Astronomy Glossary

A · B · C · D · E · F · G · H · I · K · L · M · N · O · P · Q · R · S · T · U · V · W · Z

A

• Accretion — The building-up of larger bodies as dust and rock collide and stick together; how planets grew from the disk around the young Sun. See How It Formed.
• Aphelion — The point in an orbit farthest from the Sun. (Its opposite is perihelion.)
• Asteroid — A small rocky or metallic body orbiting the Sun, most of them in the Asteroid Belt. See Asteroid Belt.
• Asteroid Belt — The ring of rocky debris between Mars and Jupiter, holding the dwarf planet Ceres. See Asteroid Belt.
• Astronomical Unit (AU) — The average Earth–Sun distance, about 150 million km — the standard measure for Solar System distances.
• Aurora — The northern and southern lights: glowing curtains in the upper atmosphere, caused by charged particles from the Sun. See Sunspots & the Solar Cycle.
• Axial Tilt — The angle a planet's spin axis leans from upright; the cause of seasons, and extreme on Uranus. See Uranus.
• Aperture — The diameter of a telescope's main lens or mirror; how much light it gathers, and the number that matters most. See Telescope Guide.
• Averted Vision — Looking slightly to the side of a faint object so its light lands on the more sensitive edge of the retina, revealing detail your direct gaze misses. See How to Observe.
• Accretion Disk — A flat, swirling disk of gas spiraling into a compact object such as a black hole, heated by friction until it glows; the part of a black-hole system we actually see. See What Is a Black Hole?.

B

• Binary Star — Two stars orbiting a common center of mass; most stars are in multiple systems.
• Black Hole — A region where gravity is so strong that nothing, not even light, can escape past its event horizon. Formed when a massive star collapses, or grown to millions of suns at a galaxy's center.
• Bok Globule — A small, dense, dark cloud of gas and dust collapsing to form new stars; the compact nurseries within dark nebulae. See Dark Nebulae.
• Bortle Scale — A 1–9 rating of a site's darkness, from pristine wilderness (1) to inner city (9); the higher the number, the less you'll see. See How to Observe.
• Brown Dwarf — A "failed star" between about 13 and 80 Jupiter masses, too light to sustain ordinary hydrogen fusion; the middle ground between the largest planets and the smallest stars. See Main Sequence.
• Big Bang — The hot, dense beginning of the universe about 13.8 billion years ago; not an explosion in space but the expansion of space itself. See The Big Bang.
• Big Freeze (Heat Death) — The leading long-term fate of the universe: endless expansion and a slow cooling toward a cold, uniform, low-energy state of maximum entropy. See Fate of the Universe.

C

• Cassini Division — The dark gap dividing Saturn's two brightest rings, visible in a small telescope. See Planetary Rings.
• Coma — The glowing cloud of gas and dust around a comet's nucleus, formed as ice turns to vapor near the Sun. See Comets.
• Comet — An icy "dirty snowball" that grows a coma and tails when it nears the Sun. See Comets.
• Conjunction — When two bodies appear close together in the sky, or a planet lines up with the Sun.
• Constellation — One of 88 official patterns of stars used to map the sky. See Greek Myths in the Sky.
• Corona — The Sun's faint outer atmosphere, visible during a total solar eclipse. See The Sun.
• Cryovolcano — An "ice volcano" that erupts water, ammonia, or methane instead of molten rock, as on Enceladus and Triton. See Enceladus.
• Collimation — Aligning a reflector's mirrors for a sharp image; a quick, routine adjustment, not a chore to fear. See Telescope Types.
• Chandrasekhar Limit — The maximum mass of a white dwarf, about 1.4 solar masses; beyond it, electron degeneracy fails and the star collapses or detonates as a Type Ia supernova. See White Dwarfs.
• Cosmic Microwave Background (CMB) — The oldest light in the universe, released 380,000 years after the Big Bang; a faint microwave glow filling all of space and the practical edge of the observable universe. See Cosmic Microwave Background.
• Cosmic Web — The largest-scale structure of the universe: galaxies strung along vast filaments and walls around enormous empty voids. See The Cosmic Web.
• Cosmological Constant — A constant energy of empty space, once added by Einstein and now the leading description of dark energy — a steady outward pressure driving accelerating expansion. See Dark Energy.
• Cosmology — The study of the universe as a whole — its origin, contents, structure, and fate. See Cosmology.

