Astronomy

Planetshine — illumination of a moon by light reflected from its planet Planetshine Planetshine is the faint illumination of a moon’s dark side caused by sunlight reflected from its parent planet; Earthshine on the Moon is the most familiar example.

Pleiades (M45): the Seven Sisters open star cluster Pleiades The Pleiades (M45) are a nearby open star cluster in Taurus, dominated by hot blue stars. Visible to the naked eye worldwide, they are roughly 100 million years old and important i…

Plutino (2:3 resonant Kuiper belt object) Plutino A plutino is a trans‑Neptunian object locked in a 2:3 mean‑motion resonance with Neptune, sharing Pluto’s characteristic semimajor axis near 39–40 astronomical units from the Sun.

Pluto: the dwarf planet at the edge of the Solar System Pluto Overview of Pluto: physical traits, orbit, discovery and reclassification, exploration by New Horizons, and its role among Kuiper belt objects.

Plutoid (trans‑Neptunian dwarf planet) Plutoid A plutoid is a dwarf planet that orbits the Sun beyond Neptune. It is massive enough to be rounded by self‑gravity but has not cleared its orbital neighborhood; four bodies are cur…

Polaris — the North Star: identity, system, history and navigational role Polaris (star) Polaris (Alpha Ursae Minoris) is the current northern pole star. This article explains its properties, multiple-star system, role as a Cepheid variable, navigational importance, an…

Pollux (star) — the bright orange giant of Gemini Pollux (star) Pollux is the brightest star in Gemini, an evolved K-type orange giant roughly 34 light-years away. It has expanded after exhausting core hydrogen and is known to host at least one…

Polydeuces — Trojan moon of Saturn Polydeuces (moon) Polydeuces is a small Trojan moon that shares Dione's orbit around Saturn. Discovered by Cassini in 2004, it is a few kilometers across and librates near Dione's trailing Lagrange…

Portia — inner moon of Uranus Portia (moon) Portia is a small, inner moon of Uranus discovered by Voyager 2 in 1986. It is a prolate, low‑albedo body in the Portia group whose orbit is slowly decaying and may eventually form…

Praxidike (Jupiter XXVII) Praxidike (moon) Praxidike is a small, irregular, retrograde moon of Jupiter discovered in 2000. It belongs to the Ananke group, orbits far from Jupiter with a high inclination, and is likely a fra…

Procyon (Alpha Canis Minoris) Procyon Procyon is a nearby binary star system in Canis Minor, visible as the eighth-brightest star. It consists of a bright F-type star and a faint white dwarf companion and marks a verte…

Prometheus (moon of Saturn) Prometheus (moon) Prometheus is a small, irregular moon of Saturn that shepherds the inner edge of the F ring and creates dynamic structures through gravitational interaction.

Proper motion in astronomy Proper motion Proper motion is the apparent angular shift of a star across the sky caused by its true motion through space; measured in arcseconds per year, it helps reveal stellar distances, ki…

Prospero — an irregular retrograde moon of Uranus Prospero (moon) Prospero is a small, retrograde, non-spherical irregular moon of Uranus discovered in 1999 and named after Shakespeare's sorcerer; it may belong to a dynamical cluster with Sycorax…

Proteus — Neptune’s large inner moon Proteus (moon) Proteus is Neptune’s second-largest moon and the largest of its inner satellites. Discovered by Voyager 2 in 1989, it is dark, heavily cratered, non-spherical, and offers clues to…

Protostar: the formative stage of a star Protostar A protostar is the contracting, pre–main-sequence object formed from a collapsing interstellar cloud. It gathers mass, powers itself by contraction and brief deuterium burning, and…

Provisional designation in astronomy Temporary alphanumeric labels assigned to newly observed celestial objects until their orbits and identities are secured and a permanent number or name is given.

Proxima Centauri — the nearest star to the Sun Proxima Centauri Proxima Centauri is a nearby red dwarf star in the constellation Centaurus. It is the closest known star to the Sun and hosts at least one Earth-sized planet; it is notable for str…

Psamathe — an irregular moon of Neptune Psamathe (moon) Psamathe is a small, retrograde irregular satellite of Neptune, discovered in 2003. It orbits far from the planet, is non-spherical, and may share an origin with the moon Neso.

PSR B1257+12 B Extrasolar planet orbiting the pulsar PSR B1257+12 in Virgo; one of the earliest confirmed exoplanets, a few times the mass of Earth with a short-period orbit around its pulsar hos…

Puck (moon) — an inner satellite of Uranus Puck (moon) Puck is a small inner moon of Uranus discovered in 1985 by Voyager 2. It is a heavily cratered, low‑albedo body about 81 km in radius, named after Shakespeare's sprite and designat…

Pulsar A pulsar is a rapidly rotating neutron star that emits concentrated beams of electromagnetic radiation seen as regular pulses; used to study extreme physics, test relativity, and p…

Puppis — the Stern of the Ship Argo Puppis Puppis is a southern constellation representing the stern of the mythical ship Argo. It contains bright hot stars, several Messier clusters and nebulae, and was formed when Argo Na…

Purple Mountain Observatory Purple Mountain Observatory in Nanjing, China, is a major astronomical research center noted for optical telescopes, surveys, and discovery of numerous asteroids and minor planets.

