The Astronomer's Atlas

The six people look up most

Black holeA region where gravity is so strong that nothing, not even light, escapes.

RedshiftLight stretched to longer wavelengths: how we measure cosmic distance and expansion.

Light-yearNot a time but a distance: how far light travels in a year, 9.46 trillion km.

SpectrumSplit starlight into colours and it tells you what a star is made of.

Main sequenceThe long, stable, hydrogen-burning middle age of a star. The Sun is in it.

Dark matterUnseen mass, five times the ordinary kind, holding galaxies together.

Browse by theme26

Stars17

Binary star: Two stars orbiting each other. Roughly half of all Sun-like stars have at least one companion.
Brown dwarf: A 'failed star' — bigger than a planet but too small to ignite hydrogen fusion. It glows dimly with leftover heat.
Cepheid variable: A pulsing giant star whose rhythm reveals its true brightness — astronomy's classic distance ruler.
Flare star: A star (usually a red dwarf) prone to sudden brightening as its magnetic field snaps — bad news for close-orbiting planets.
Globular cluster: A dense ball of hundreds of thousands of ancient stars orbiting a galaxy — some nearly as old as the universe.
Main sequence: The long, stable adulthood of a star, fusing hydrogen in its core. The Sun is about halfway through its ~10-billion-year run.
Molecular cloud: A cold, dense cloud of gas and dust — the raw material and birthplace of stars.
Nebula: A cloud of gas and dust in space — a star nursery, a dying star's shed skin, or an explosion's wreckage.
Planetary nebula: The glowing shells a dying Sun-like star exhales — nothing to do with planets; early telescopes just saw round disks.
Red dwarf: A small, cool, dim star — the most common type in the galaxy, burning so slowly it can live trillions of years.
Red giant: The swollen late-life phase of a Sun-like star — cooler at the surface but enormously bigger and brighter. The Sun's turn comes in ~5 billion years.
Spectral type: A star's temperature class — O, B, A, F, G, K, M from hottest blue to coolest red. The Sun is G2. (Mnemonic: Oh Be A Fine Girl/Guy, Kiss Me.)
Star: A ball of gas massive enough to ignite nuclear fusion in its core and shine by its own light.
Star cluster: Stars born together and still traveling together — loose 'open' clusters like the Pleiades, or dense ancient globulars.
Supergiant: A star in the most massive stars' brief, brilliant late phase — like Betelgeuse — destined for a supernova.
Variable star: A star whose brightness changes — by pulsing, erupting, or being eclipsed by a companion.
White dwarf: The Earth-sized, still-glowing ember left when a Sun-like star dies — a teaspoon weighs tonnes; it will cool for trillions of years.

Planets13

Atmosphere: The layer of gases held around a world by its gravity.
Axial tilt: The lean of a planet's spin axis relative to its orbit. Earth's 23.4° tilt is why seasons exist.
Crater: A bowl-shaped scar left by an impact. Airless worlds keep them for billions of years.
Exoplanet: A planet orbiting a star other than the Sun. Nearly 6,000 are confirmed, with thousands more candidates.
Exosphere: An atmosphere so thin its atoms almost never collide — Mercury and the Moon have only this.
Gas giant: A planet made mostly of hydrogen and helium with no solid surface — Jupiter and Saturn.
Ice giant: A planet like Uranus or Neptune: a deep fluid interior of water, methane and ammonia under a hydrogen envelope.
Planet: A world orbiting a star that is round under its own gravity and has cleared its orbital lane. The Solar System has eight.
Protoplanetary disk: The spinning disk of gas and dust around a newborn star, from which planets condense — many are now directly photographed.
Regolith: The layer of loose, broken rock and dust blanketing airless worlds — lunar regolith is powder-fine and razor-sharp.
Rings: Disks of orbiting ice and rock particles around a planet. All four giant planets have them; Saturn's are simply the masterpiece.
Rogue planet: A planet wandering the galaxy without a star, ejected during its system's rough youth — microlensing suggests billions exist.
Terrestrial planet: A rocky world like Mercury, Venus, Earth or Mars — metal core, rocky mantle, solid surface.

