Melnick 34 (BAT99-116) — Massive Wolf–Rayet Binary in the Tarantula Nebula Melnick 34 Melnick 34 (BAT99-116, Mk34) is a very massive Wolf–Rayet binary in the Tarantula Nebula of the Large Magellanic Cloud, notable for extreme luminosity, strong winds and bright X-ra…
Mensa (constellation) Mensa is a small, faint southern constellation named by Nicolas‑Louis de Lacaille after Table Mountain. It contains few bright stars and overlaps part of the Large Magellanic Cloud…
Mercury: Smallest and Innermost Planet of the Solar System Mercury (planet) Mercury is the smallest and innermost planet, notable for extreme temperature swings, a heavily cratered surface, a large iron core, a weak magnetic field, a very thin exosphere an…
Messier 100 (M100) — a grand-design spiral galaxy in the Virgo Cluster Messier 100 Messier 100 (M100, NGC 4321) is a nearly face-on grand-design spiral in Coma Berenices and a prominent Virgo Cluster member, notable for well-defined arms, active star formation an…
Messier 15 (M15): a dense, core-collapsed globular cluster in Pegasus Messier 15 Messier 15 (M15) is a compact, core‑collapsed globular cluster in Pegasus about 33,600 light‑years away, notable for its dense core, many variable stars and pulsars, and the planet…
Messier 32 (M32): the compact dwarf elliptical companion of Andromeda Messier 32 Messier 32 (M32) is a nearby compact dwarf elliptical galaxy orbiting the Andromeda system. It is notable for its dense stellar core, scarcity of gas and dust, and a central superm…
Messier 38 (M38) — Open star cluster in Auriga Messier 38 Messier 38 (NGC 1912) is a bright open cluster in Auriga. Visible to binoculars, it lies roughly 3,400–3,500 light-years away and forms a trio with M36 and M37 for winter sky obser…
Messier 49 (NGC 4472): a giant elliptical galaxy in Virgo Messier 49 Messier 49 (NGC 4472) is a luminous giant elliptical galaxy in Virgo, an early-discovered member of the Virgo Cluster notable for its rich globular cluster system, central X-ray so…
Messier 52 (M52) — Open star cluster NGC 7654 in Cassiopeia Messier 52 Messier 52 (NGC 7654) is a rich open star cluster in Cassiopeia, visible with binoculars and small telescopes. Discovered by Charles Messier in 1774, it lies several thousand light…
Messier 54 (M54) — A globular cluster linked to the Sagittarius dwarf galaxy Messier 54 Messier 54 (M54, NGC 6715) is a bright, dense globular cluster discovered by Charles Messier. It lies toward Sagittarius and is physically associated with the Sagittarius dwarf ell…
Messier 7 (NGC 6475) — the Ptolemy Cluster Messier 7 Messier 7 (NGC 6475), the Ptolemy Cluster, is a bright open star cluster in Scorpius visible to the naked eye. This article covers its appearance, structure, history, observation t…
Messier 74 (NGC 628) — the Phantom Galaxy Messier 74 A nearly face-on grand-design spiral galaxy in Pisces about 32 million light-years away, noted for its symmetric spiral arms, numerous star-forming regions, and low-surface-brightn…
Messier 81 (Bode's Galaxy): nearby grand-design spiral in Ursa Major Messier 81 Messier 81 (M81 or NGC 3031) is a bright, nearby grand-design spiral galaxy in Ursa Major, notable for its dust-rich arms, active nucleus, and importance for extragalactic studies…
Messier 82 (M82) — the Cigar Galaxy Messier 82 Messier 82 (M82) is a nearby starburst galaxy in Ursa Major. Noted for an intense central starburst, a bipolar superwind, ultraluminous X-ray sources, many young massive clusters a…
Messier 87 (M87): Giant Elliptical Galaxy in the Virgo Cluster Messier 87 Messier 87 is a nearby supergiant elliptical galaxy in the Virgo Cluster, notable for its powerful relativistic jet and a supermassive black hole imaged by the Event Horizon Telesc…
Messier Catalogue A historic list of bright deep-sky objects compiled by Charles Messier to help comet hunters avoid confusion; still popular with amateur astronomers for observation and outreach.
Meteor: definition, origins, composition, observation, and impacts Meteor A meteor is the luminous phenomenon produced when a meteoroid enters an atmosphere. This article explains terminology, composition, origins, observation methods, historical signifi…
Meteor shower: causes, observation, and notable displays Meteor shower An overview of meteor showers: how streams of cometary or asteroidal debris produce meteors, how showers are named and measured, notable examples, and practical observing guidance.
Metis — Jupiter's innermost moon Metis (moon) Metis is Jupiter’s closest known moon, a small irregular satellite discovered in 1979. It orbits within the planet’s main ring and likely contributes dust to that ring system.
Metonic cycle (nineteen-year lunisolar period) Metonic cycle Nineteen-year cycle in which 19 solar years closely equal 235 lunar months, used historically and today to align lunar months with the solar year in calendars and astronomical calc…
Micro black hole Small, hypothetical black holes whose masses and sizes are far below stellar scale; they may evaporate by Hawking radiation and could form in the early universe or in high-energy p…
Midnight sun: polar daylight phenomenon, causes, regions, and impacts Midnight sun The midnight sun is continuous daylight experienced in high latitudes during local summer. This article explains its cause, where and when it occurs, its effects on people and natu…
Milky Way — the galaxy that contains the Solar System Milky Way Overview of the Milky Way: its structure, size, contents, history of study, neighbors in the Local Group, and its future interactions — including the expected encounter with Androm…
Milky Way–Andromeda collision The predicted merger of the Milky Way and Andromeda galaxies in about four billion years: observations, dynamics, stages, and likely consequences for stars, gas, and the Local Grou…
Mimas: Saturn's heavily cratered inner moon, home of Herschel crater Mimas (moon) Mimas is a small, icy moon of Saturn, notable for the giant Herschel impact crater and its role in shaping the Cassini Division through orbital resonance.