D

• Dwarf Planet — A round, Sun-orbiting body that has not cleared its orbital neighborhood — Ceres, Pluto, Eris, Haumea, and Makemake. See Dwarf Planets.
• Dobsonian — A Newtonian reflector on a simple, sturdy rocker-box mount; the best aperture-per-dollar and the usual first serious telescope. See Telescope Guide.
• Direct Imaging — The rare exoplanet-detection method that actually photographs a planet by masking the star's glare; works only for young, giant planets in wide orbits. See Finding Exoplanets.
• Drake Equation — Frank Drake's 1961 chain of factors for estimating how many communicating civilizations might exist in the galaxy; a framework for organizing our ignorance more than a solvable formula. See Habitable Zone.
• Degeneracy Pressure — A quantum effect that resists compression and holds up dead stars: electron degeneracy supports white dwarfs, neutron degeneracy supports neutron stars. See White Dwarfs.
• Dark Nebula — A cold, dense dust cloud that blocks the light behind it, appearing as a black silhouette on the sky — and a birthplace of stars. See Dark Nebulae.
• Dark Energy — The unknown influence, about 68% of the universe, driving its expansion to accelerate; the biggest open mystery in cosmology. See Dark Energy.
• Dark Matter — Invisible mass, about 27% of the universe, detected only by its gravity; it holds galaxies together and forms the scaffolding of the cosmic web. See Dark Matter.

E

• Eccentricity — A measure of how stretched (non-circular) an orbit is; 0 is a perfect circle.
• Eclipse (Solar) — When the Moon passes in front of the Sun, casting its shadow on Earth. See Solar Eclipses.
• Ecliptic — The Sun's yearly path across the sky and the plane of Earth's orbit; the Moon and planets stay near it. See Zodiacal Cloud.
• Elongation — The angle between a planet and the Sun in our sky; greatest elongation is the best time to spot Mercury or Venus. See Mercury.
• Emission Nebula — A glowing cloud of gas (an H II region) energized to shine its own light by hot young stars within; the star-forming nebulae like Orion. See Emission Nebulae.
• Event Horizon — A black hole's point of no return: the boundary within which the escape velocity exceeds the speed of light, so nothing, not even light, can get back out. See What Is a Black Hole?.
• Event Horizon Telescope (EHT) — A planet-scale network of radio observatories linked into one virtual Earth-sized telescope; it produced the first images of black holes (M87* in 2019, Sagittarius A* in 2022). See Black Holes.
• Elliptical Galaxy — A smooth, featureless ball of mostly old stars with little gas and almost no new star formation; the largest galaxies in the universe are giant ellipticals built by mergers. See Galaxy Types.
• Exoplanet — A planet that orbits a star other than the Sun; more than 6,000 are now confirmed. See Exoplanets.
• Expansion of Space — The growth of the universe in which the distance between galaxies increases over time — galaxies are not so much moving through space as space is growing between them. See The Expanding Universe.
• Einstein, Albert — The physicist whose 1915 theory of general relativity underlies all of modern cosmology. His equations implied an expanding universe; the "cosmological constant" he added to resist that and later regretted is now the leading description of dark energy. See Dark Energy.

F

• Frost Line — The distance from the young Sun beyond which ices could condense — the divide between the rocky planets and the giants. See How It Formed.
• Fusion (Nuclear) — The merging of light atomic nuclei into heavier ones, releasing energy; how the Sun shines. See The Sun.
• Focal Length — The distance a telescope's optics take to bring light to a focus; longer focal length gives more magnification with a given eyepiece.
• Focal Ratio (f/number) — Focal length divided by aperture; lower ("faster") ratios give wider, brighter fields, higher ratios suit high-power planetary views.
• Fermi Paradox — The tension between a galaxy full of ancient, planet-rich star systems and the apparent absence of any sign of other civilizations: "where is everybody?" See Habitable Zone.

