Astronomy Glossary: Space and Universe Terms Explained

A

Accretion disk: A rotating disc of gas and dust spiralling onto a compact object such as a black hole, neutron star or forming star. Friction inside the disc converts gravitational energy into heat and light, sometimes across the whole spectrum.
Active galactic nucleus: A galaxy's centre powered by matter accreting onto a supermassive black hole. AGN can outshine the rest of the host galaxy across the electromagnetic spectrum. Quasars are extreme AGN seen at cosmological distances.
Adaptive optics: A technology that uses fast, flexible mirrors to correct for the blurring of light by Earth's atmosphere. Adaptive optics lets ground-based telescopes reach resolutions approaching those of space observatories at infrared and visible wavelengths.
Albedo: The fraction of light a surface reflects, ranging from 0 (perfectly black) to 1 (perfectly white). Fresh snow and Venus's cloud tops have very high albedo; asphalt and dark asteroids have very low albedo.
Altitude: The angle of an object above the horizon, measured in degrees from 0° at the horizon to 90° straight overhead. Together with azimuth, altitude locates any point in the local sky from a specific observing site.
Aperture: The diameter of a telescope's primary light-collecting element — usually the main mirror or lens. Aperture sets both how much light the telescope collects and the smallest angular detail it can resolve.
Apparent magnitude: A logarithmic scale describing how bright a celestial object appears from Earth. Lower numbers are brighter: the Sun is about −27, a full Moon around −12, and the faintest stars visible to the naked eye about +6. Apparent magnitude depends on distance and does not measure a star's true output.
Artificial satellite: A crewed or uncrewed spacecraft placed in orbit around Earth or another body. Sputnik 1 in 1957 was the first. Thousands of artificial satellites now support communication, navigation, weather forecasting and science.
Asterism: A recognisable pattern of bright stars that is not itself an official constellation. The Big Dipper is an asterism inside the constellation Ursa Major. Asterisms are useful for finding your way around the night sky.
Asteroid: A rocky or metallic body smaller than a planet that orbits the Sun. Most asteroids reside in the asteroid belt between Mars and Jupiter; others share planetary orbits or cross Earth's path. They are leftovers from the Solar System's formation.
Asteroid belt: The doughnut-shaped region between Mars and Jupiter, roughly 2.2 to 3.2 astronomical units from the Sun, containing most known asteroids. Despite popular images, its objects are so widely separated that a spacecraft can cross it without special evasive manoeuvres.
Astrometry: The precise measurement of positions and motions of celestial objects. In exoplanet science, astrometry catches the tiny sky-plane wobble a planet forces on its star. ESA's Gaia mission is beginning to deliver astrometric detections of long-period giants.
Astronomical unit: The average distance from Earth to the Sun, roughly 149.6 million kilometres. The astronomical unit is the standard measuring stick for distances within the Solar System. Neptune, for example, orbits at about 30 astronomical units.
Astronomy: The scientific study of everything beyond Earth's atmosphere — stars, planets, galaxies, the interstellar medium and the universe as a whole. Astronomy is a natural science and is distinct from astrology, which has no empirical basis.
Astrophysics: The branch of astronomy that applies physics and chemistry to explain how celestial objects work: how stars produce energy, how galaxies rotate, how black holes distort spacetime. Astrophysics turns observed light into models of matter, energy and gravity.
Atmosphere: A layer of gases bound to a planet, moon or star by gravity. Planetary atmospheres are shaped by outgassing from the interior, delivery by impacts, escape to space and, on Earth, biological processes.
Axial tilt: The angle between a planet's rotation axis and the perpendicular to its orbital plane. Earth's axial tilt of about 23.4° drives the seasons; Uranus is tipped so far — around 98° — that its poles alternately face the Sun over its orbit.

B

Barred spiral galaxy: A spiral galaxy with a bar-shaped concentration of stars crossing its central region, from which the spiral arms emerge. The Milky Way is a barred spiral, as are many nearby giant galaxies.
Big Bang: The prevailing scientific model in which the universe began from an extremely hot, dense state around 13.8 billion years ago and has been expanding and cooling ever since. The Big Bang is a description of expansion, not an explosion in a pre-existing space.
Binary star: A system of two stars gravitationally bound and orbiting a common centre of mass. Most Sun-like stars are members of binary or higher-multiplicity systems, which are essential laboratories for measuring stellar masses.
Black hole: A region of spacetime where gravity is so strong that not even light can escape. A black hole is bounded by an event horizon; its mass sets its size. Black holes do not “suck in” matter — objects far away orbit them like any other mass.
Brown dwarf: A substellar object more massive than a giant planet but not massive enough to sustain hydrogen fusion in its core. Brown dwarfs cool slowly over billions of years and glow mainly in the infrared.