Minor planet designation: numbering and naming of small Solar System bodies Minor planet designation How the Minor Planet Center assigns permanent number-and-name identifiers to asteroids, dwarf planets and other small Solar System bodies, and how these differ from provisional and…
Mir: Soviet–Russian Modular Space Station (1986–2001) Mir Mir was a Soviet and later Russian modular space station operated from 1986 to 2001. It hosted long-duration crews, international partners and research in microgravity, and shaped…
Mira (Omicron Ceti) — the pulsating red giant and its companion Mira Mira (Omicron Ceti) is a nearby binary in Cetus whose red giant primary is the prototype of Mira-type variables, pulsating with a cycle of roughly 330 days while a compact companio…
Miranda (moon of Uranus) Miranda (moon) Miranda is a small, geologically complex moon of Uranus noted for its patchwork surface, dramatic cliffs and coronae. It was discovered in 1948 and imaged only once by Voyager 2 in…
Mneme (Jupiter XL) — an irregular, retrograde moon of Jupiter Mneme (moon) Mneme is a small, irregular retrograde moon of Jupiter discovered in 2003. It orbits at ~21.4 million km, has a period of ~641 days, belongs to the Ananke group and is ~2 km across…
Molecular cloud A molecular cloud is a cold, dense region of the interstellar medium where molecules—chiefly H2—form; these clouds host star formation and are studied through dust, infrared and ra…
Monoceros — the Unicorn Constellation Monoceros Monoceros is a faint equatorial constellation whose name means “unicorn.” Introduced in the 17th century, it lies near Orion and contains a number of interesting stars, nebulae and…
Montes Taurus — lunar mountain region east of Mare Serenitatis Montes Taurus Montes Taurus is a rugged highland mountain range on the Moon's near side, east of Mare Serenitatis. It includes peaks up to ~3 km, several craters, and the Taurus–Littrow valley v…
Month: calendar unit, lunar origin, and modern uses Month A month is a common calendar unit roughly one twelfth of a year. It originated from the Moon's phases and appears in lunar, lunisolar, and solar calendars with varying definitions…
Moon (Earth's natural satellite) Moon The Moon is Earth's only natural satellite: a rocky, airless body about one quarter of Earth's diameter, influential in tides, human exploration, and cultural history.
Moon landing: history, technology, and significance Moon landing Arrival of a spacecraft on the Moon's surface, covering crewed and robotic landings, key missions, technology, scientific goals, and historical milestones from Luna 2 to Apollo and…
Moons of Haumea Hiʻiaka and Namaka are two icy moons of the dwarf planet Haumea, discovered in 2005. They formed from a collisional event and offer clues to Haumea's composition, history and orbit…
Moons of Mars: Phobos and Deimos Moons of Mars An overview of Mars' two small moons, Phobos and Deimos, covering their features, orbits, origin theories, exploration history, and scientific importance.
Pluto’s Moon System: Charon, Nix, Hydra, Kerberos, and Styx Moons of Pluto Overview of Pluto’s five moons—Charon, Nix, Hydra, Kerberos and Styx—including sizes, orbits, rotations, compositions, New Horizons findings, and ongoing searches for faint dust or…
Moons of Saturn An overview of Saturn's satellite system: counts and sizes, major moons, classification, discoveries and exploration, roles in ring dynamics, and astrobiological interest.
Most distant objects in the observable universe Most distant things Overview of the most distant known sources of light and matter: how distance is measured, the kinds of objects found at highest redshift, historical progress, observational methods…
Mount Stromlo Observatory Mount Stromlo Observatory, located near Canberra, Australia, is a major research and teaching facility of the Australian National University specializing in optical astronomy, inst…
Mu Cephei (Garnet Star): red hypergiant in Cepheus Mu Cephei Mu Cephei, nicknamed the Garnet Star, is a red hypergiant in the constellation Cepheus. It is one of the largest known stars, a semiregular variable and an important object in stel…
Multiverse: concepts, types, history, and scientific significance Multiverse An overview of the multiverse idea: definitions, major categories, historical origin, scientific debates, applications in cosmology and philosophy, and notable criticisms.
Mundilfari (Saturn moon) Mundilfari (moon) Mundilfari is a small irregular, retrograde moon of Saturn discovered in 2000. It orbits at about 18.36 million km with a ~929-day period and is thought to be linked to Phoebe; nam…
Musca: a small southern constellation near the Southern Cross Musca Musca is a compact constellation in the far southern sky, notable for dark nebulae and a handful of bright B‑type stars. It lies close to Crux and Carina and is one of the 88 moder…
MY Cephei — a red supergiant in the constellation Cepheus MY Cephei MY Cephei is an evolved red supergiant star in the constellation Cepheus, with an estimated radius roughly 1,134 times that of the Sun and characteristics typical of late-stage mas…
mya (million years ago): notation and usage in geology, paleontology, and astronomy Mya (unit) Explanation of mya (million years ago): meaning, conversion, notation vs Ma and Myr, and recommended usage across geology, paleontology and astronomy.
