What the words mean

How bright a star would look if it were moved to a standard distance of 10 parsecs.

It lets stars be compared fairly. The Sun's is 4.83: from 10 parsecs it would be a faint, ordinary star.

See Apparent magnitude, Luminosity, Parsec

Growth by gathering: dust sticking into pebbles, pebbles into boulders, and so on up to planets.

See Protoplanetary disc, Differentiation

A flat, spinning disc of gas spiralling in towards a star or a black hole.

Friction in the disc can heat the gas to millions of °C, which is how black holes, dark themselves, come to be among the brightest things in the sky.

See Black hole, X-rays, AccretionLesson What a black hole is

A galaxy whose centre outshines all its stars, powered by gas falling towards its central black hole.

About one galaxy in ten is active. Seyfert galaxies, radio galaxies, quasars and blazars are all kinds of active galaxy.

See Supermassive black hole, Accretion disc, Quasar, Seyfert galaxy, BlazarLesson Active galaxies and quasars

The share of the light falling on a surface that it reflects.

The Moon looks bright but reflects only about 12%, much the same as worn tarmac.

See Luminosity

The width of a telescope's main lens or mirror. It sets how much light the telescope collects and how fine a detail it can show.

See Arcsecond, WavelengthLesson Telescopes

The point of an orbit furthest from the Sun.

The Earth reaches it in early July, at about 152.1 million km.

See Perihelion, ApoapsisLesson Kepler's three laws

The furthest point of an orbit from whatever is being orbited.

See Periapsis, Aphelion

How bright a star or planet looks from the Earth, on a scale where smaller numbers mean brighter.

Five magnitudes is a factor of exactly 100 in brightness. Sirius is -1.46, and the faintest stars the eye can see from a dark place are about 6.

See Absolute magnitude, Luminosity

A very small angle, one 3,600th of a degree.

The full Moon is about 1,800 arcseconds wide. A one pound coin looks one arcsecond wide from nearly 5 km away.

See Parallax, Parsec

A small body of rock or metal in orbit round the Sun.

Most are leftovers from the building of the planets.

See Asteroid belt, Trojan, Meteoroid

The region between the orbits of Mars and Jupiter where most known asteroids are found.

It is mostly empty space. Everything in it together has about 3% of the mass of the Moon.

See Asteroid, Resonance

The average distance between the Earth and the Sun, used as the ruler inside the Solar System.

It is fixed by definition at exactly 149,597,870.7 km, which light crosses in about 8 minutes 19 seconds.

See Light year, Parsec, Light timeLesson Measuring distance: kilometres, au, light years and parsecs

The layer of gas held round a planet or moon by its gravity.

See Greenhouse effect, Magnetosphere

Light in the sky near a planet's magnetic poles, made when charged particles strike the upper air.

Jupiter, Saturn, Uranus and Neptune have them too.

See Solar wind, Magnetosphere

The lean of a planet's spin axis away from upright on its orbit.

The Earth's is 23.44°, which gives it seasons. Uranus leans by about 98°, so it rolls round the Sun on its side.

See Solstice, Equinox, PrecessionLesson Seasons

The balance point that two bodies both go round.

For the Earth and Moon it lies inside the Earth, about 4,670 km from the centre. For the Sun and Jupiter it lies just outside the Sun's surface.

See Orbit, Mass

The hot, dense early state from which the universe has been expanding and cooling for 13.8 billion years.

It was not an explosion at one place in space. Space itself was expanding, everywhere at once.

See Cosmic microwave background, Hubble constant, RedshiftLesson The expanding universe

The making of the first elements, in the first few minutes of the universe, when all of it was hot enough for nuclear fusion.

It left about a quarter of ordinary matter as helium and nearly all the rest as hydrogen, with traces of deuterium and lithium.

See Big Bang, Nuclear fusion, Hydrogen, HeliumLesson The first minutes and the oldest light

Two stars held together by gravity, each going round the other. Three or more together make a multiple star.

The size of the orbit and the time for one turn give the masses of the two stars, which is how stars are weighed.

See Barycentre, Orbital period, MassLesson Double and multiple stars

Something we could measure from a distance that would be hard to explain without life.

Oxygen and methane together in a planet's air is one example. None has been confirmed anywhere but the Earth.

See Exoplanet, Habitable zone, AtmosphereLesson Are we alone?

A white dwarf that has cooled until it no longer shines. None is known, and the universe is thought to be too young to hold many yet.

See White dwarfLesson White dwarfs

A place where so much mass is packed so tightly that nothing which goes in, not even light, can come out.

Some form when the heaviest stars die. Far larger ones sit at the centres of galaxies.

See Event horizon, Escape velocity, Neutron starLesson What a black hole is

An ideal object that soaks up all the light that falls on it and glows with a colour set only by its temperature.

Stars come close. Cool ones glow red, the Sun yellow-white, and the hottest blue-white.

See Kelvin, Photosphere

An active galaxy with a jet pointing almost straight at the Earth.