C

Celestial equator: The projection of Earth's equator onto the celestial sphere. Stars along the celestial equator rise due east and set due west from any point on Earth. It is the reference plane for the declination coordinate.
Celestial sphere: An imaginary sphere centred on Earth on which every star appears to sit at the same distance. Astronomers use it as a mathematical model for locating stars, planets and satellites in the sky using coordinates such as right ascension and declination.
Chromosphere: A relatively thin layer of a star's atmosphere just above the photosphere, visible in specific spectral lines. During a total solar eclipse, the Sun's chromosphere is briefly seen as a reddish rim around the disc.
Circumbinary planet: An exoplanet that orbits both stars of a binary system rather than a single one. Kepler-16b, sometimes called the “Tatooine planet”, is the best-known example. Circumbinary orbits are dynamically constrained and are typically wide.
Comet: A small icy body from the outer Solar System that develops a temporary atmosphere and long tails when heated near the Sun. Comets shed dust and gas that stream away, driven by sunlight and the solar wind, producing the tail we see from Earth.
Conjunction: The apparent close approach of two objects in the sky as seen from Earth. A conjunction of Jupiter and Saturn simply means they line up along our sight-line; they remain hundreds of millions of kilometres apart in space.
Constellation: One of 88 officially recognised regions of the sky, each traditionally associated with a named pattern of stars. Modern constellations are boundaries on the celestial sphere rather than physical groupings of stars, which are often at very different distances.
Corona: The outermost, extremely hot layer of a star's atmosphere. The Sun's corona reaches millions of degrees despite the surface being only about 5,800 K, an ongoing puzzle in solar physics. It streams outward as the solar wind.
Coronagraph: An optical device that blocks the light of a bright central source so that nearby faint objects — a stellar corona, exoplanets, or a debris disc — become visible. Coronagraphs are central to direct imaging of exoplanets.
Cosmic inflation: A very brief epoch in the first fraction of a second after the Big Bang during which the universe is believed to have expanded exponentially. Inflation explains the observed uniformity of the cosmic microwave background.
Cosmic microwave background: The thermal afterglow of the hot early universe, released about 380,000 years after the Big Bang and now visible in every direction as microwave light. Precise maps of the CMB constrain the age, geometry and contents of the universe.
Cosmic web: The largest-scale structure of the universe: galaxies collect along filaments and sheets separated by huge nearly empty voids. Dark matter provides the gravitational scaffolding on which the cosmic web grew.
Cosmological redshift: The stretching of light to longer wavelengths as it travels through expanding space. Unlike a Doppler shift, it is not caused by motion through space but by the growth of space itself. High redshift means we see the object as it was long ago.
Crater: A bowl-shaped depression on a solid surface, usually formed by an impact or by volcanic collapse. Impact craters dominate airless worlds like Mercury and the Moon because there is no wind or water to erase them.

D

Dark energy: The name given to whatever is causing the universe's expansion to accelerate. It accounts for about 68 % of the total energy content and is often modelled as a cosmological constant, though its physical nature is unknown.
Dark matter: A form of matter that does not emit, absorb or reflect light but interacts gravitationally. Its presence is inferred from galaxy rotation curves, gravitational lensing and the cosmic microwave background. It makes up about 27 % of the universe's energy content.
Declination: The celestial equivalent of latitude, measuring how far an object lies north (+) or south (−) of the celestial equator in degrees. Together with right ascension, declination fixes an object's position on the celestial sphere.
Direct imaging: A detection technique that photographs an exoplanet as a separate source of light from its star, usually after a coronagraph or starshade suppresses the star's glare. It works best for young, hot, wide-orbit giant planets.
Dwarf planet: A body that orbits the Sun and is massive enough for gravity to pull it into a roughly round shape, but which has not cleared other objects from its orbital neighbourhood. Pluto, Eris, Haumea, Makemake and Ceres are recognised examples.

