Astronomy

Parallax: apparent displacement, measurement, and applications Parallax Parallax is the apparent displacement of an object when viewed from different positions. It underlies astronomical distance measurement, binocular depth perception, surveying, phot…

Parallel universe A parallel universe is a hypothetical separate reality that coexists with our observable universe; explored in physics, philosophy and fiction. This article outlines concepts, theo…

Parker Solar Probe NASA robotic probe launched in 2018 to study the Sun's outer atmosphere and solar wind, approaching within ~6.2 million km and setting records for spacecraft speed.

Parsec: definition, measurement, and role in astronomy Parsec A parsec is the astronomical distance defined by one arcsecond of parallax. This article explains its geometric basis, conversions, historical origin, practical use in astronomy, a…

Pasiphaë (Jupiter moon) Pasiphaë (moon) Pasiphaë is a distant, irregular retrograde moon of Jupiter discovered in 1908. It is a dark, likely captured object and the namesake of the Pasiphae group of Jovian satellites.

Pasiphaë group — Jupiter’s retrograde irregular moons Pasiphaë group A family of retrograde, non-spherical irregular satellites of Jupiter with similar distant, inclined orbits, believed to be fragments of a captured body. Includes Pasiphaë, Sinope…

Pasithee (Jupiter moon) Pasithee (moon) Pasithee (Jupiter XXXVIII) is a small, irregular retrograde moon of Jupiter discovered in 2001. It belongs to the Carme group and follows an elongated, high-inclination orbit far f…

Paul Oswald Ahnert: German astronomer and author of the Kalender für Sternfreunde Paul Oswald Ahnert Paul Oswald Ahnert (1897–1989) was a German astronomer who worked at Sonneberg Observatory and published the influential annual observing guide 'Kalender für Sternfreunde'. Asteroi…

Pavo (constellation) Pavo is a modern southern constellation representing the peacock. It contains several bright stars and notable deep-sky objects and is best observed from southern latitudes.

Pease 1 — a planetary nebula in the globular cluster Messier 15 Pease 1 Pease 1 is a faint planetary nebula located within the globular cluster Messier 15 in the constellation Pegasus. It is the first known planetary nebula found inside a globular clus…

Pegasus (constellation) Pegasus is a large northern constellation best known for the Great Square asterism, several bright stars such as Markab and Enif, prominent galaxies and for hosting 51 Pegasi, the…

Pelican Nebula (IC 5067/5070): a nearby star-forming emission region Pelican nebula IC 5067/5070, the Pelican Nebula in Cygnus, is an emission (H II) nebula about 1,800 light-years away, notable for dark dust lanes, active star formation and striking H‑alpha struc…

Per aspera ad astra Latin motto meaning 'through hardships to the stars'; history, grammar, variants, and modern uses as a symbol of striving and achievement.

Perdita — inner moon of Uranus Perdita (moon) Perdita is a small inner satellite of Uranus, discovered in Voyager 2 images and later confirmed by Hubble. It belongs to the Portia group and remains little known beyond its orbit…

Perihelion: the nearest point of an orbit to the Sun Perihelion Perihelion is the point in an object's solar orbit where it is closest to the Sun. This article explains its definition, orbital causes, effects on climate and visibility, historic…

Periodic comet: classification, naming, and orbital behavior Periodic comet Overview of periodic comets: orbital classes (short/long, Jupiter-family, Halley-type), naming and numbering conventions, orbital evolution, examples and how astronomers observe an…

Perseids (Annual Meteor Shower) Perseids The Perseids are a bright, reliable meteor shower produced by debris from comet Swift–Tuttle. Visible annually from mid‑July to late August with a strong peak in early to mid‑Augus…

Perseus (constellation) Perseus is a northern constellation notable for the variable star Algol, the bright star Mirfak, the Double Cluster and the radiant of the Perseids meteor shower.

Perseus Arm — a major spiral arm of the Milky Way Perseus Arm The Perseus Arm is one of the Milky Way’s two main spiral arms. It hosts abundant star-forming regions, open clusters and nebulae and is a key feature used to map the Galaxy’s larg…

Persian calendar (Solar Hijri) — structure, history and uses Persian calendar The Persian (Solar Hijri) calendar is a solar, observation-based calendar used in Iran and Afghanistan. It begins on the vernal equinox and aligns closely with the tropical year an…

Phases of the Moon: cause, names, cycle, and significance Phases of the Moon The lunar phases are the changing illuminated shapes of the Moon as seen from Earth across a synodic month (~29.5 days). Causes, phase names, calendar role, eclipses, and notable f…

Philae (spacecraft) — Rosetta's comet lander Philae (spacecraft) Philae was the European Space Agency's robotic lander that made the first soft landing on a comet in 2014, carrying instruments to study composition, structure and activity of 67P/…

Phobos (moon) Phobos is the larger, irregular moon of Mars. It orbits very close to the planet, is heavily cratered and grooved, and is slowly spiraling inward; its origin and long-term fate rem…

Phoebe (satellite of Saturn) Phoebe (moon) Phoebe is a small, dark, irregular moon of Saturn with a distant retrograde orbit. Likely a captured primitive body, it is a roughly equal mix of rock and ice and was explored by t…

Phoenix (constellation) Phoenix is a small, faint constellation of the southern sky, named for the mythical phoenix. Introduced by late 16th-century European mapmakers, it contains a few bright stars and…

Phoenix (spacecraft) Phoenix was a stationary NASA lander that touched down on Mars in 2008 to examine polar soil, confirm near-surface water ice, and assess the planet's habitability for microbial lif…

Phoenix Dwarf — a Local Group dwarf galaxy Phoenix Dwarf A compact overview of the Phoenix Dwarf, a nearby Local Group dwarf galaxy with mixed stellar populations, its discovery, properties, and significance for galaxy evolution studies.