Looking down the jet makes a blazar seem brighter and more changeable than other active galaxies. Most of the active galaxies found in gamma rays are blazars.

See Active galaxy, Quasar, Gamma raysLesson Active galaxies and quasars

A ball of gas too heavy to be a planet but too light to keep hydrogen fusion going, so it never becomes a true star.

The dividing line is at about 8 per cent of the Sun's mass, roughly 80 times the mass of Jupiter.

See Star, Nuclear fusion, Gas giantLesson What a star is

The Earth's equator pushed outward onto the sky.

It is tilted 23.4° to the ecliptic. The two cross at the points where the Sun stands at the equinoxes.

See Ecliptic, Equinox, Declination

One of the two points in the sky straight above the Earth's poles, around which the stars appear to turn.

Polaris sits less than a degree from the northern one at present. The southern one has no bright star nearby.

See Celestial sphere, Precession

An imaginary ball around the Earth with the stars painted on the inside, used for mapping the sky.

It works because the stars are so far off that they keep their pattern wherever on Earth you stand.

See Celestial equator, Celestial pole, Right ascension, Declination

A supergiant star that pulses on a steady beat of days or weeks. The longer the beat, the more luminous the star, which makes Cepheids markers of distance.

The rule was found by Henrietta Leavitt in the Magellanic Clouds. Polaris is the nearest Cepheid.

See Variable star, Standard candle, Distance ladder, LuminosityLesson Variable stars and the distance ladder

The most mass a white dwarf can have: about 1.4 times the mass of the Sun.

Above it the electrons that hold a white dwarf up can no longer carry the weight. Named after Subrahmanyan Chandrasekhar, who worked it out.

See White dwarf, Neutron star, SupernovaLesson White dwarfs

The thin layer just above the Sun's visible surface, seen as a red rim during a total eclipse.

See Photosphere, Corona

The thin cloud of gas and dust around the solid heart of an active comet.

It can grow wider than a planet, round a nucleus only a few kilometres across.

See Comet, Comet tail

A small body of ice, dust and rock that grows a cloud of gas and a tail when it comes near the Sun.

See Coma, Comet tail, Kuiper belt, Oort cloud

Gas and dust pushed off a comet by sunlight and the solar wind.

A tail always points away from the Sun, whichever way the comet is travelling.

See Comet, Coma, Solar wind

When two bodies appear close together in the sky, or when a planet lines up with the Sun.

See Opposition, Transit

One of the 88 named areas into which the whole sky is officially divided.

The stars of a constellation only line up from where we stand. Most are at very different distances and have nothing to do with one another.

See Celestial sphere, Zodiac

The outer third of the Sun, where heat is carried up by rising and sinking gas, like water at a rolling boil.

See Radiative zone, Granule, PhotosphereLesson What the Sun is

The inner quarter of the Sun's radius, the only part hot and dense enough for fusion.

The temperature at the centre is about 15.7 million kelvin.

See Nuclear fusion, Radiative zoneLesson What the Sun is

The Sun's thin outer atmosphere, which at over a million kelvin is far hotter than the surface beneath it.

Why it is so hot is not fully settled. By eye it can be seen only during a total solar eclipse.

See Chromosphere, Solar wind, X-rays

A huge cloud of plasma thrown off the Sun, which can reach the Earth in one to three days.

When one arrives it can set off bright aurorae and disturb power grids and satellites.

See Solar flare, Aurora, Solar wind

The oldest light there is: a faint glow of microwaves coming from every direction in the sky.

It was released about 380,000 years after the Big Bang, when the universe first became clear enough for light to travel freely.

See Big Bang, Redshift, Look-back timeLesson The expanding universe

The pattern galaxies make on the largest scales: clusters joined by long threads, wrapped round nearly empty voids.

It formed as gravity drew matter towards wherever there was already slightly more of it.

See Galaxy cluster, Supercluster, VoidLesson The cosmic web

Eruptions of water, ammonia or methane in place of molten rock.

Enceladus sprays water from cracks near its south pole, found by the Cassini spacecraft in 2005.

See Subsurface ocean, Tidal heating

The stretch of about 200 million years after the oldest light was released and before the first stars lit.

The universe then was hydrogen and helium gas, slowly gathering under gravity.

See Recombination, Star, Big BangLesson The first minutes and the oldest light

The name for whatever is making the expansion of the universe speed up.

It was found in 1998. It makes up about 68 per cent of everything in the universe, and what it is remains unknown.

See Dark matter, Hubble constant, Big BangLesson Dark matter and dark energy

Matter that gives out no light at all and is known only by the pull of its gravity.

There seems to be about five times as much of it as of ordinary matter. Nobody yet knows what it is made of.

See Dark energy, Gravitational lensing, Galaxy clusterLesson Dark matter and dark energy

The sky's version of latitude, from +90° at the north celestial pole to -90° at the south.