E

Eclipse: An event in which one celestial body's shadow falls on another, or one body passes in front of another and blocks its light. Solar eclipses occur when the Moon covers the Sun; lunar eclipses occur when Earth's shadow falls on the Moon.
Ecliptic: The apparent yearly path of the Sun across the sky, which is also the plane of Earth's orbit projected onto the celestial sphere. The Moon and planets stay near the ecliptic because the Solar System is nearly flat.
Elliptical galaxy: A smooth, roughly ellipsoidal galaxy with little cold gas and mostly old stars. Ellipticals range from dwarfs to the giants that dominate the centres of galaxy clusters. Most formed through mergers of earlier disc galaxies.
Equinox: One of two moments each year when the Sun crosses the celestial equator, and day and night are approximately equal at every latitude. The spring (vernal) equinox and autumn equinox mark the start of astronomical spring and autumn respectively.
Event horizon: The boundary of a black hole beyond which no signal — including light — can reach the outside universe. The event horizon is not a physical surface but a mathematical one-way membrane in spacetime.
Exomoon: A moon orbiting an exoplanet. Despite intense searches — for example around Kepler-1625b — no exomoon has yet been confirmed. Detecting them requires unusually clean and repeated transit signals.
Exoplanet: A planet that orbits a star other than the Sun, or a free-floating planetary-mass object unbound to any star. More than 5,900 exoplanets have been confirmed and thousands of candidates await follow-up.

G

Galaxy: A gravitationally bound system of stars, gas, dust and dark matter that ranges in mass from a million to more than a trillion solar masses. Galaxies come in spiral, elliptical, lenticular and irregular shapes.
Galaxy cluster: A group of hundreds to thousands of galaxies bound together by gravity, filled with hot X-ray-emitting gas and dominated by dark matter. Galaxy clusters are the largest gravitationally bound structures in the universe.
Galaxy merger: A gravitational encounter in which two or more galaxies combine over hundreds of millions of years. Mergers can trigger bursts of star formation, feed central black holes and transform discs into ellipticals.
Gas giant: A large planet made mostly of hydrogen and helium, with no solid surface accessible from outside. Jupiter and Saturn are the Solar System's gas giants. Their apparent “surfaces” are just the top of a deep atmosphere.
Globular cluster: A dense, spherical star cluster typically containing hundreds of thousands of very old stars. Globular clusters orbit the halos of galaxies. The Milky Way is home to about 150 known globulars.
Gravitational microlensing: A detection method that spots an exoplanet when its gravity, and that of its host star, temporarily magnifies a background star. Microlensing is uniquely sensitive to planets far from their stars and to rogue planets.
Gravity assist: A manoeuvre that uses a planet's gravity to change a spacecraft's speed and direction without expending fuel. Voyager 2's Grand Tour and Cassini's route to Saturn depended on gravity assists at multiple planets.

H

Habitable zone: The range of distances from a star where a rocky planet with a suitable atmosphere could hold liquid water on its surface. A habitable-zone orbit does not confirm habitability or the presence of life — it is a screening criterion.
Hawking radiation: A predicted feeble thermal emission from black holes that arises from quantum effects near the event horizon. It has not yet been directly detected. Hawking radiation implies black holes evaporate over immense timescales.
Heliopause: The outer boundary of the heliosphere, where the solar wind is stopped and turned aside by the pressure of the interstellar medium. Voyager 1 crossed the heliopause in 2012 and Voyager 2 in 2018, entering interstellar space.
Heliosphere: The vast bubble of solar wind and magnetic field carved out of the interstellar medium by the Sun. It reaches well beyond Pluto and shields the inner Solar System from most galactic cosmic rays.
Hertzsprung–Russell diagram: A plot of stellar luminosity against surface temperature or spectral class. Most stars lie on a diagonal band called the main sequence; giants and dwarfs occupy their own regions. The diagram is the map of stellar evolution.
Hot Jupiter: A gas giant of roughly Jupiter's mass that orbits its star in only a few days. Intense irradiation heats the atmosphere to hundreds or thousands of degrees. Hot Jupiters were the first exoplanets discovered around Sun-like stars.