Orbits11

Aphelion: The point in an orbit farthest from the Sun.
Barycenter: The shared balance point two bodies actually orbit around. For Pluto and Charon it lies in open space between them.
Eccentricity: How stretched an orbit is: 0 is a perfect circle; closer to 1 is a long ellipse like a comet's.
Escape velocity: The speed needed to leave a world's gravity without further pushing: 11.2 km/s from Earth, 2.4 km/s from the Moon.
Lagrange point: One of five places where two bodies' gravity and orbital motion balance, letting spacecraft 'park'. JWST lives at Sun–Earth L2.
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L1 (SOHO's sunward perch) and L2 are meta-stable; L4/L5 are stable and collect Trojan asteroids.
Orbit: The path of one body around another — falling forever and always missing.
Perihelion: The point in an orbit closest to the Sun.
Prograde & retrograde: Moving with (prograde) or against (retrograde) the usual direction of spin or orbit. Venus spins retrograde; Triton orbits retrograde.
Roche limit: The distance inside which tides tear a moon apart — stray closer and you become a ring.
Tidal locking: When tides brake a body's spin until one face permanently faces its partner — the Moon to Earth, and many exoplanets to their stars.
Yarkovsky effect: The tiny push sunlight gives a spinning asteroid as it re-radiates heat — enough to reshape orbits over millennia, and measured directly on Bennu.

Galaxies11

Blazar: A galaxy whose central black hole fires a jet of particles almost straight at Earth, making it appear brilliantly bright and variable.
Dwarf galaxy: A small galaxy of millions to a few billion stars. They are the most common galaxies — and big galaxies grow by eating them.
Elliptical galaxy: A smooth, egg-shaped galaxy of mostly old stars, usually built from galaxy mergers.
Galaxy: A gravitationally bound city of stars, gas, dust and dark matter — from dwarfs of a few million stars to giants of trillions.
Galaxy cluster: Hundreds to thousands of galaxies bound together — the largest gravitationally settled structures in the universe.
Interstellar medium: The thin gas and dust between stars — about one atom per cubic centimeter, yet enough to build new suns.
Irregular galaxy: A galaxy without tidy spiral or elliptical shape — often small, gas-rich, and star-forming, like the Magellanic Clouds.
Milky Way: Our home galaxy of 100–400 billion stars — and the glowing band across dark skies, which is its disk seen from inside.
Quasar: The blazing core of a distant galaxy whose supermassive black hole is feeding — often outshining its entire galaxy from billions of light-years away.
Spiral galaxy: A rotating disk galaxy with winding arms of stars and gas — like the Milky Way and Andromeda.
Supercluster: A collection of galaxy clusters — like Laniakea, the 'immeasurable heaven' our Milky Way belongs to.

Light11

Blueshift: The squeezing of light waves toward bluer colors when a source moves toward you.
Doppler effect: The change in waves from a moving source — sound rising as an ambulance nears, starlight shifting color as a star moves.
Electromagnetic spectrum: The full family of light: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma-ray. Our eyes catch one narrow slice.
Infrared: Heat-light with waves longer than red. It pierces dust clouds and carries the stretched light of the first galaxies — JWST's specialty.
Photon: A particle of light — the messenger carrying almost everything we know about the universe.
Radio astronomy: Astronomy with radio waves — revealing pulsars, quasars, the CMB, black-hole images, and gas invisible to optical telescopes.
Redshift: The stretching of light waves toward red as a source recedes — for distant galaxies, space itself expanding en route does the stretching, making redshift cosmology's odometer.
Spectroscopy: Splitting light into its rainbow of colors to read chemical fingerprints — how we know what stars are made of without visiting.
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Every element absorbs/emits at signature wavelengths. Spectroscopy also yields temperatures, speeds, magnetic fields, and exoplanet air.
Ultraviolet: Light with waves shorter than violet — sunburn light — revealing hot stars and energetic processes; mostly blocked by our atmosphere.
Wavelength: The distance between wave crests — what distinguishes radio from visible from gamma-ray light.
X-ray astronomy: Astronomy with X-rays — the light of million-degree gas around black holes, neutron stars, and galaxy clusters. Must be done from space.