See Right ascension, Celestial equator

Mass divided by volume: how tightly a thing is packed.

Water is 1,000 kg per cubic metre and the Earth averages 5,513. Saturn averages 687, less than water.

See Mass

The sorting of a young, molten world into layers, with heavy metal sinking to the middle and lighter rock floating above it.

It is why the Earth has an iron core, a rocky mantle and a thin crust.

See Terrestrial planet, AccretionLesson Rock, gas, ice: three kinds of planet

The chain of methods used to measure distances in space, each one set by the one that reaches less far.

Parallax sets the Cepheids, Cepheids set the Type Ia supernovae, and the supernovae set the Hubble constant.

See Parallax, Cepheid, Standard candle, Hubble constantLesson Variable stars and the distance ladder

The change in the wavelength of light from something moving towards you or away from you.

Light from an approaching source is squeezed towards blue, and from a receding one stretched towards red. The size of the shift gives the speed.

See Redshift, Spectral line

A body that orbits the Sun and is round, but shares its orbit with many others.

The International Astronomical Union names five: Ceres, Pluto, Haumea, Makemake and Eris. Several more almost certainly qualify.

See Planet, Kuiper belt, Trans-Neptunian object

A number from 0 up to 1 that says how stretched an orbit is. 0 is a perfect circle.

The Earth's is 0.0167, Mars's is 0.0935 and Halley's Comet's is 0.967.

See Ellipse, Perihelion, AphelionLesson Kepler's three laws

The line the Sun appears to follow against the stars through the year, which is the plane of the Earth's orbit seen from inside it.

The planets and the Moon always stay near it, because the Solar System is nearly flat.

See Inclination, Celestial equator, Zodiac

The whole range of light, from radio waves through the colours we see to gamma rays.

Visible light is a narrow slice of it. Most of what astronomy has learned in the last century came from the rest.

See Radio waves, Infrared, Ultraviolet, X-rays, WavelengthLesson Light is the only messenger

A stretched circle, the shape of every closed orbit.

An ellipse has two focus points. The body being orbited sits at one of them and the other is empty.

See Eccentricity, Semi-major axis, Kepler's lawsLesson Kepler's three laws

A smooth, rounded galaxy with no disc or arms, made mostly of old stars.

The largest galaxies known are ellipticals, built up from collisions between smaller galaxies.

See Galaxy, Spiral galaxy, Galaxy clusterLesson Kinds of galaxy

One of the two days each year when the Sun stands over the equator and day and night are close to equal everywhere.

They fall on about 20 March and 22 or 23 September.

See Solstice, Celestial equatorLesson Seasons

The speed needed to leave a body for good without any further push.

It is 11.2 km/s from the surface of the Earth, 2.4 km/s from the Moon and about 618 km/s from the visible surface of the Sun.

See Orbital velocity, Gravity

The edge of a black hole: the boundary from inside which nothing can get out.

It is not a surface. For a black hole with the Sun's mass it would be a sphere about 6 km across.

See Black hole, Singularity, Escape velocityLesson The event horizon

A planet that orbits a star other than the Sun.

The first found round a Sun-like star was 51 Pegasi b, in 1995.

See Transit, Habitable zone

Moving under gravity alone, with nothing pushing back.

Astronauts float because they and their spacecraft are falling together. At the height of the International Space Station gravity is still nearly 90% as strong as on the ground.

See Orbit, WeightLesson What an orbit is

How many crests of a wave pass each second.

For light, shorter wavelength means higher frequency and more energy in each photon.

See Wavelength, Photon

The distance from a young star beyond which it is cold enough for water to freeze into solid grains.

In the young Solar System it is thought to have lain between where Mars and Jupiter are now, which goes a long way to explaining why the giants are outside it.

See Protoplanetary disc, Ice giant, Gas giantLesson Rock, gas, ice: three kinds of planet

A huge family of stars, gas and dust held together by gravity.

The smallest hold a few thousand stars and the largest more than a trillion.

See Milky Way, Spiral galaxy, Elliptical galaxyLesson Kinds of galaxy

Hundreds or thousands of galaxies held together by gravity.

Clusters are the largest things that gravity holds together. The nearest large one, in Virgo, is about 54 million light years away.

See Galaxy, Local Group, Dark matterLesson Kinds of galaxy

Two galaxies falling together under gravity and settling into one.

Hardly any stars hit one another, but the shapes of both galaxies are changed. Two spirals of similar size usually leave an elliptical.

See Galaxy, Elliptical galaxy, Tidal tail, StarburstLesson When galaxies collide

Light with the shortest wavelengths and the most energy of all, made by the most violent events in the universe.

See Electromagnetic spectrum, X-raysLesson The whole spectrum: light we cannot see

A very large planet made mostly of hydrogen and helium, with no solid surface: Jupiter and Saturn.

See Ice giant, Terrestrial planetLesson Rock, gas, ice: three kinds of planet

A tight ball of several thousand to millions of very old stars, held firmly together by gravity.