G

• Galaxy — A vast, gravitationally bound system of stars, gas, and dust; our own is the Milky Way. See Andromeda Galaxy.
• Gas Giant — A large planet made mostly of hydrogen and helium with no solid surface — Jupiter and Saturn. See Jupiter.
• Gegenschein — A faint glow directly opposite the Sun, from sunlight reflecting straight back off interplanetary dust. See Zodiacal Cloud.
• Giant Impact — A collision between growing worlds; the leading explanation for the Moon's origin. See The Moon.
• Great Red Spot — A storm on Jupiter wider than Earth, observed for over 150 years. See Jupiter.
• Gravitational Waves — Ripples in spacetime produced by violent events such as merging black holes, first detected directly in 2015; a new way to "hear" black holes rather than see them. See Stellar Black Holes.
• Gamow, George — Physicist who, with Ralph Alpher and Robert Herman, predicted in the late 1940s that a hot Big Bang would leave a faint afterglow — the cosmic microwave background, detected in 1964 and identified in 1965. See Cosmic Microwave Background.
• Geller, Margaret — Astronomer who, with John Huchra, made the first 3-D maps of the nearby universe in the 1980s, revealing its frothy, weblike large-scale structure and the "Great Wall." See The Cosmic Web.

H

• Heliocentric — Sun-centered; the model, confirmed by Galileo's observations, in which the planets orbit the Sun.
• Hydrogen-alpha (H-alpha) — A red wavelength of light used in special solar telescopes to reveal prominences and flares. See The Sun.
• H II Region — A cloud of ionized hydrogen glowing near hot young stars — the technical name for an emission nebula and a marker of active star formation. See Emission Nebulae.
• Habitable Zone (Goldilocks Zone) — The band of distances around a star where a planet's surface could hold liquid water — not too hot, not too cold. Being in it is necessary for surface water, not sufficient for life. See Habitable Zone.
• Hot Jupiter — A gas-giant exoplanet orbiting scaldingly close to its star, with a "year" lasting only days; the first class of exoplanet discovered. See Famous Exoplanets.
• Hertzsprung–Russell Diagram — The master chart of stars, plotting brightness against temperature; most stars fall along its diagonal "main sequence," and a star's position reveals its stage of life. See Main Sequence.
• Heat Death — The far-future state of maximum entropy in which energy is spread evenly and no further work is possible; the endpoint of the Big Freeze. See Fate of the Universe.
• Hubble's Law — The observation that more distant galaxies recede faster, in direct proportion to distance; its slope — the Hubble constant — measures the expansion rate of the universe. See The Expanding Universe.
• Hubble Tension — The unresolved disagreement between two precise measurements of the cosmic expansion rate (about 67 versus 73), a leading open problem in modern cosmology. See The Expanding Universe.
• Hubble, Edwin — The astronomer who proved other galaxies exist (1924) and that the universe is expanding (1929); both the Hubble Space Telescope and the Hubble constant carry his name. See The Expanding Universe.
• Hoyle, Fred — Astronomer and champion of the rival "steady state" theory who coined the term "Big Bang" in 1949 — mockingly, for an idea he opposed. The name stuck; the theory won. See The Big Bang.

I

• Ice Giant — A giant planet richer in heavier "ices" (water, ammonia, methane) than in gas — Uranus and Neptune. See Neptune.
• Ion Tail — A comet's straight, bluish tail of charged gas, blown directly away from the Sun by the solar wind. See Comets.

K

• Kirkwood Gaps — Nearly empty lanes in the Asteroid Belt, swept clear by resonances with Jupiter's gravity. See Asteroid Belt.
• Kuiper Belt — The icy belt beyond Neptune, home to Pluto and the short-period comets. See Kuiper Belt.