Sky events11

Conjunction: When two objects appear close together in the sky — like the Moon passing a bright planet.
Eclipse: One body passing through another's shadow — the Moon darkening in Earth's shadow, or the Sun hidden behind the Moon.
Elongation: A planet's apparent angle from the Sun. Big elongation = high, dark-sky views; small = lost in twilight.
Equinox: The two moments each year when the Sun crosses the celestial equator and day and night are nearly equal everywhere (around March 20 and September 22).
Lunar eclipse: The full Moon passing through Earth's shadow, often turning sunset-red — safely visible to everyone on the night side.
Meteor shower: Many meteors radiating from one point, seen when Earth crosses a comet's dust trail on the same dates each year.
Occultation: One body hiding another — the Moon covering a star, or an asteroid briefly blocking one (how rings around small worlds were found).
Opposition: When an outer planet sits opposite the Sun in our sky — rising at sunset, up all night, and closest to Earth. Prime viewing.
Solar eclipse: The Moon covering the Sun. Totality — day turning to night, corona blazing — is widely called nature's greatest spectacle.
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Only safe to look at with proper eclipse glasses except during the seconds-to-minutes of totality itself.
Solstice: The Sun's northernmost or southernmost day of the year (around June 20 and December 21) — longest and shortest days.
Zodiacal light: A faint pyramid of light on the horizon after dusk or before dawn — sunlight scattered off comet and asteroid dust in the plane of the Solar System.

Measuring10

Absolute magnitude: How bright a star would look from a standard distance of 32.6 light-years — a fair way to compare stars' true power.
Albedo: The fraction of light a surface reflects. Fresh snow is near 0.9; charcoal-dark asteroids sit near 0.04.
Apparent magnitude: How bright something looks from Earth. Confusingly, smaller numbers are brighter — magnitude 1 outshines magnitude 6.
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The scale is logarithmic: 5 magnitudes = 100× brightness. Negative values are brightest: the Sun is −26.7, Sirius −1.46; the faintest naked-eye stars are ~+6.
Astronomical unit (AU): The average Earth–Sun distance: 149.6 million km. The Solar System's everyday ruler.
Density: How much mass is packed into a given space. Saturn's average density is lower than water's.
Light-year: The distance light travels in one year: 9.46 trillion km. A ruler for the space between stars.
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Light speed makes telescopes time machines: see something 1,000 light-years away and you see it as it was 1,000 years ago.
Luminosity: An object's true total power output, regardless of distance.
Parallax: The tiny apparent shift of a nearby star against the background as Earth circles the Sun — geometry that yields true distances.
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The parsec (3.26 ly) is defined from it. ESA's Gaia has measured parallaxes for nearly two billion stars.
Parsec: 3.26 light-years — the professional unit of stellar distance, defined by parallax geometry.
Proper motion: A star's slow drift across the sky from its real velocity — Barnard's Star, the fastest, crosses a Moon-width in 180 years.

Cosmology9

Big Bang: The event 13.8 billion years ago when the universe began expanding from an extremely hot, dense state — the start of space, time, matter and energy as we know them.
Cosmic microwave background: The oldest light in existence: a faint glow from when the universe first became transparent, 380,000 years after the Big Bang.
Cosmology: The study of the universe as a whole — its origin, contents, shape and fate.
Dark energy: The name for whatever is making the universe's expansion speed up. It behaves like an energy of empty space — and nobody knows what it is.
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Discovered in 1998 via distant supernovae; it makes up ~68% of the universe's energy budget. Its nature is arguably the biggest open problem in physics.
Dark matter: Invisible material that outweighs normal matter five to one. We map it by its gravity — galaxies spin and cluster as if held by unseen mass.
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Candidates range from undiscovered particles (WIMPs, axions) to primordial black holes. Every direct-detection experiment so far has come up empty — the mystery is real and quantified.
Hubble constant: The universe's current expansion rate. Two rigorous methods disagree by ~8% — the 'Hubble tension', possibly a crack pointing at new physics.
Inflation: The proposed burst of exponential expansion in the universe's first fraction of a second — explaining its smoothness and seeding all structure. Strongly motivated, not yet directly proven.
Multiverse: The speculative idea that our universe is one of many. Some versions follow naturally from inflation theory — none is currently testable, which is exactly what 'Speculative' means on this site.
Universe: Everything there is: all space, time, matter and energy. The observable part spans ~93 billion light-years — the whole may be far larger, possibly infinite.

