987A SN 1987A, discovered in February 1987 in the Large Magellanic Cloud, is the nearest modern supernova. Its neutrino detections, unusual progenitor, and evolving ring nebula transfor…

SN 1994D: a well-observed Type Ia supernova in NGC 4526 SN 1994D Overview of SN 1994D, a Type Ia supernova discovered near the galaxy NGC 4526 in 1994; its observational record, physical characteristics, host environment, and scientific importan…

Solar calendar A calendar system that measures the year by the Earth's orbit around the Sun. Includes tropical and sidereal variants and underlies most civil calendars (e.g., Gregorian, Julian, I…

Solar eclipse: causes, types, observations, history and safety Solar eclipse An explanation of solar eclipses: how they occur, the different types, observable effects, cultural and scientific significance, prediction cycles and basic viewing safety.

Solar eclipses on Mars Solar eclipses on Mars are caused by its tiny moons Phobos and Deimos. They produce brief transits and small shadows rather than the dramatic total eclipses seen on Earth, and have…

Solar flare A solar flare is a sudden, intense release of magnetic energy in the Sun's atmosphere that produces broad-spectrum radiation and can affect space and Earth technologies.

Solar mass — unit of mass used in astronomy Solar mass The solar mass (M☉) is the standard mass unit in astronomy equal to the mass of the Sun (≈1.99×10^30 kg). It is used to express masses of stars, planets, black holes and galaxies.

Solar radius (R☉) Solar radius The solar radius is the characteristic radius of the Sun used as a unit in astronomy. It equals about 695,700 km (432,300 mi) and serves to compare sizes of stars and planets.

Solar System: Structure, origin, major bodies and notable facts Solar System Overview of the Sun and the bodies that orbit it, the system's structure and formation, major planets and moons, belts and minor bodies, historical context and key distinguishing f…

Solar telescope: instruments for observing the Sun Solar telescope A solar telescope is built to observe the Sun safely and in selected wavelengths. This article covers design, filters, ground and space examples, scientific uses, history, and safe…

Solar time Measurement of time based on the Sun's apparent motion: solar day, apparent vs mean solar time, equation of time, sundials, and relation to civil and sidereal time.

Solar wind The solar wind is a continuous flow of charged particles and magnetic field from the Sun that shapes the heliosphere, drives space weather, and interacts with planets, comets and s…

Solar Wind Composition Experiment (Apollo lunar foil collectors) Solar Wind Composition Experiment Apollo-era experiment that exposed foil collectors on the lunar surface to capture solar wind ions and return them to Earth for isotopic analysis of solar gases.

Solstice — astronomical cause, seasonal effects and cultural significance Solstice A solstice is either of two yearly moments when the Sun reaches its extreme declination, producing the longest or shortest day and marking important seasonal and cultural transitio…

South Pole–Aitken basin A vast impact basin on the Moon's far side, the South Pole–Aitken basin is one of the largest and oldest craters in the Solar System and a key target for lunar geology and explorat…

Soyuz (spacecraft) Soyuz is a long-running series of Soviet and Russian crewed spacecraft noted for its three-part design, role ferrying crews to space stations, and status as one of the world’s most…

Space colonization: human settlement beyond Earth Space colonization An overview of space colonization: concepts, locations, technical needs, history, challenges, uses and ethical issues involved in establishing permanent human communities beyond Ea…

Space debris: causes, risks, detection, and mitigation Space debris Objects left in orbit—defunct satellites, spent rockets and fragments—that pose collision risks to spacecraft. Overview of sources, tracking, hazards, removal methods and policy ch…

Space exploration: history, methods, and contemporary goals Space exploration Survey of space exploration: aims, technology, key milestones like Sputnik, Gagarin and Apollo, robotic probes, benefits, and future directions including international and commerci…

Space Observatory: Instruments and Science Beyond Earth’s Atmosphere Space observatory A space observatory is an astronomical instrument located beyond Earth’s atmosphere used to observe celestial objects and wavelengths that are inaccessible or degraded from the gro…

Space Oddity (1969): David Bowie's breakthrough song about astronaut Major Tom Space Oddity Explore David Bowie's 1969 breakthrough Space Oddity, the atmospheric pop single introducing fictional astronaut Major Tom, its Apollo 11-era release, reception, and enduring influ…

Space opera (science fiction subgenre) Space opera Space opera: a dramatic, large-scale subgenre of science fiction featuring interstellar settings, heroic conflicts, exotic technology, and sweeping narratives from early pulps to m…

Space probe An unmanned spacecraft sent beyond Earth to study planets, moons, asteroids and interplanetary space; mission types include flybys, orbiters, landers and rovers that return scienti…

Space Race Mid-20th-century rivalry between the United States and the Soviet Union to achieve superior spaceflight capability, producing first satellites, human orbital flights, Moon landings…

Space station A crewed artificial satellite in orbit that supports long-term human habitation and scientific research; overview of design, history, operations, notable examples and future direct…