L

• Lemaître, Georges — Belgian priest and physicist who proposed the expanding universe (1927) and traced it to an origin he called the "primeval atom" — the seed of Big Bang theory — two years before Hubble’s observations confirmed it. See The Big Bang.
• Local Group — The small cluster of galaxies our Milky Way belongs to, dominated by the Milky Way and Andromeda plus dozens of smaller members. See Galaxies.
• Libration — The slight rocking of the Moon that lets us see a little more than half its surface over time. See The Moon.
• Light-year — The distance light travels in one year, about 9.5 trillion km (5.9 trillion miles); the nearest exoplanet, Proxima b, lies about 4.2 light-years away. See Famous Exoplanets.

M

• Magnitude — A measure of brightness; lower (even negative) numbers are brighter. A dark-sky limit for the naked eye is about magnitude 6.
• Maria — The dark plains of ancient hardened lava on the Moon (Latin for "seas"). See The Moon.
• Messier Object — One of 110 bright deep-sky objects catalogued by Charles Messier, like M31 (Andromeda) and M42 (Orion Nebula). See Andromeda Galaxy.
• Meteor — The streak of light ("shooting star") as a meteoroid burns up in the atmosphere. See Meteor Showers.
• Meteorite — A piece of space rock that survives the fall and lands on the ground. See Meteor Showers.
• Meteoroid — A small rock or dust grain in space, before it enters the atmosphere. See Meteor Showers.
• Meteor Shower — A yearly event when Earth passes through a comet's dust trail and many meteors radiate from one point. See Meteor Showers.
• Magnification — How much larger a telescope makes an object appear; limited by aperture and the atmosphere, so far less important than sellers' big numbers suggest. See Telescope Guide.
• Magnetar — A neutron star with an extreme magnetic field, trillions of times Earth's — the most powerful magnets known, capable of galaxy-spanning flares. See Neutron Stars & Pulsars.
• Messier Catalog — A list of 110 fuzzy sky objects compiled by comet-hunter Charles Messier in the 1700s — first to avoid mistaking them for comets, now the classic to-do list of deep-sky observers. See Nebulae.
• Main Sequence — The long, stable stage where a star fuses hydrogen into helium in its core, spending about 90% of its life; the Sun is here now. See Main Sequence.
• Molecular Cloud — A vast, cold cloud of interstellar gas and dust that is the birthplace of stars; when its dense knots collapse, new stars form. See Star Formation.
• Milky Way — Our home galaxy — a barred spiral about 100,000 light-years across holding 100–400 billion stars, seen from inside as the band of light across a dark sky. See The Milky Way.

N

• Nebula — A cloud of gas and dust in space; some are stellar nurseries, others the wreckage of dying stars. See Orion Nebula.
• Nebular Hypothesis — The theory that the Solar System formed from a collapsing, spinning cloud of gas and dust. See How It Formed.
• Node — A point where the Moon's tilted orbit crosses Earth's orbital plane; eclipses happen only near a node. See Solar Eclipses.
• Nucleus (Comet) — The solid, icy core of a comet, usually just a few kilometers across. See Comets.
• Neutron Star — The ultra-dense collapsed core left by a supernova when the star is too light to form a black hole; a teaspoon of its material would weigh billions of tons. See Neutron Stars & Pulsars.
• Nucleosynthesis — The forging of chemical elements inside stars and in stellar explosions; nearly every atom heavier than helium was made this way. See Supernovae.

O

• Occultation — When one body passes in front of and hides another, such as the Moon covering a star, or an object crossing a distant star to reveal its size.
• Oort Cloud — A vast spherical shell of icy bodies far beyond the Kuiper Belt, source of the long-period comets. See Scattered Disk & Oort Cloud.
• Opposition — When a planet is opposite the Sun in our sky, at its brightest and best for viewing. See Mars.
• Orbit — The curved path one body follows around another under gravity.
• Observable Universe — The region of the cosmos whose light has had time to reach us — a sphere about 93 billion light-years across, centered on the observer. See The Observable Universe.
• Olbers' Paradox — The puzzle of why the night sky is dark rather than blazing with starlight; resolved by the universe having a finite age and by its expansion. See The Observable Universe.
• Olbers, Heinrich — German astronomer who in 1823 popularized the puzzle now called Olbers’ paradox: why the night sky is dark rather than ablaze with starlight. See The Observable Universe.

