Astronomy

Right ascension — celestial coordinate for locating objects Right ascension Right ascension (RA) is the angular coordinate used along the celestial equator to locate objects on the sky, measured eastward from the vernal equinox and usually expressed in hou…

Ring galaxies: structure, formation, and notable examples Ring galaxy Overview of ring galaxies, their defining features, formation theories, observation methods, and well-known examples such as Hoag's Object, the Cartwheel Galaxy, and NGC 1291.

Rings of Jupiter: composition, structure, discovery and significance Rings of Jupiter A concise encyclopedia overview of Jupiter’s faint ring system: its components, origin, discovery by Voyager 1, later study, and how it differs from the prominent rings of other pl…

Rings of Neptune Neptune's faint ring system is composed of several narrow rings and distinctive clumps called ring arcs. Discovered and studied by ground observations and Voyager 2, the rings reve…

Rings of Rhea: the proposed ring system around Saturn’s moon Rings of Rhea A summary of the 2008 proposal that Saturn’s moon Rhea hosts narrow rings: the observational basis, proposed properties, later searches, possible origins, and why the idea matters…

Rings of Saturn Saturn's rings are a vast, thin system of ice-rich particles and gaps that encircle the planet; they reveal disk dynamics, shepherding moons, and an active, still-debated history.

Rings of Uranus: structure, discovery, origin and significance Rings of Uranus An overview of Uranus’s narrow, dark rings: discovery by stellar occultation and Voyager 2, later Hubble detections, composition and particle sizes, formation theories, dynamics, a…

Roche limit The Roche limit is the minimum distance at which a body held together by self-gravity can orbit a larger primary without being torn apart by tidal forces.

Roman calendar The system of months and dating used in ancient Rome, evolving from an early lunar scheme to the solar Julian calendar and influencing modern Western calendars.

Rosalind (moon) — a small inner satellite of Uranus Rosalind (moon) Rosalind is a small inner moon of Uranus (Uranus XIII), discovered in Voyager 2 images in 1986. It is a member of the Portia group and remains poorly characterized beyond size, alb…

Rosetta (spacecraft): ESA mission to comet 67P/Churyumov–Gerasimenko Rosetta (spacecraft) ESA's Rosetta rendezvoused with comet 67P, deployed the Philae lander, studied the comet's composition and activity through perihelion, and ended with a controlled landing in 2016.

Ross 154 — Nearby Flare M Dwarf in Sagittarius Ross 154 Ross 154 (V1216 Sgr) is a nearby red dwarf and UV Ceti–type flare star in Sagittarius, about 9.69 light‑years away. It is X‑ray active, young, low‑mass, and one of the closest star…

Ross 248 (HH Andromedae): a nearby red-dwarf flare star Ross 248 Ross 248, also HH Andromedae and Gliese 905, is a nearby red dwarf and UV Ceti–type flare star in Andromeda, about 10.3 light-years away and too faint to see without a telescope.

Rotation period (astronomy) Rotation period The rotation period is the time an astronomical body takes to spin once about its axis. This article explains definitions, measurement methods, examples (Earth, stars, asteroids),…

Rover (space exploration) A rover is a mobile vehicle designed to travel across the surface of another world. Articles covers purpose, design features, history, examples and how rovers are powered and opera…

RR Lyrae — prototype of RR Lyrae variable stars RR Lyrae RR Lyrae is the eponymous short-period pulsating variable in Lyra. It typifies old, low-mass horizontal-branch stars used as standard candles in studies of the Milky Way and globul…

RS Persei: a red supergiant variable in the Double Cluster RS Persei RS Persei is a luminous red supergiant and variable star located in the Double Cluster (h and χ Persei). It is notable for its cool temperature, large radius, semiregular variabili…

RSGC1 — Massive Young Red Supergiant Cluster in Scutum RSGC1 RSGC1 is a heavily obscured, massive young open cluster in the Milky Way that contains numerous red supergiants and a yellow hypergiant. It lies in Scutum about 6.6 kpc from the Su…

RW Cephei — an orange hypergiant in the constellation Cepheus RW Cephei RW Cephei is a luminous orange hypergiant star in Cepheus. Extremely large and unstable, it illustrates late-stage evolution of massive stars through intense mass loss, dust format…

S Doradus S Doradus is a luminous blue hypergiant in the Large Magellanic Cloud and the prototype of the S Doradus (LBV) class, noted for dramatic brightness, temperature and size changes du…

S Orionis: a Mira variable red giant in the constellation Orion S Orionis S Orionis is a pulsating Mira-type red giant star in Orion. It undergoes large brightness and size changes as an evolved asymptotic giant branch star and reaches a radius several h…

S Persei — a red supergiant near the Double Cluster in Perseus S Persei S Persei is a luminous red supergiant in Perseus, located near the Double Cluster. It is a variable star with a dense circumstellar envelope and is an important target for maser an…

S-type asteroid Stony (silicaceous) asteroids: composition, spectral traits, distribution in the asteroid belt, notable examples and scientific importance.