The Milky Way has over 150 of them, in its outskirts.

See Star cluster, Open cluster, Milky WayLesson Star clusters

The top of a column of hot gas rising at the Sun's surface, about 1,000 to 1,500 km across.

Each lasts ten minutes or so before it cools, sinks at its edges and is replaced.

See Convective zone, Photosphere

The bending of light by gravity as it passes something massive.

A cluster of galaxies can bend the light of galaxies behind it into arcs. How strongly it bends shows how much mass is there, seen or unseen.

See Gravity, Dark matter, Galaxy clusterLesson Dark matter and dark energy

A ripple in space itself, sent out when very heavy things move violently.

The first to be detected, in 2015, came from two black holes merging about 1.3 billion light years away.

See Black hole, Neutron star, Gravity, InterferometerLesson Gravitational waves

The pull that every mass has on every other mass.

It weakens with the square of the distance: twice as far away, a quarter of the pull.

See Mass, Orbit, Inverse square lawLesson What an orbit is

Passing close to a moving planet to gain or lose speed without using fuel.

Voyager 2 used four in a row to reach Neptune in twelve years.

See Transfer orbit, GravityLesson Leaving and arriving: how spacecraft get about

The warming of a planet when gases in its air let sunlight in but hold back the heat trying to leave.

It keeps the Earth about 33 °C warmer than it would otherwise be. On Venus it keeps the ground at about 464 °C, hot enough to melt lead.

See Atmosphere, Albedo

The range of distances from a star at which a planet with a suitable atmosphere could keep liquid water on its surface.

Being in it is no promise of anything. Venus sits near the inner edge of the Sun's.

See Exoplanet, Greenhouse effect

The edge of the heliosphere, where the solar wind is stopped by the gas between the stars.

It is not the edge of the Solar System. The Sun's gravity holds comets very much further out.

See Heliosphere, Oort cloud

The bubble that the solar wind blows in the thin gas between the stars.

Voyager 1 crossed its edge in 2012 at about 121 au from the Sun, and Voyager 2 in 2018 at about 119 au.

See Solar wind, Heliopause

The second lightest element, and what the Sun makes from hydrogen.

It is about 25% of the Sun by mass. Everything heavier adds up to under 2%.

See Hydrogen, Nuclear fusion

A chart of stars with temperature along the bottom and true brightness up the side. Most stars fall on one band, the main sequence.

See Main sequence, Luminosity, Spectral type, Red giant, White dwarfLesson Colour and brightness

The region around a planet where its own gravity wins over the Sun's, so a moon can stay.

The Earth's reaches out about 1.5 million km, roughly four times the distance of the Moon.

See Lagrange point, Irregular moon

A giant planet that orbits very close to its star, with a year lasting only a few days.

The Solar System has nothing like one. They probably formed further out and moved inwards.

See Exoplanet, Gas giant, Frost lineLesson Planets round other stars

The number that says how fast the universe is expanding today.

It is about 70 km/s for every megaparsec of distance, which is about 21 km/s for every million light years. Two ways of measuring it disagree by nearly a tenth.

See Redshift, Big Bang, ParsecLesson The expanding universe

The lightest and commonest element, a single proton with a single electron.

It makes up about 73% of the Sun by mass.

See Helium, Nuclear fusion

A large planet made mostly of water, ammonia and methane under a hydrogen and helium atmosphere: Uranus and Neptune.

Planetary scientists call those three materials ices because of where they freeze, not because they are frozen. Inside the planets they are hot, dense fluids.

See Gas giant, Frost lineLesson Rock, gas, ice: three kinds of planet

A bowl blasted out of a surface by something striking it at many kilometres per second.

Counting craters is the main way of telling how old a surface is: the more there are, the longer it has lain undisturbed.

See Regolith, Meteorite

The tilt of an orbit against a chosen flat plane, usually the ecliptic or the equator of the planet being orbited.

The Moon's orbit is tilted by about 5.1° to the ecliptic, which is why there is not an eclipse every month.

See Node, Ecliptic

Light with wavelengths a little longer than red, which we feel as heat.

Cool things such as planets, dust and newborn stars shine mostly in infrared, which is why the James Webb Space Telescope looks there.

See Electromagnetic spectrum, Ultraviolet

An instrument that splits a beam of light in two, sends the halves along different paths and compares them when they meet again.

The tiniest change in the length of either path shows up, which is how LIGO's 4 km arms catch gravitational waves.

See Gravitational wave, WavelengthLesson Gravitational waves

The rule that light, and gravity, spread out and weaken with the square of the distance.

Twice as far is a quarter as strong. Sunlight at Jupiter, 5.2 times further out than the Earth, is 27 times weaker.

See Luminosity, Gravity

A small moon on a wide, tilted and often backwards orbit.

Irregular moons were almost certainly captured. They make up most of the moons of Jupiter and Saturn by number.