P

• Penumbra — The lighter, outer part of a shadow, where an eclipse appears only partial. See Solar Eclipses.
• Perihelion — The point in an orbit closest to the Sun. (Its opposite is aphelion.)
• Phases — The changing sunlit fractions we see of the Moon (or of Mercury and Venus) through a cycle. See The Moon.
• Photosphere — The Sun's visible "surface," the layer from which its light escapes. See The Sun.
• Planet — A round body that orbits the Sun and has cleared its orbital neighborhood of other objects. See The Planets.
• Planetesimal — A kilometer-sized building block of planets, formed by accretion in the early disk. See How It Formed.
• Plasma — A hot, ionized gas of charged particles; the state of matter the Sun is made of. See The Sun.
• Prominence — A loop or arc of glowing plasma rising above the Sun's surface. See Sunspots & the Solar Cycle.
• Planetary Nebula — The glowing shell of gas a dying Sun-like star sheds as it becomes a white dwarf; a historical misnomer — it has nothing to do with planets. See White Dwarfs.
• Protoplanetary Disk — The flattened, spinning disk of gas and dust around a young star, in which planets form. See How It Formed.
• Protostar — A star-to-be: a collapsing, heating clump of cloud material that glows from its own contraction but has not yet ignited nuclear fusion. See Star Formation.
• Pulsar — A rapidly spinning neutron star whose beams of radiation sweep past Earth as clockwork pulses, like a cosmic lighthouse. See Neutron Stars & Pulsars.
• Penzias, Arno & Robert Wilson — The two Bell Labs radio engineers who accidentally discovered the cosmic microwave background in 1964–65, at first mistaking it for interference; they shared the 1978 Nobel Prize. See Cosmic Microwave Background.
• Perlmutter, Schmidt & Riess — Saul Perlmutter, Brian Schmidt, and Adam Riess — leaders of the two teams that discovered the accelerating universe in 1998, the evidence for dark energy — who shared the 2011 Nobel Prize. See Dark Energy.

Q

• Quasar — An intensely bright galactic core powered by a feeding supermassive black hole; its blazing accretion disk can outshine an entire galaxy and be seen across the universe. See Supermassive Black Holes.

R

• Radiant — The single point in the sky from which a meteor shower's streaks appear to spread. See Meteor Showers.
• Red Giant — A large, cool, luminous late stage of a Sun-like star, such as Arcturus; the Sun's fate in about five billion years. See Red Giants.
• Retrograde — Backward motion — either a body's reverse spin (Venus) or a planet's apparent backward drift against the stars. See Venus.
• Ring System — A disk of countless icy and rocky particles orbiting a planet; all four giants have one. See Planetary Rings.
• Radial Velocity — An exoplanet-detection method that measures the tiny wobble a planet's gravity induces in its star, seen as a rhythmic Doppler color-shift; the method behind the first discovery. See Finding Exoplanets.
• Red Dwarf — A small, cool, long-lived star, the most common type in the galaxy; many known exoplanets, including Proxima b and TRAPPIST-1, orbit red dwarfs. See Famous Exoplanets.
• Reflector — A telescope that gathers light with a mirror (the Newtonian design); the cheapest route to a big aperture. See Telescope Types.
• Refractor — A telescope that gathers light with a lens at the front; low-fuss and crisp on the Moon and planets. See Telescope Types.
• Reflection Nebula — A dust cloud that shines by scattering the light of a nearby star, glowing blue like the daytime sky; e.g. the haze around the Pleiades. See Reflection Nebulae.
• Redshift — The stretching of light to longer, redder wavelengths as it crosses expanding space; the main way astronomers measure cosmic distance and expansion. See The Expanding Universe.
• Rubin, Vera — American astronomer whose 1970s measurements of flat galaxy rotation curves made the case for dark matter undeniable; widely regarded as overdue for a Nobel Prize she never received. See Dark Matter.

