Astronomy

Spacesuit: design, function and types of protective garments for spaceflight Space suit A spacesuit is a sealed, wearable life‑support system that lets humans survive and work outside the atmosphere. This article summarizes components, operating principles, types, ope…

Spacecraft: vehicles for travel and operations beyond Earth's atmosphere Spacecraft Overview of spacecraft: definition, main types, structure, propulsion, launch and operation, history, uses and distinctions between crewed and uncrewed vehicles.

Spaceship Earth — the global stewardship metaphor Spaceship Earth A neutral overview of the Spaceship Earth concept: its meaning, origins, applications in environmentalism and systems thinking, cultural echoes, and related topics.

Spacewatch — University of Arizona survey of minor planets and near‑Earth objects Spacewatch Spacewatch is a long-running astronomical survey based at the University of Arizona that discovers, tracks, and characterizes asteroids, comets and other small bodies in the Solar…

Spheroid: definition, types, geometry and natural examples Spheroid A spheroid is an ellipsoid of revolution formed by rotating an ellipse. This article explains its geometry, oblate and prolate forms, mathematical properties, natural occurrences a…

Spica (Alpha Virginis): a bright blue star of the constellation Virgo Spica Spica (Alpha Virginis) is the brightest star in Virgo, a blue-white B-type star and close binary system long important for navigation, stellar astrophysics, and cultural astronomy.

Spiral galaxy: structure, types, formation and significance Spiral galaxy A spiral galaxy is a rotating, disk-shaped galaxy with a central bulge and winding arms. This article explains its structure, main types, formation ideas, role in astronomy and how…

Spitzer Space Telescope — NASA infrared observatory and its legacy Spitzer Space Telescope NASA's Spitzer Space Telescope (launched 2003) was a cooled infrared observatory that revealed dusty star formation, measured exoplanet thermal emission, mapped cool objects, and l…

Sponde (Jupiter moon) Sponde (moon) Sponde (Jupiter XXXVI) is a small irregular, retrograde moon discovered in 2001. About 2 km across, it orbits Jupiter at ~24.25 million km in a highly inclined, eccentric orbit and…

Sputnik 2: the Soviet satellite that carried Laika into orbit Sputnik 2 Sputnik 2 was the second Soviet satellite and the first to carry a living animal, the dog Laika. Launched in 1957, it advanced space biology, provoked ethical debate, and influence…

Standard asteroid physical characteristics and how they are measured Standard asteroid physical characteristics Overview of the common physical properties used to describe asteroids, methods of measurement, and why these characteristics matter for science, missions, and hazard assessment.

Star: Structure, Life Cycle, Types, and Cosmic Importance Star A clear, concise encyclopedia overview of stars: their makeup, how nuclear fusion powers them, their life cycle and types, and their role in producing light and chemical elements.

Star chart A star chart is a map of the night sky showing stars, constellations and celestial objects; used across history for navigation, ritual, timekeeping and today for astronomy, educati…

Star cluster: types, formation, dynamics and astronomical importance Star cluster Overview of star clusters, their main types (open and globular), formation, evolution, observational methods, notable examples and their role in stellar and galactic studies.

Star formation: how stars are born in molecular clouds Star formation An accessible overview of star formation: the physical stages, triggers, timescales, observation methods, and why the process matters for planets and galaxies.

Star system (stellar system) Star system A star system (stellar system) is a small number of stars bound by gravity that orbit a common center. This article describes types, dynamics, formation, notable examples and how t…

Starburst galaxy A galaxy undergoing a short, intense episode of star formation that exhausts its gas rapidly; often triggered by interactions and important for understanding galaxy evolution.

Starfish (Sea Stars): biology, ecology, and significance Starfish Starfish, or sea stars (class Asteroidea), are marine echinoderms. This article explains their anatomy, water vascular system, feeding, reproduction, ecological roles and conservat…

Starfleet: The United Federation of Planets' space service in Star Trek Starfleet Starfleet is the exploratory, scientific, diplomatic and defensive organization of the United Federation of Planets in the Star Trek fictional universe, operating starships, starba…

Stella Stella — Latin for "star" — is a feminine given name and a versatile label across culture, commerce and technology, used for people, brands, software, vehicles, instruments, and wo…

Stellar classification: organizing stars by spectrum and temperature Stellar classification System that groups stars by spectral characteristics and temperature. Describes spectral types (O–M), numeric subclasses, luminosity classes, history (Harvard, MK), uses, and notab…

Stellar evolution: the life cycles of stars Stellar evolution Stellar evolution describes how stars form, change, and end their lives. It explains physical processes, stages from nebula to remnant, timescales that depend on mass, and why star…

Stellar precession (precession of the equinoxes) Stellar precession Slow, long-term change in the orientation of Earth's rotation axis that makes the apparent positions of the stars drift over millennia and shifts the identity of the pole star.