See Regular moon, Retrograde, Hill sphere

The temperature scale that starts at absolute zero, the coldest anything can be.

Its steps are the same size as degrees Celsius, and 0 °C is 273.15 K.

See Blackbody

Three rules, found by Johannes Kepler between 1609 and 1619, that describe how planets move round the Sun.

Orbits are ellipses with the Sun at one focus; a planet sweeps out equal areas in equal times; and the square of the period goes as the cube of the orbit's size.

See Ellipse, Semi-major axis, Orbital periodLesson Kepler's three laws

A wide ring of icy bodies beyond Neptune, from about 30 to about 50 au from the Sun.

Pluto is its best known member.

See Trans-Neptunian object, Dwarf planet, Oort cloud

One of five places where a small object can keep step with two larger ones.

The James Webb Space Telescope circles the point called L2, about 1.5 million km from the Earth on the side away from the Sun.

See Trojan, Hill sphere

How long light takes to cross a gap, used as a way of saying how big the gap is.

The Moon is about 1.3 light seconds away, the Sun 8.3 light minutes, and Neptune a little over 4 light hours.

See Speed of light, Light year, Look-back timeLesson Measuring distance: kilometres, au, light years and parsecs

The distance light travels in one year, about 9.46 million million km.

It measures distance, not time. One light year is about 63,241 au, and the nearest star beyond the Sun is 4.25 light years away.

See Astronomical unit, Parsec, Speed of lightLesson Measuring distance: kilometres, au, light years and parsecs

The small family of galaxies that the Milky Way belongs to.

It has more than 50 known members spread over about 10 million light years. The Milky Way and Andromeda are much the largest.

See Milky Way, Galaxy clusterLesson Kinds of galaxy

How long ago the light you are seeing set out.

Looking far away always means looking into the past. We see the Andromeda Galaxy as it was about 2.5 million years ago.

See Speed of light, Light timeLesson Light is the only messenger

The total power a star gives out, in every direction and at every wavelength.

The Sun's is 3.828 × 10²⁶ watts.

See Apparent magnitude, Absolute magnitude, Inverse square law

When the Moon passes through the Earth's shadow.

The Moon turns a dull red, lit only by sunlight bent through the Earth's air: the light of every sunrise and sunset at once.

See Solar eclipse, Umbra, NodeLesson Phases and eclipses

The region round a magnet, or round moving electric charge, in which other magnets and charges feel a force.

Planets make theirs with moving, conducting fluid deep inside: liquid iron in the Earth, metallic hydrogen in Jupiter.

See Magnetosphere, Plasma

The space round a planet ruled by its own magnetic field, which turns the solar wind aside.

Jupiter's is the largest structure in the Solar System after the heliosphere.

See Magnetic field, Solar wind, Aurora

The long, steady part of a star's life while it fuses hydrogen in its core.

The Sun is about 4.6 billion years into a main-sequence life of roughly 10 billion.

See Star, Nuclear fusion

How much matter a thing holds. It is the same wherever you take it.

See Weight, Gravity, Density

The streak of light made when a meteoroid burns up in the air.

See Meteoroid, Meteorite

A night or more with many meteors, all seeming to come from one part of the sky, as the Earth passes through dust left along a comet's orbit.

See Meteor, Comet, MeteoroidLesson Falling rocks: meteors, meteorites and impacts

A piece of a meteoroid or asteroid that has survived the fall and reached the ground.

See Meteor, Meteoroid

A piece of rock or metal in space, smaller than an asteroid: anything from a grain of dust to a boulder.

See Meteor, Meteorite

The galaxy we live in. From inside it shows as a pale band of light across a dark sky.

It is a flat disc of stars about 100,000 light years across, with the Sun roughly half way out from the centre.

See Galaxy, Spiral galaxy, Local GroupLesson The Milky Way

A cold, dark cloud of gas and dust in which stars form.

One cloud holds from 1,000 to 10 million times the mass of the Sun and can be hundreds of light years across.

See Nebula, ProtostarLesson Nebulae and star birth

A natural body that orbits a planet, a dwarf planet or an asteroid.

Written with a capital M it means the Earth's own.

See Regular moon, Irregular moon, Tidal locking

An asteroid or comet whose orbit brings it close to the Earth's.

More than 35,000 are known. None of the large ones is on course to hit the Earth in the next century or more.

See Asteroid, Comet, Impact craterLesson Falling rocks: meteors, meteorites and impacts

A cloud of gas and dust in space. Some are where stars form and some are what dying stars throw off.

See Molecular cloud, Planetary nebula, Protostar, SupernovaLesson Nebulae and star birth

A particle with almost no mass that passes through ordinary matter nearly untouched.

Fusion in the Sun's core makes them in huge numbers. Tens of billions pass through each square centimetre of you every second, day and night.

See Nuclear fusion, Core (of the Sun)

The crushed core left by an exploded star: more than the Sun's mass in a ball about 20 km across.

A spinning neutron star whose beams of radio waves sweep past the Earth is called a pulsar.