Photosphere — the light-emitting layer of a star Photosphere The photosphere is the visible, light-emitting layer of a star where photons escape. This article describes its definition, structure, observable phenomena, methods of study, and i…

Physical cosmology: the study of the universe’s structure and evolution Physical cosmology Branch of astronomy that uses physical laws and observations to describe the universe’s origin, composition, large-scale structure, and evolution, including Big Bang, dark matter,…

Pi1 Gruis (π1 Gruis) Pi1 Gruis Pi1 Gruis is an evolved red giant star in the southern constellation Grus. It displays large convective surface cells, mass loss, and is an important object for studying late stell…

Pictor (constellation) Pictor Pictor is a small southern constellation representing a painter's easel. It is best known for the nearby star Beta Pictoris with its debris disk and directly imaged exoplanets and…

Pioneer 10: NASA's first probe to Jupiter and the outer Solar System Pioneer 10 Pioneer 10, launched in March 1972, was the first spacecraft to traverse the asteroid belt and make close observations of Jupiter. Its instruments and iconic plaque shaped later de…

Pioneer 11 — NASA's early outer-planet explorer Pioneer 11 Pioneer 11 was a NASA robotic probe launched in 1973 that flew past Jupiter and became the first spacecraft to explore Saturn. It returned unique data on planets, rings and magneto…

Pioneer program (NASA unmanned probes, 1958–1978) Pioneer program NASA's Pioneer program was a series of unmanned space missions from 1958 to 1978, notable for early lunar attempts, the first probes to Jupiter and Saturn, and the Pioneer plaques…

Pisces (astrology) Pisces is the twelfth sign of the tropical zodiac, represented by two fish. Associated with the water element and mutable quality, it is linked to imagination, empathy and the Nept…

Pisces (constellation) Pisces is a zodiac constellation crossing the celestial equator, depicted as two fish joined by a cord; faint externally but notable for Messier 74 and its role in celestial histor…

Piscis Austrinus (Southern Fish) — constellation overview Piscis Austrinus Piscis Austrinus, the Southern Fish, is a small southern-hemisphere constellation best known for the bright star Fomalhaut; visible near Aquarius and noted in classical and Arabic…

Pistol Star — a luminous blue hypergiant near the Galactic center Pistol Star Pistol Star is an extremely luminous blue hypergiant in the Quintuplet cluster, obscured by dust near the Milky Way's center and surrounded by a nebula formed by past mass loss.

Planck (spacecraft) ESA's Planck was a space observatory that mapped the cosmic microwave background with unprecedented sensitivity and resolution, producing definitive measurements of key cosmologica…

Planck epoch — the universe’s earliest quantum-dominated era Planck epoch The Planck epoch is the earliest interval after the Big Bang (up to the Planck time), when quantum gravitational effects dominated and standard physics breaks down; it precedes gra…

Planet A planet is a large body that orbits a star, shaped by its own gravity into a rounded form. Planets occur in our Solar System and around other stars and vary widely in composition,…

Planetarium: domes, projection systems, uses and terminology Planetarium A planetarium is a theater or device that recreates the night sky for education, training and entertainment. This article covers domes, projection technologies, sizes, uses, histor…

Planetary nebula: glowing shells from dying low- and intermediate-mass stars Planetary nebula A planetary nebula is an expanding shell of ionized gas ejected by a low- to intermediate-mass star near the end of its life. This article explains their formation, appearance, var…

Planetary ring A planetary ring is a disc of particles that orbits a planet. This article explains composition, structure, origin theories, shepherd moons, notable examples and what spacecraft ha…

Planetary science: study of planets, their systems, and processes Planetary science Planetary science (planetology) examines planets, moons, rings and minor bodies: their formation, composition, dynamics, evolution and exploration using telescopes, probes and comp…

Planetary system: structure, discovery, and significance Planetary system A concise, neutral overview of planetary systems: components, how exoplanets are found, nearby examples, the habitable zone, and why these systems matter for astrobiology.

Planetesimal A planetesimal is a solid body formed in a protoplanetary disk that acts as a building block of planets; ranges from meter-size objects to hundreds of kilometers and includes aster…

Planets beyond Neptune Overview of objects orbiting the Sun beyond Neptune: history of searches for additional planets, the Kuiper belt and trans‑Neptunian objects, the Planet Nine hypothesis, and scient…