Pyxis (constellation) Pyxis Pyxis is a small, faint southern constellation representing a mariner's compass, introduced in the 18th century. It lies near Vela and Puppis and contains few bright stars or notab…

Quasar: powerful active galactic nuclei and their cosmic role Quasar Quasars are extremely luminous active galactic nuclei powered by accretion onto supermassive black holes. This article explains their structure, observations, history, and importan…

Queen of Outer Space (1958 film) Queen of Outer Space 1958 science fiction film directed by Edward Bernds, noted for its CinemaScope visuals, campy space-opera plot, Zsa Zsa Gabor's starring role, and its place in 1950s genre cinema.

R Apodis R Apodis is a pulsating red giant variable star in the southern constellation Apus. It illustrates late stages of stellar evolution—pulsation, mass loss and dust formation—and is o…

R Doradus — a nearby red giant with an unusually large apparent size R Doradus R Doradus is a cool red giant star in the constellation Dorado, notable for its very large apparent angular diameter and an extended, pulsating atmosphere.

R Leporis (Hind's Crimson Star) R Leporis R Leporis, also called Hind's Crimson Star, is a carbon-rich red giant in Lepus known for its intense red color, large size (hundreds of times the Sun) and strong variability.

R Scuti (variable star in Scutum) R Scuti R Scuti is a luminous, pulsating variable star in the Scutum constellation. It shows alternating deep and shallow minima typical of RV Tauri variables and serves as a laboratory fo…

R136a1 — Extremely Massive Wolf–Rayet Star in the Tarantula Nebula R136a1 R136a1 is the most massive and luminous star known: a hydrogen-rich Wolf–Rayet in the R136 cluster of the Tarantula Nebula, in the Large Magellanic Cloud. Estimated ~265 M☉ and ~8.…

R136c — a very massive Wolf–Rayet star in the R136 cluster R136c R136c is an extremely luminous WN5h Wolf–Rayet star in the R136 young cluster of the Tarantula Nebula (Large Magellanic Cloud), notable for its high mass, strong winds and uncertai…

Radio astronomy: study of celestial radio emission and observational techniques Radio astronomy Radio astronomy observes celestial objects at radio wavelengths, using single dishes and interferometers to study galaxies, pulsars, the cosmic microwave background, spectral lines…

Radio galaxy A radio galaxy is an active galaxy that emits powerful radio-frequency radiation from jets and lobes driven by a central black hole; important for jet physics, galaxy evolution and…

Radio telescope A radio telescope is an antenna and receiver system used in radio astronomy to detect and analyse radio-frequency emission from celestial sources, enabling study of phenomena invis…

Red dwarf star Red dwarf Red dwarfs are the smallest, coolest common stars. This article explains their properties, internal structure, lifetimes, observational challenges, and relevance for planets and as…

Red giant (stellar evolution stage) Red giant A red giant is an evolved star that has exhausted core hydrogen and expanded; this article explains its properties, formation, notable examples, and observational significance.

Red shift in astronomy Red shift Red shift is the lengthening of light's wavelength from distant sources. It reveals motion, cosmic expansion, and gravity effects and is measured by comparing shifted spectral line…

Red supergiant A red supergiant is a very large, cool, luminous evolved massive star (spectral types K–M). These stars have huge radii, strong winds, often vary in brightness and commonly end as…

Reflection nebula A reflection nebula is an interstellar cloud of dust visible by scattering starlight. It is often blue, differs from emission nebulae, and frequently occurs near star-forming regio…

Regulus (Alpha Leonis): the brightest star of Leo Regulus Regulus (Alpha Leonis) is the blue-white, B-type star that marks the heart of Leo. A rapidly rotating, multiple-star system roughly 79 light-years away, notable for its brightness,…

Relativistic jet — powerful, near‑light astrophysical plasma streams Relativistic jet Narrow, high‑speed plasma outflows launched from accreting compact objects. Relativistic jets produce radio lobes and gamma rays, show beaming and superluminal effects, and affect…

Reticulum (constellation) Reticulum Reticulum is a small, modern southern constellation representing the reticle of a telescope eyepiece. It contains modest stars and faint deep-sky objects and was introduced during…

Retrograde and Direct Motion in Astronomy Retrograde and direct motion Overview of prograde (direct) and retrograde motion: definitions, real vs apparent retrograde, causes, historical context, examples from planets, moons, asteroids, and observationa…

Rhea: Saturn's Second-Largest Moon Rhea (moon) Rhea is Saturn's second-largest natural satellite, composed mainly of water ice with some rock. It has an ancient, heavily cratered surface and was examined closely by the Cassini…

Rho Cassiopeiae — the yellow hypergiant in Cassiopeia Rho Cassiopeiae Rho Cassiopeiae is a luminous yellow hypergiant in Cassiopeia, notable for its enormous radius, extreme mass loss and episodic eruptions that illustrate late, unstable stages of ma…

Rigel (Beta Orionis): a blue supergiant in Orion Rigel Rigel is a luminous blue supergiant star in the constellation Orion. It is a multiple system, far larger and more luminous than the Sun and visible from both hemispheres as a bluis…