See Supernova, Black hole, White dwarfLesson Neutron stars and pulsars

One of the two points where a tilted orbit crosses the plane it is measured against.

Eclipses can only happen when the Moon is near one of its nodes at new or full Moon.

See Inclination, Solar eclipse, Lunar eclipseLesson Phases and eclipses

The joining of light atomic nuclei into heavier ones, which releases energy.

Each second the Sun turns about 600 million tonnes of hydrogen into helium, and about 4 million tonnes of that leaves as light.

See Core (of the Sun), Hydrogen, Helium, NeutrinoLesson What the Sun is

When a nearer body passes in front of a farther one and hides it.

Timing how long a star vanishes behind a distant world gives that world's size. This is how Eris was measured.

See Transit, Solar eclipse

A vast, thin shell of icy bodies thought to surround the Solar System out to tens of thousands of au.

Nothing has ever been seen in it. It is worked out from the orbits of comets that fall in from every direction.

See Comet, Kuiper belt, Heliopause

A loose group of a few dozen to a few thousand stars, usually young, whose members slowly scatter.

The Pleiades are one. The Milky Way has more than a thousand.

See Star cluster, Globular clusterLesson Star clusters

When a planet is on the opposite side of the Earth from the Sun, so it rises at sunset and is at its nearest and brightest.

See Conjunction, Synodic period

The path one body follows around another, held there by gravity.

A body in orbit is falling the whole time, but moving sideways fast enough that it keeps missing.

See Gravity, Free fall, Ellipse, Orbital periodLesson What an orbit is

The time taken to go once round an orbit.

For the Earth it is 365.256 days measured against the stars.

See Semi-major axis, Synodic period, Kepler's laws

The speed a body needs to stay in a given orbit.

Close above the Earth it is about 7.8 km/s. The Earth itself goes round the Sun at 29.8 km/s.

See Escape velocity, OrbitLesson What an orbit is

The small shift of a nearby star against far ones as the Earth moves round the Sun.

The size of the shift gives the distance directly. Even the nearest star, Proxima Centauri, shifts by only 0.77 arcseconds.

See Parsec, ArcsecondLesson Measuring distance: kilometres, au, light years and parsecs

A distance of about 3.26 light years, the unit astronomers use most between the stars.

It is how far away you would have to stand for the gap between the Earth and the Sun to look one arcsecond wide. One parsec is 206,265 au.

See Parallax, Arcsecond, Light yearLesson Measuring distance: kilometres, au, light years and parsecs

The paler outer part of a shadow, where the light source is only partly hidden.

See Umbra, Solar eclipse

The nearest point of an orbit to whatever is being orbited.

It takes a special name for some bodies: perihelion round the Sun, perigee round the Earth, perijove round Jupiter.

See Apoapsis, Perihelion

The point of an orbit nearest the Sun.

The Earth reaches it in early January, at about 147.1 million km.

See Aphelion, Periapsis, EccentricityLesson Kepler's three laws

How much of the sunlit half of a moon or planet is turned towards us.

The Moon is always half lit. Its phases are only our changing view of that half.

See Synodic period, TerminatorLesson Phases and eclipses

The smallest packet light comes in.

Bluer light comes in packets that each carry more energy than redder light.

See Wavelength, Electromagnetic spectrum

The layer of the Sun that we see, where the gas turns clear enough for light to get out.

It is a few hundred kilometres deep and about 5,772 K. The Sun has no solid surface, and this is the nearest thing to one.

See Granule, Sunspot, ChromosphereLesson What the Sun is

A body that orbits the Sun, is massive enough to have pulled itself round, and has cleared other bodies from around its orbit.

This is the International Astronomical Union's definition of 2006. It gives the Solar System eight.

See Dwarf planet, Terrestrial planet, Gas giant, Ice giant, ExoplanetLesson Rock, gas, ice: three kinds of planet

The glowing shell of gas blown off by a dying star like the Sun. It has nothing to do with planets.

The name comes from early telescopes, in which some of these clouds looked like the round discs of planets.

See Nebula, White dwarf, Red giantLesson Nebulae and star birth

One of the bodies, from about a kilometre to hundreds of kilometres across, that planets were built from.

Asteroids and comets are planetesimals that were never used.

See Accretion, Protoplanetary disc, AsteroidLesson How the planets formed

Gas so hot that its atoms have lost electrons, so it carries electric current and is steered by magnetic fields.

The Sun is plasma all the way through, and so is most of the ordinary matter in the universe.

See Star, Magnetic fieldLesson What the Sun is

A step of ten times bigger or ten times smaller, the only practical way to count the sizes of things in space.

From a person to the edge of the visible universe is about 27 such steps.

See Astronomical unit, Light yearLesson How big is space

The slow circling of a spinning body's axis, like the wobble of a spinning top.

The Earth's axis takes about 26,000 years to go round once, so the pole star changes over the centuries.