Sagittarius Dwarf Elliptical Galaxy (Sag DEG) Sag DEG A small, loop-shaped satellite of the Milky Way, Sag DEG is a tidally disrupted dwarf spheroidal galaxy notable for associated globular clusters and the stellar stream it supplies…

Sagitta (constellation) Sagitta Sagitta is a small northern constellation whose Latin name means “arrow.” Long recognized since antiquity, it forms a compact arrow-shaped pattern between Cygnus and Aquila and is…

Sagittarius (astrology) Sagittarius — the Archer — is a fire sign of the zodiac associated with adventure, freedom, and philosophy. Common sun‑sign dates and traits, mythology, uses in horoscopes, and dis…

Sagittarius (constellation) — the Archer and the Galactic Center Sagittarius (constellation) Sagittarius is a southern zodiac constellation representing an archer (often a centaur). It lies toward the densest part of the Milky Way and contains numerous star clusters, nebul…

Sagittarius A — the Milky Way's central radio complex Sagittarius A Sagittarius A (Sgr A) is the complex radio and infrared source at the center of the Milky Way, comprising a supernova remnant, an ionized gas spiral, and the compact object Sagitta…

Sagittarius A*: the Milky Way’s central radio source and black hole Sagittarius A* Sagittarius A* (Sgr A*) is the compact, variable radio source at the center of the Milky Way, identified with a supermassive black hole of roughly four million solar masses and stu…

Sakigake — Japan's first interplanetary probe Sakigake Sakigake was Japan's first interplanetary spacecraft, launched by ISAS in 1985 as a technology demonstrator and to study the interplanetary medium during the Halley comet era.

Sao — an irregular outer moon of Neptune Sao (moon) Sao is a small, non-spherical irregular satellite of Neptune (Neptune XI), with a distant, inclined, and eccentric orbit. It was discovered in 2002 and later named after a Nereid f…

Saros cycle The Saros cycle is an astronomical period of about 18 years 11 days 8 hours after which lunar and solar eclipses repeat with similar geometry. It underpins eclipse prediction and c…

Artificial satellite Satellite (artificial) An artificial satellite is a human-made object placed into orbit to observe, communicate, navigate, or conduct experiments; this article explains types, orbits, history, and common…

Satellite flare (satellite glint) Satellite flare A satellite flare is a brief bright reflection of sunlight from a man-made satellite. It is predictable for many spacecraft and can be very bright, sometimes disrupting astronomica…

Saturn: the Ringed Gas Giant Saturn Saturn, the sixth planet from the Sun, is a large gas giant known for its extensive ring system, numerous moons (including Titan), layered interior, and key role in studies of plan…

Saturn Nebula (planetary nebula in Aquarius) Saturn Nebula The Saturn Nebula is a bright planetary nebula in Aquarius, famed for ring-like features and small jet-like 'handles'. It lies at roughly 2,400 light‑years and illustrates the late…

Saturn's Gallic group of prograde irregular satellites Saturn's Gallic group of satellites A small family of prograde irregular moons of Saturn with similar orbits and reddish surfaces. Includes Albiorix, Bebhionn, Erriapo and Tarvos; likely collisional fragments from a…

Norse group — the retrograde irregular satellites of Saturn Saturn's Norse group of satellites A cluster of Saturn’s outer irregular moons that orbit retrograde on eccentric, often highly inclined paths. Mostly small, likely captured objects named from Norse mythology; Phoeb…

Scattered disc (Scattered disk) – distant trans‑Neptunian region Scattered disc A remote region of the Solar System of icy minor planets on eccentric, often highly inclined orbits, produced largely by gravitational interactions with the giant planets.

Sceptrum Brandenburgicum — an obsolete constellation Sceptrum Brandenburgicum Sceptrum Brandenburgicum was a short-lived 17th‑century constellation created by Gottfried Kirch to represent the scepter of Brandenburg; its stars were later absorbed into neighbo…

Scholz's Star (WISE J072003.20−084651.2): a nearby binary that passed through the Oort cloud Scholz's Star Scholz's Star (WISE J072003.20−084651.2) is a nearby low-mass binary — a late M-type red dwarf with a brown dwarf companion — that likely traversed the outer Oort cloud ~70,000 yea…

Scorpio (astrology): sign, characteristics, history, and uses Scorpio (astrology) Scorpio is the eighth sign of the tropical zodiac (roughly Oct 23–Nov 22). A fixed water sign symbolized by the scorpion, associated with intensity, transformation, secrecy, and ru…

Scorpius — the Scorpion constellation Scorpius Scorpius is a prominent southern zodiac constellation near the Milky Way center, known for its curved shape, bright star Antares, and many deep-sky objects visible to southern obse…

Scorpius–Centaurus Association (Sco–Cen): nearest OB association Scorpius–Centaurus Association A nearby OB association of young, massive and low-mass stars roughly 380–470 ly away. Includes Antares, several southern bright stars, and evidence of past supernovae shaping the l…

Sculptor (constellation) Sculptor is a faint southern constellation, created by Nicolas-Louis de Lacaille. It contains the Sculptor Galaxy and nearby Sculptor Group and hosts the Sculptor Dwarf, a Milky Wa…

Sculptor Galaxy (NGC 253) — the Silver Coin Galaxy Sculptor galaxy NGC 253, the Sculptor or Silver Coin Galaxy, is a bright, nearly edge-on spiral with a central starburst, prominent dust lanes, and membership of the nearby Sculptor Group, about 1…

Scutum (constellation) Scutum Scutum is a small southern constellation on the Milky Way, known for the Wild Duck Cluster and for giving its name to Delta Scuti variables and a nearby spiral arm.

Search for Extra-Terrestrial Intelligence (SETI) Search for Extra-Terrestrial Intelligence SETI encompasses scientific searches for signs of intelligent life beyond Earth, using radio, optical and data-driven methods, public participation projects, and specialized observ…