Black holes7

Accretion disk: A spinning disk of gas spiraling into a massive object like a black hole, heated to glowing by friction.
Black hole: An object so dense that inside a boundary called the event horizon, nothing — not even light — can escape its gravity.
Event horizon: A black hole's point of no return — the boundary where escape would require faster-than-light travel.
Jet: A beam of particles flung out near light speed along a feeding black hole's or neutron star's spin axis.
Singularity: The point of infinite density general relativity predicts at a black hole's center — usually read as the place where the theory itself breaks down and quantum gravity must take over.
Spaghettification: The stretching of anything falling into a small black hole, as gravity pulls far harder on your feet than your head. Real physics, real term.
Supermassive black hole: A black hole of millions to billions of solar masses, found at the center of nearly every big galaxy.

Small bodies7

Asteroid: A rocky object orbiting the Sun, smaller than a planet — most live in the belt between Mars and Jupiter.
Comet: A mountain-sized body of ancient ice and dust that grows a glowing head and tails when sunlight heats it.
Interstellar object: A body passing through our Solar System from another star — three known so far: 'Oumuamua, Borisov, and 3I/ATLAS.
Meteor: The streak of light — a 'shooting star' — as a space pebble burns up in the atmosphere.
Meteorite: A piece of space rock that survives the fall and lands. (In space: meteoroid → burning: meteor → on the ground: meteorite.)
Meteoroid: A small rock or particle traveling through space — sand-grain to boulder-sized.
Trojan asteroid: An asteroid sharing a planet's orbit at the stable Lagrange points 60° ahead or behind — Jupiter has over 10,000 known.

Solar System6

Asteroid belt: The zone between Mars and Jupiter holding millions of asteroids — leftovers that Jupiter's gravity never let become a planet.
Dwarf planet: A round world orbiting the Sun that hasn't cleared its orbital lane — like Pluto, Eris, and Ceres.
Heliosphere: The vast bubble blown by the solar wind, enclosing all the planets. Both Voyagers have crossed its edge into interstellar space.
Kuiper Belt: The ring of icy worlds beyond Neptune — home of Pluto, Arrokoth, and millions of frozen leftovers.
Oort Cloud: The hypothesized shell of trillions of comets extending halfway to the next star — the deep-freeze source of long-period comets. Inferred from comet orbits; never directly seen.
Planetesimal: A building block of planets — the km-scale bodies that merged into worlds 4.5 billion years ago.

Extreme events6

Gamma-ray burst: The brightest explosions known: seconds-long flashes from collapsing giant stars or colliding neutron stars, visible across the universe.
Magnetar: A neutron star with the strongest magnetic field known — a trillion times Earth's — capable of galaxy-visible outbursts.
Neutron star: The city-sized, Sun-massed corpse of an exploded giant star — matter at nuclear density, one teaspoon outweighing a mountain.
Nova: A sudden brightening when a white dwarf's stolen hydrogen detonates on its surface — the star survives to flash again.
Pulsar: A spinning neutron star sweeping lighthouse beams of radiation past Earth — some keep time better than atomic clocks.
Supernova: The explosion of a star — collapsing giant or detonating white dwarf — briefly outshining its whole galaxy and forging heavy elements.
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Type II: core collapse of a giant. Type Ia: white dwarf detonation — standardizable candles that mapped cosmic acceleration.

Life5

Astrobiology: The science of life in the universe: how it starts, where it could survive, and how we would detect it.
Biosignature: A measurable clue that life exists somewhere — like oxygen and methane coexisting in a planet's atmosphere.
Habitable zone: The distance band around a star where a planet with the right atmosphere could keep liquid water on its surface — neither boiled nor frozen.
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A necessary-but-not-sufficient test: Venus and Mars technically skirt the Sun's zone. Atmosphere, magnetic field, and geology decide the rest.
SETI: The Search for Extraterrestrial Intelligence — listening for signals deliberately or accidentally broadcast by other civilizations. Decades of silence so far constrain only a tiny slice of possibilities.
Technosignature: Evidence of technology elsewhere — a radio signal, city lights, industrial pollution in an atmosphere. None found; the search is young.