See Axial tilt, Celestial pole

Moving or spinning the same way as most things in the system.

In the Solar System that is anticlockwise seen from above the Sun's north pole.

See Retrograde

The flat, spinning disc of gas and dust round a newborn star, out of which its planets grow.

See Accretion, Frost lineLesson Rock, gas, ice: three kinds of planet

A star in the making: a ball of gas still shrinking and heating up, before fusion has begun in its core.

See Nebula, Protoplanetary disc, Nuclear fusionLesson How stars live and die

The most luminous kind of active galaxy, so bright that its centre can be seen across billions of light years.

A quasar can give out thousands of times the energy of the Milky Way. The first, 3C 273, was recognised in 1963. Over a million are now known.

See Active galaxy, Supermassive black hole, Look-back timeLesson Active galaxies and quasars

The thick layer round the Sun's core where energy works its way outward as light, absorbed and given out again countless times.

The gas is so dense that the journey takes many thousands of years.

See Core (of the Sun), Convective zoneLesson What the Sun is

Light with the longest wavelengths, from about a millimetre to many kilometres.

Cold gas between the stars gives off radio waves, which pass through the dust that blocks ordinary light.

See Electromagnetic spectrum, Infrared

The time, about 380,000 years after the Big Bang, when the universe cooled enough for electrons to settle onto nuclei and make the first whole atoms.

With the loose electrons gone the universe turned clear, and the light set free is the cosmic microwave background.

See Cosmic microwave background, Plasma, Big BangLesson The first minutes and the oldest light

A small, cool, faint star. Red dwarfs are by far the commonest kind of star.

They have less than about half the Sun's mass and burn so slowly that they can last for trillions of years.

See Star, Main sequence, Spectral typeLesson What a star is

A star near the end of its life that has swollen to many times its old size and cooled to a red glow.

The Sun will become one in about 5 billion years.

See Main sequence, White dwarf, Hertzsprung-Russell diagramLesson How stars live and die

A stretching of light to longer, redder wavelengths.

Light from distant galaxies is redshifted because space itself has expanded while the light was on its way.

See Doppler shift, Look-back time

The layer of loose dust and broken rock that covers the solid ground of an airless world.

The Moon's was made by billions of years of impacts, large and small.

See Impact crater

A moon on a near-circular orbit close to its planet's equator, going the same way as the planet spins.

Such moons formed in place, from a disc round the young planet.

See Irregular moon, Prograde

When the periods of two orbiting bodies are in a simple ratio, so they tug each other at the same places again and again.

For every orbit of Ganymede, Europa makes two and Io makes four. Pluto makes two orbits for every three of Neptune.

See Orbital period, Tidal heating

Moving or spinning the opposite way to most things in the system.

Venus spins backwards. Triton goes round Neptune backwards, the best sign that it was captured and did not form there.

See Prograde, Irregular moon

The sky's version of longitude, measured eastward along the celestial equator.

It is counted in hours, from 0 to 24, because the sky turns through one hour of it in about one hour of time.

See Declination, Celestial equator

A flat disc of countless small pieces of ice or rock in orbit round a planet.

All four giant planets have one. Saturn's main rings are about 280,000 km across and mostly only about 10 metres thick.

See Roche limit, Shepherd moon

The distance inside which a planet's tides would pull apart a moon held together only by its own gravity.

For a liquid moon as dense as its planet it is about 2.44 times the planet's radius. Saturn's main rings lie inside Saturn's.

See Tide, Ring system

Half the longest width of an elliptical orbit, which serves as its average size.

Kepler's third law ties it to the time one orbit takes.

See Ellipse, Orbital period, Kepler's lawsLesson Kepler's three laws

The commonest and least powerful kind of active galaxy.

It looks like a normal galaxy in ordinary light but is bright in infrared. Named after Carl Seyfert, who first picked them out in 1943.

See Active galaxy, InfraredLesson Active galaxies and quasars

A small moon whose gravity keeps the edge of a ring sharp or holds a narrow ring together.

Prometheus and Pandora orbit on either side of Saturn's thin F ring.

See Ring system

The time a planet takes to turn once measured against the stars.

For the Earth it is 23 hours 56 minutes 4 seconds, about four minutes shorter than the day on the clock.

See Solar day

The point at the centre of a black hole where today's physics says density becomes endless.

Most physicists read this as a sign that the theory has stopped working there, not as a description of something real.

See Black hole, Event horizonLesson The event horizon

The rise and fall in the number of sunspots, flares and storms on the Sun, which takes about eleven years.

The Sun's magnetic poles swap over at each peak, so the full magnetic cycle is about 22 years.

See Sunspot, Solar flare

The time from one noon to the next: on Earth, 24 hours on average.

It is longer than the sidereal day because the Earth moves along its orbit as it spins, and has to turn a little further to bring the Sun back overhead.

See Sidereal day

When the Moon passes between the Earth and the Sun and its shadow falls on the Earth.