S

• Saros Cycle — The roughly 18-year rhythm in which eclipses repeat. See Solar Eclipses.
• Scattered Disk — A region of stretched, tilted icy orbits beyond the Kuiper Belt, home to Eris. See Scattered Disk & Oort Cloud.
• Solar Cycle — The roughly 11-year rise and fall in the Sun's activity, tracked by sunspots. See Sunspots & the Solar Cycle.
• Solar Wind — The steady stream of charged particles flowing out from the Sun; it shapes comet tails and drives auroras. See The Sun.
• Sunspot — A cooler, darker patch on the Sun where magnetic fields suppress heat. See Sunspots & the Solar Cycle.
• Supergiant — Among the largest, most luminous stars, like Betelgeuse — destined to explode as a supernova. See Betelgeuse.
• Supernova — The catastrophic explosion of a massive star at the end of its life; core-collapse (Type II) or a white-dwarf detonation (Type Ia). See Supernovae.
• Supernova Remnant — The expanding, glowing wreckage of an exploded star, such as the Crab and Veil nebulae; scatters heavy elements into space. See Supernova Remnants.
• Spiral Galaxy — A galaxy with a flat rotating disk, central bulge, and arms of gas and young stars — the classic pinwheel; most also have a central bar. See Galaxy Types.
• Schwarzschild Radius — The radius of a black hole's event horizon, set entirely by its mass; double the mass and you double the radius. See What Is a Black Hole?.
• Singularity — The center of a black hole, where matter is crushed (in the equations) to infinite density and known physics breaks down; the great unsolved problem at a black hole's core. See What Is a Black Hole?.
• Spaghettification — The stretching of an object into a thin strand by the extreme difference in gravity near a black hole; strongest for the smallest black holes. See What Is a Black Hole?.
• Super-Earth — A rocky exoplanet larger than Earth but smaller than Neptune; possibly the most common planet type in the galaxy, though we have none in our Solar System. See What Is an Exoplanet?.
• Supermassive Black Hole — A black hole of millions to billions of solar masses found at the center of most large galaxies, including the Milky Way's Sagittarius A*. See Supermassive Black Holes.
• Sagan, Carl — Astronomer and science communicator whose book and series "Cosmos" (1980) brought the universe to a mass audience; famous for "we are made of star-stuff" and the "Pale Blue Dot." See Cosmology.
• Smoot, George — Astrophysicist who led the COBE satellite team that mapped the tiny ripples in the cosmic microwave background in 1992 — the seeds of all cosmic structure — sharing the 2006 Nobel Prize. See Cosmic Microwave Background.

T

• Terminator — The line dividing day from night on a moon or planet; where shadows are longest and detail is sharpest. See The Moon.
• Terrestrial Planet — A small, dense, rocky planet with a solid surface — Mercury, Venus, Earth, Mars. See The Planets.
• Tidal Locking — Rotating once per orbit, so the same face always points at the partner, as the Moon does to Earth. See The Moon.
• Tiger Stripes — The warm south-polar fractures on Enceladus from which its water geysers erupt. See Enceladus.
• Trans-Neptunian Object (TNO) — Any icy body orbiting beyond Neptune, in the Kuiper Belt or scattered disk. See Kuiper Belt.
• Transit (Method) — The most productive exoplanet-detection method: watching for the small, repeating dip in a star's brightness when a planet crosses in front of it. The dip's depth reveals the planet's size. See Finding Exoplanets.
• Tidal Tail — A long streamer of stars and gas pulled out of a galaxy by the gravity of a passing or merging galaxy — the signature of a galactic collision. See Galaxy Collisions.

U

• Ultramassive Black Hole — The most extreme supermassive black holes, of tens of billions of solar masses, like TON 618.
• Umbra — The dark, inner part of a shadow, where an eclipse is total. See Solar Eclipses.

V

• Variable Star — A star whose brightness changes over time, whether by pulsation (like Betelgeuse) or eclipses. See Betelgeuse.

W

• White Dwarf — The dense, Earth-sized ember left when a Sun-like star sheds its outer layers, often lighting a planetary nebula. See White Dwarfs.

Z

• Zwicky, Fritz — Swiss astronomer who in 1933 found galaxy clusters moving too fast for their visible mass and coined the term "dark matter" (dunkle Materie) — ignored for forty years before being vindicated. See Dark Matter.