Stellar wind: outflows from stars and their effects Stellar wind Overview of stellar winds: mechanisms that drive mass outflows from stars, types across stellar classes, observational methods, and their effects on stellar evolution, planetary en…

Stephano (moon) Stephano is a small, retrograde irregular moon of Uranus discovered in 1999. Named after a character in Shakespeare's The Tempest, it is non-spherical and likely a captured object…

Stickney Crater (Phobos) Stickney (crater) Stickney is the largest crater on Phobos, Mars's inner moon. This prominent impact basin dominates Phobos's surface, influences its geology, and is a focal point for studies of sma…

Styx (moon) — small satellite of the dwarf planet Pluto Styx (moon) Styx is a small irregular moon of the dwarf planet Pluto, discovered in 2012 with the Hubble Space Telescope. It orbits within Pluto’s system of small satellites and was named afte…

Subaru Telescope An 8.2-m optical–infrared observatory operated by the National Astronomical Observatory of Japan on Mauna Kea, noted for wide-field imaging, adaptive optics, and surveys of galaxie…

Subsolar point: where the Sun is directly overhead Subsolar point The subsolar point is the location on a planet where the Sun appears at the zenith. This article explains its motion, connection to axial tilt and declination, climate effects, and…

Suisei — Japan's first interplanetary spacecraft (Halley's Comet probe) Suisei Suisei was Japan's first interplanetary probe, launched in 1985 by ISAS to observe Halley's Comet in 1986. It carried ultraviolet and plasma instruments to study the comet's coma a…

Summer solstice The summer solstice is the annual moment when the Sun reaches its greatest declination toward a hemisphere, producing the longest day and inspiring scientific study and cultural ce…

Sun (Sol): The star at the center of the Solar System Sun Overview of the Sun: its structure, energy source, activity (sunspots, flares, solar wind), role for Earth, life cycle and key facts. Includes further reading links.

Sunrise Sunrise is the instant the Sun's upper limb becomes visible above the horizon, signaling the transition from night to morning. Timing is affected by Earth's rotation, axial tilt, l…

Sunset: definition, causes, phases, and cultural significance Sunset Sunset is the daily event when the Sun disappears below the western horizon. This article explains its geometry, the atmospheric causes of color, twilight phases, practical uses, a…

Sunspot A sunspot is a relatively cool, magnetically active region on the Sun's visible surface. Sunspots follow an approximately 11-year cycle and influence solar activity and space weath…

Super star cluster A compact, massive aggregate of very young, luminous stars that forms in intense starbursts and may evolve into long-lived cluster systems like globular clusters.

Super-Earth (exoplanet class) Super-Earth A super-Earth is an extrasolar planet with mass larger than Earth's but substantially less than ice giants; the label denotes mass only and does not imply composition or habitabili…

Supercluster (large-scale cosmic structure) Supercluster A supercluster is an enormous assembly of galaxy groups and clusters that forms one of the largest known patterns of matter, arranged into filaments, walls and surrounding voids ac…

Supergiant (stellar class) Supergiant Supergiant stars are extremely luminous, massive stars (luminosity class I). This entry explains their properties, classification, life cycles, roles in nucleosynthesis and superno…

Supermassive black hole Extremely massive black holes found in galactic centers, with masses millions to billions of times the Sun; central to active galactic nuclei, galaxy evolution and high-energy astr…

Supernova: stellar explosions, types, and cosmic significance Supernova A supernova is a catastrophic explosion that ends the life of certain stars. This article explains causes, types, observable features, historical examples, and their role in the co…

Surface gravity Surface gravity is the gravitational acceleration felt at or near the visible surface of a planet, star, or other object. It determines weight, atmospheric retention, and appears i…

Surtur (Saturn XLVIII) — moon S/2006 S 7 Surtur (moon) Surtur (Saturn XLVIII, S/2006 S 7) is a small, irregular and likely captured retrograde moon of Saturn, discovered in 2006 and named after the fire giant Surtr of Norse mythology.

Suttungr — a small irregular moon of Saturn Suttungr (moon) Suttungr (Saturn XXIII) is a small, retrograde irregular moon discovered in 2000. It orbits far from Saturn, is likely a fragment from Phoebe, and belongs to the Norse group of sat…

Suzaku (ASTRO-EII) — Japanese X‑ray astronomy satellite Suzaku (satellite) Suzaku (ASTRO‑EII) was a Japanese X‑ray astronomy satellite launched on July 10, 2005 by ISAS. It carried X‑ray telescopes and detectors to study high‑energy astrophysical phenomen…

Sycorax (Uranian moon) Sycorax (moon) Sycorax is the largest retrograde irregular satellite of Uranus, discovered in 1997. This article covers its discovery, orbit and physical traits, origin theories, and place among…

Synchronous orbit An orbit whose period matches the rotation period of the central body. Commonly used for communications and weather satellites; geostationary orbits are a special, equatorial case.

Tadpole Galaxy (UGC 10214, Arp 188) Tadpole galaxy A disturbed barred spiral galaxy in Draco, notable for a long tidal tail and active star formation resulting from a past galactic encounter; imaged in detail by the Hubble Space Te…