I

Ice giant: A giant planet dominated by heavier volatile ices — water, methane and ammonia — beneath a hydrogen and helium atmosphere. Uranus and Neptune are ice giants. Despite the name, most of their “ice” is actually a hot, high-pressure fluid.
Interferometry: A technique that combines light or radio waves from several separated telescopes to produce images with resolution equivalent to a much larger single instrument. The Event Horizon Telescope used radio interferometry to image M87*.
Intermediate-mass black hole: A black hole with a mass between roughly 100 and 100,000 solar masses — filling the gap between stellar-mass and supermassive black holes. Only a small number of solid candidates are known.
Irregular galaxy: A galaxy that lacks the clear spiral or elliptical structure of larger systems. Irregulars are often gas-rich, actively forming stars and distorted by past interactions with a larger neighbour, as with the Large Magellanic Cloud.

K

Kuiper Belt: The ring of icy bodies beyond Neptune, extending from about 30 to 50 astronomical units. It is home to dwarf planets including Pluto, Haumea and Makemake, and to the short-period comets that visit the inner Solar System.

L

Lagrange point: One of five positions in a two-body system where a small third body can maintain a stable position relative to the two larger bodies. The James Webb Space Telescope orbits the Sun–Earth L2 point, about 1.5 million kilometres from Earth.
Launch vehicle: A rocket that carries a payload from Earth's surface into space. Modern launch vehicles range from small solid-fuel rockets to giant multi-stage boosters like the Saturn V, Falcon Heavy and SLS.
Lava world: A rocky exoplanet close enough to its star that its dayside surface is hot enough to melt. Some models predict oceans of magma and rock-vapour atmospheres. 55 Cancri e is the best-known example under study.
Light-year: The distance that light travels in one Earth year — about 9.46 trillion kilometres. A light-year is a unit of distance, not time. Proxima Centauri, the nearest star to the Sun, is about 4.24 light-years away.
Local Group: The gravitationally bound cluster of galaxies that includes the Milky Way, the Andromeda Galaxy, the Triangulum Galaxy and dozens of dwarfs. It spans about 10 million light-years across and is falling toward its centre of mass.
Luminosity: The total amount of energy an object radiates per second, independent of distance. Luminosity is often expressed in watts or in units of the Sun's luminosity. A distant star and a nearby one can have the same apparent magnitude but very different luminosities.

M

Magnetosphere: The region around a planet dominated by its own magnetic field. Earth's magnetosphere deflects most of the solar wind and traps charged particles that light up the aurorae. Jupiter's is the largest structure in the Solar System after the heliosphere.
Main-sequence star: A star that produces energy by fusing hydrogen into helium in its core. Most stars, including the Sun, spend the largest fraction of their lives on the main sequence. Their surface temperatures and luminosities fall on a well-defined band of the H–R diagram.
Meteor: The bright streak of light produced when a meteoroid enters Earth's atmosphere and burns up. A meteor is the flash itself — not the object — which is why the popular name “shooting star” is doubly misleading: it is neither shooting nor a star.
Meteorite: A fragment of a meteoroid that survives the fiery passage through Earth's atmosphere and reaches the ground. Meteorites are the only extraterrestrial samples most people ever hold and preserve chemical records of the early Solar System.
Meteoroid: A rocky or metallic object in space, smaller than an asteroid — from grains of dust up to a metre or so. A meteoroid becomes a meteor when it enters an atmosphere and a meteorite if any part reaches the ground.
Mini-Neptune: A common class of exoplanet larger than Earth but smaller than Neptune, usually with a substantial hydrogen–helium atmosphere over a rocky or icy core. Often used interchangeably with sub-Neptune, though authors may set slightly different boundaries.
Moon: A natural satellite orbiting a planet, dwarf planet or asteroid. Earth's Moon is capitalised. Other planets host moons ranging from tiny captured asteroids to worlds larger than Mercury, such as Jupiter's Ganymede.

N

Neutron star: The extraordinarily dense remnant left behind when a massive star's core collapses in a supernova. A neutron star packs more mass than the Sun into a sphere about 20 kilometres across, held up against gravity by neutron degeneracy pressure.
Nuclear fusion: The process by which light atomic nuclei merge into heavier ones, releasing energy. Stars power themselves by fusing hydrogen into helium and, in later stages, fusing heavier elements. Fusion produces the elements up to iron in the universe.