Space weather5

Aurora: Glowing curtains of light near a planet's poles, painted by solar particles guided along magnetic field lines.
Coronal mass ejection: A billion-tonne eruption of magnetized solar gas. If one hits Earth, expect auroras — and stressed power grids.
Magnetosphere: The magnetic bubble around a planet that deflects charged particles. Earth's is life's invisible shield.
Solar flare: A magnetic explosion on the Sun releasing the energy of billions of bombs in minutes, flashing across the spectrum.
Solar wind: The million-km/h stream of charged particles flowing off the Sun — sculptor of comet tails and auroras.

Sky patterns4

Asterism: A recognizable star pattern that isn't an official constellation — like the Big Dipper (part of Ursa Major) or the Summer Triangle.
Circumpolar: Stars that never set from your latitude, endlessly circling the celestial pole — like the Big Dipper from most of Europe and North America.
Constellation: One of 88 official regions the sky is divided into, each named for the figure its brightest stars suggested to earlier cultures.
Zodiac: The band of constellations along the ecliptic that the Sun, Moon and planets travel through. Astronomy maps it; astrology (a belief system, not a science) borrows its names.

Sky coordinates4

Celestial equator: Earth's equator projected onto the sky, dividing it into northern and southern halves.
Celestial pole: The point the sky appears to rotate around — directly above Earth's pole. Polaris sits almost exactly on the northern one.
Ecliptic: The Sun's yearly path across the sky — really Earth's orbital plane. The planets always wander close to this line.
Zenith: The point straight overhead.

The Sun4

Chromosphere: The thin reddish layer of the Sun's atmosphere just above its visible surface, glimpsed during total eclipses.
Corona: The Sun's wispy outer atmosphere, visible during total eclipses — mysteriously heated to over a million degrees.
Photosphere: The Sun's visible 'surface' — the layer where its gas becomes transparent, at about 5,500 °C.
Sunspot: A cooler, darker patch on the Sun where magnetic fields choke the heat flow — some grow larger than Earth.

Gravity4

Gravitational lens: Mass bending light like a glass lens: massive galaxies can magnify, distort and multiply the images of things behind them.
Gravitational wave: A ripple in space-time itself, launched by accelerating masses like colliding black holes — first detected in 2015.
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LIGO's detectors measure arm-length changes of 1/10,000th a proton's width. The 2017 neutron-star merger was seen in both waves and light, opening 'multi-messenger' astronomy.
Gravity: The attraction between masses — the force that shapes orbits, stars, galaxies and the universe's fate. In Einstein's picture, mass curves space-time and things follow the curves.
Tides: The stretching of one body by another's gravity varying across it — raising Earth's oceans twice daily and heating moons like Io and Europa.

Physics3

Hydrostatic equilibrium: The balance between gravity squeezing inward and pressure pushing outward — what makes stars and planets round.
Nuclear fusion: Squeezing light atomic nuclei together into heavier ones, releasing energy — the engine of every star and the origin of most elements.
Plasma: Gas so hot its electrons roam free — the state of matter stars are made of, and most visible matter in the universe.

Particles2

Cosmic ray: An atomic nucleus or particle traveling near light speed, launched by supernovae and other cosmic accelerators.
Neutrino: A ghostly particle that barely interacts with matter — trillions from the Sun pass through you every second. Supernovae announce themselves in neutrinos first.

History2

Geocentric: The ancient model placing Earth at the universe's center — overturned by Copernicus, Galileo and Kepler.
Heliocentric: The Sun-centered model of the Solar System — Copernicus's revolution.

Exoplanet hunting2

Radial velocity: The wobble-speed of a star along our line of sight — reading it from starlight's Doppler shift revealed the first exoplanets.
Transit: A planet crossing its star's face, dimming it slightly on a rhythm — the method that has found most known exoplanets, and a window into their atmospheres.

Observing1

Light pollution: Artificial light washing out the night sky. A third of humanity can no longer see the Milky Way.

Spaceflight1

Satellite: Anything orbiting something else — moons are natural satellites; thousands of artificial ones now circle Earth.

Space settlement1

Terraforming: The theoretical engineering of a planet into an Earth-like state. Even for Mars, the accessible CO₂ falls far short — a concept for centuries hence, not decades.

Speculative physics1

Wormhole: A theoretical tunnel connecting distant points of space-time, allowed by Einstein's equations under exotic conditions never observed. Firmly Theoretical — traversable versions require unknown 'negative energy' matter.