O

Observable universe: The portion of the universe from which light has had time to reach us since the Big Bang. It is a sphere about 93 billion light-years across today. What lies beyond it may extend much farther, but is not observable in principle.
Oort Cloud: The hypothesised spherical reservoir of icy bodies surrounding the Solar System at distances from a few thousand to perhaps 100,000 astronomical units. The Oort Cloud is thought to supply long-period comets. No object has been directly imaged there.
Opposition: The moment when an outer planet lies directly opposite the Sun in Earth's sky, rising as the Sun sets. Planets at opposition are typically closest to Earth and brightest, making them prime targets for observation.
Orbital eccentricity: A number between 0 and 1 describing how much an orbit deviates from a perfect circle. A circular orbit has eccentricity 0; a very elongated ellipse approaches 1. Mercury's eccentricity of 0.21 is the largest of the planets.
Orbital inclination: The tilt of an orbit relative to a reference plane, usually the ecliptic or the equator of the central body. High-inclination orbits reach far above and below the reference plane; Solar System planetary inclinations are all less than about 7°.
Orbiter: A spacecraft that enters orbit around a planet, moon or asteroid rather than flying past. Cassini at Saturn and Mars Reconnaissance Orbiter are prime examples, delivering long-term coverage rather than a brief snapshot.

P

Parallax: The apparent shift of a nearby object against more distant background as an observer's viewpoint changes. Astronomers use Earth's orbit as a baseline to measure the parallax of nearby stars, giving direct distances in parsecs and light-years.
Payload: The scientific instruments, satellites, crew or cargo carried by a launch vehicle. Every kilogram of payload costs energy to lift, so mission designers spend enormous effort trimming mass.
Periapsis: The point in an orbit where the orbiting body is closest to the object it orbits. Around the Sun this point is called perihelion; around Earth, perigee; around the Moon, perilune. The opposite point is apoapsis.
Photon sphere: A region around a non-rotating black hole where gravity bends light into unstable circular orbits. Photons that enter the photon sphere either fall in or escape; images of M87* and Sgr A* revealed the bright ring produced just outside it.
Photosphere: The visible surface of a star — the layer from which most of its light escapes into space. The Sun's photosphere is only a few hundred kilometres thick and has a temperature of about 5,800 K.
Planet: Under the current IAU definition, a body that orbits the Sun, is massive enough to be nearly round, and has cleared other objects from its orbital neighbourhood. Eight bodies in our Solar System qualify. Planets that orbit other stars are exoplanets.
Planetary nebula: An expanding shell of gas cast off by a dying low- to intermediate-mass star as it becomes a white dwarf. The name is a historical accident from 18th-century telescopes; planetary nebulae have nothing to do with planets.
Planetary ring: A disc of dust and icy or rocky particles held in orbit around a planet by gravity. Saturn's rings are the most spectacular but Jupiter, Uranus and Neptune also have faint ring systems. Ring particles range from micrometres to metres in size.
Probe: An uncrewed spacecraft sent to observe or measure another body or region of space. Probes may fly by, orbit, land or plunge — as with the Cassini mission's Huygens probe on Titan.
Prograde orbit: An orbit that revolves in the same direction as the central body rotates. Almost all of the Solar System's planets and major moons follow prograde orbits, a signature of formation from a shared spinning disc of gas and dust.
Protostar: A young stellar object that has begun to collapse under gravity from a dense clump of a molecular cloud but has not yet reached temperatures high enough to sustain hydrogen fusion in its core.
Pulsar: A rapidly rotating neutron star that emits beams of radio or higher-energy radiation from its magnetic poles. If the beam sweeps across Earth we see a very regular pulse, sometimes accurate to microseconds.

R

Radial velocity: The component of an object's velocity directed toward or away from the observer. Precise measurements of a star's radial velocity reveal the Doppler wobble a planet imprints on its motion and yield the planet's minimum mass.
Radio telescope: A telescope designed to detect radio waves from cosmic sources such as hydrogen gas, pulsars and quasars. Radio dishes can operate day and night and are often combined into vast arrays like the VLA or ALMA.
Red dwarf: A small, cool, low-mass main-sequence star of spectral type M. Red dwarfs are by far the most common stars in the galaxy and can live for hundreds of billions of years. Many host detected exoplanets.
Red giant: A late stage in the life of a low- to intermediate-mass star, after core hydrogen is exhausted. The outer layers expand enormously while the surface cools, giving a large red star hundreds of times the Sun's radius.
Redshift: A shift of features in a spectrum toward longer, redder wavelengths. Redshift can be caused by an object moving away from us, by gravitational effects, or by the expansion of space (cosmological redshift).
Reflecting telescope: A telescope that uses a curved mirror to focus light. Nearly all modern research telescopes are reflectors because large mirrors are cheaper, lighter and free from the chromatic aberration of lenses.
Refracting telescope: A telescope that uses lenses to bend light to a focus. Refractors dominated 19th-century astronomy; today they are common in amateur and educational telescopes, while research increasingly uses reflectors.
Relativistic jet: A narrow, collimated stream of plasma launched at close to the speed of light from the magnetised environment near a black hole or neutron star. Relativistic jets can extend for hundreds of thousands of light-years.
Resolution: The smallest angular detail an instrument can distinguish. Resolution improves with larger aperture and shorter wavelength. It is not the same as image quality: adaptive optics, seeing conditions and detector sampling all matter too.
Retrograde orbit: An orbit that revolves in the opposite direction to the central body's rotation. Neptune's largest moon, Triton, follows a retrograde orbit, a strong sign that it was captured from the Kuiper Belt rather than forming in place.
Right ascension: The celestial equivalent of longitude, measured eastward along the celestial equator in hours, minutes and seconds. Combined with declination, right ascension gives a unique position for any object on the celestial sphere.
Roche limit: The minimum distance at which a moon or comet held together only by its own gravity can survive the tidal pull of a nearby larger body. Cross it and the smaller body is torn apart, potentially forming a ring.
Rocky planet: A planet composed mainly of silicate rock and metal with a solid surface. Mercury, Venus, Earth and Mars are the Solar System's rocky planets. Many exoplanets are also rocky, though composition must be measured, not assumed.
Rogue planet: A planetary-mass object that drifts through the galaxy unbound to any star. Some were probably ejected from planetary systems; others may have formed like very low-mass brown dwarfs. Most are detected through gravitational microlensing.
Rover: A wheeled spacecraft designed to drive on the surface of another world. NASA has operated a growing family of Mars rovers — Sojourner, Spirit, Opportunity, Curiosity and Perseverance — that together have transformed our view of the planet.

S

Singularity: A mathematical point at the centre of a black hole where general relativity predicts infinite density. Physicists interpret it as a sign that current theory breaks down; a full description requires quantum gravity. It is not something we can image or visit.
Solar flare: A sudden brightening of a region of the Sun's atmosphere caused by the release of energy stored in twisted magnetic fields. Flares are often accompanied by coronal mass ejections that can drive geomagnetic storms at Earth.
Solar wind: A continuous stream of charged particles — mostly electrons, protons and alpha particles — flowing outward from the Sun's corona at hundreds of kilometres per second. The solar wind shapes planetary magnetospheres and comet tails.
Solstice: One of two moments each year when the Sun reaches its greatest angular distance from the celestial equator, giving the longest or shortest day. The June solstice marks summer in the Northern Hemisphere and winter in the Southern.
Space station: A crewed spacecraft designed to remain in orbit for long periods and support ongoing science, engineering and habitation. Salyut, Skylab, Mir, the International Space Station and Tiangong are the best-known examples.
Space telescope: A telescope operating above Earth's atmosphere, avoiding weather, city light and the atmospheric absorption that blocks ultraviolet, X-ray and gamma-ray photons. Hubble, JWST, Chandra and Fermi are landmark space telescopes.
Spacetime: The four-dimensional continuum in which events happen: three dimensions of space plus one of time. General relativity describes gravity as the curvature of spacetime by mass and energy.
Spectral class: A category based on the pattern of absorption lines in a star's spectrum, which reflects surface temperature. The main sequence runs O, B, A, F, G, K, M from hottest to coolest. The Sun is a G-type star.
Spectrograph: An instrument that spreads light out by wavelength and records the resulting spectrum. Modern astronomical spectrographs use diffraction gratings or prisms and electronic detectors to measure faint features precisely.
Spectroscopy: The study of a source's light by analysing how its brightness varies with wavelength. Spectroscopy reveals composition, temperature, motion and magnetic fields and is arguably the most powerful diagnostic tool in astronomy.
Spectrum: Light spread out by wavelength, revealing dark or bright lines produced by atoms and molecules. Analysing a spectrum tells astronomers a source's composition, temperature, velocity along the line of sight and often its distance.
Spiral galaxy: A rotating disc galaxy with a central bulge and spiral arms of gas, dust and young stars. Spirals contain most of the ongoing star formation in the nearby universe. The Milky Way is a barred spiral galaxy.
Star: A luminous ball of gas held together by its own gravity and powered, for most of its life, by nuclear fusion in its core. Stars produce most of the elements heavier than helium and shape their host galaxies.
Star cluster: A gravitationally bound group of stars that formed together from the same molecular cloud. Open clusters like the Pleiades are young and loose; globular clusters are ancient, dense and typically contain hundreds of thousands of stars.
Stellar nursery: A dense, cold region of a molecular cloud in which new stars form. Famous stellar nurseries include the Orion Nebula and the Carina Nebula. Infrared telescopes such as JWST peer through the dust to see the youngest protostars inside.
Stellar-mass black hole: A black hole formed by the core collapse of a massive star, with a mass typically between about 3 and 100 solar masses. Many are detected through X-ray binaries or by gravitational waves from mergers.
Sub-Neptune: An exoplanet with a radius between Earth and Neptune, often about two to four Earth radii, typically retaining a hydrogen-rich atmosphere. Sub-Neptunes are the most common class in the exoplanet catalogue, though absent from our Solar System.
Super-Earth: An exoplanet larger or more massive than Earth but smaller than Neptune. The label describes size only. A super-Earth may be rocky, water-rich or gas-enveloped; it is not necessarily Earth-like or habitable.
Supermassive black hole: A black hole with a mass ranging from about a million to more than ten billion suns, found at the centre of most large galaxies. The Milky Way's Sagittarius A* holds roughly 4 million solar masses.
Supernova: The explosive death of a massive star, or the runaway thermonuclear detonation of a white dwarf that has accreted too much matter. A supernova briefly outshines an entire galaxy and disperses heavy elements into space.
Synchronous rotation: The situation in which a body rotates exactly once per orbit, so it keeps the same face pointed at its companion. Earth's Moon rotates synchronously, which is why we always see the same side.

T

Tidal locking: The process by which tidal forces slow a body's rotation until it becomes synchronous with its orbit. Once locked, one hemisphere permanently faces the companion. Many close-in exoplanets are expected to be tidally locked to their stars.
Trajectory: The path a spacecraft follows through space, shaped by gravity and by any thrust its engines apply. Interplanetary trajectories often use multiple gravity assists to reach distant targets efficiently.
Transit: The passage of a smaller body across the visible face of a larger one from an observer's viewpoint. Exoplanets found by the transit method dim their host stars by a fraction of a percent when they cross the disc.
Transit depth: The fractional decrease in a star's brightness during a transit. Transit depth is approximately the square of the planet-to-star radius ratio, so a large planet in front of a small star produces a much deeper dip than a small planet in front of a big star.
Transit-timing variation: Small departures from a perfectly regular transit schedule caused by gravitational interactions between planets in a multi-planet system. Modelling these variations can reveal additional worlds and constrain their masses.
Transmission spectroscopy: A technique for probing an exoplanet's atmosphere by measuring how starlight is filtered through the atmospheric limb during a transit. Absorption at specific wavelengths reveals molecules and cloud properties.

V

Variable star: A star whose apparent brightness changes with time. Some vary because of pulsations in their outer layers, others because of eclipses in a binary system. Cepheid variables serve as “standard candles” for measuring extragalactic distances.

W

White dwarf: The compact hot remnant left behind when a low- to intermediate-mass star sheds its outer layers as a planetary nebula. About Earth-sized but roughly half a solar mass, white dwarfs cool over many billions of years.

Z

Zenith: The point on the celestial sphere directly above an observer. An object at the zenith has an altitude of 90°. The opposite point, straight below through Earth, is the nadir.
Zodiacal light: A faint cone of glow visible along the ecliptic after sunset or before sunrise from dark sites. It is sunlight scattered by dust grains left behind by comets and asteroids in the plane of the Solar System.

Astronomy glossary

A

B

C

D

E

G

H

I

K

L

M

N

O

P

R

S

T

V

W

Z

Sources and editorial approach