A total one is possible only because the Sun is about 400 times wider than the Moon and about 400 times further away.

See Lunar eclipse, Umbra, Penumbra, NodeLesson Phases and eclipses

A sudden burst of light, from radio waves to X-rays, when tangled magnetic field above a sunspot group snaps into a simpler shape.

See Sunspot, Coronal mass ejection

A stream of charged particles that blows outward from the Sun all the time.

It passes the Earth at several hundred kilometres per second.

See Corona, Heliosphere, Aurora, Magnetosphere

One of the two days each year when one pole is tipped most directly towards the Sun, giving the longest and shortest days.

They fall on about 21 June and 21 December.

See Equinox, Axial tiltLesson Seasons

A narrow dark or bright line in a spectrum, at a colour that one kind of atom takes in or gives out.

Every element has its own pattern of lines, so a spectrum shows what a star is made of. Helium was found this way in the Sun in 1868, before anyone had found it on Earth.

See Spectrum, Doppler shiftLesson Light is the only messenger

The class a star is put in from the pattern of lines in its light. The main classes, from hottest to coolest, are O, B, A, F, G, K and M.

The Sun is type G2.

See Spectrum, Spectral line, Hertzsprung-Russell diagramLesson Colour and brightness

Light spread out in order of wavelength, as a prism or a rainbow does.

See Spectral line, Electromagnetic spectrumLesson Light is the only messenger

299,792,458 metres per second in empty space, the fastest that anything can carry news from one place to another.

The figure is exact, because the metre is now defined from it. Light crosses from the Moon to the Earth in about 1.3 seconds.

See Light time, Light year, Look-back timeLesson Light is the only messenger

A galaxy shaped like a flat, turning disc with bright arms winding out from the middle.

The arms are where new stars are forming. In about two thirds of spirals, the Milky Way among them, the arms start from a straight bar of stars.

See Galaxy, Elliptical galaxy, Milky WayLesson Kinds of galaxy

Something whose true brightness is known, so that its distance can be worked out from how faint it looks.

Cepheids and Type Ia supernovae are the two most used.

See Cepheid, Inverse square law, Luminosity, Distance ladderLesson Variable stars and the distance ladder

A ball of hot gas held together by its own gravity and shining by nuclear fusion in its core.

See Nuclear fusion, Main sequence, PlasmaLesson What the Sun is

A group of stars that formed from the same cloud and are held together by gravity.

See Open cluster, Globular cluster, NebulaLesson Star clusters

A spell in which a galaxy makes new stars far faster than usual.

Collisions between galaxies set them off by squeezing clouds of gas. The Antennae galaxies have more than a thousand young star clusters made this way.

See Galaxy merger, Molecular cloud, Star clusterLesson When galaxies collide

A layer of liquid water hidden under the icy crust of a moon or dwarf planet.

The evidence is strong for Europa, Ganymede, Callisto, Enceladus and Titan.

See Tidal heating, Cryovolcanism

A darker, cooler patch on the Sun where strong magnetism holds back the heat rising from below.

Spots are often bigger than the Earth. They only look dark beside the hotter surface around them.

See Solar cycle, Umbra, Magnetic field

A planet bigger than the Earth but smaller than Neptune.

Planets in this range are among the commonest known, yet the Solar System has none. The larger ones, with thick gas round them, are often called sub-Neptunes.

See Exoplanet, Terrestrial planet, Ice giantLesson Planets round other stars

A large collection of galaxy clusters and groups, most of them not held together by gravity.

The Milky Way lies on the outer edge of one called Laniakea, about 520 million light years across.

See Galaxy cluster, Local Group, Cosmic webLesson The cosmic web

A black hole of hundreds of thousands to billions of times the mass of the Sun, found at the centre of a galaxy.

Most large galaxies have one. The Milky Way's is 4.3 million Suns. The one in the galaxy M87 is 6.5 billion.

See Black hole, Active galaxy, Accretion discLesson Active galaxies and quasars

The explosion of a star. For a few weeks it can outshine a whole galaxy.

The commonest kind happens when the core of a star of more than about 8 Suns collapses.

See Nebula, White dwarf, Nuclear fusionLesson How stars live and die

The time for two bodies to come back to the same line-up as seen from a third.

The Moon orbits the Earth in 27.32 days, but its phases repeat every 29.53 days because the Earth has moved on round the Sun in the meantime.

See Orbital period, PhaseLesson Phases and eclipses

The line between day and night on a planet or moon.

Shadows are longest there, so it is where craters and mountains show best.

See Phase

A planet made mostly of rock and metal with a solid surface: Mercury, Venus, the Earth and Mars.

See Gas giant, Ice giant, DifferentiationLesson Rock, gas, ice: three kinds of planet

Heat made inside a moon as changing tides squeeze and stretch it.

It makes Io the most volcanic body known, and keeps water liquid under the ice of Europa and Enceladus.

See Tide, Resonance, Subsurface oceanLesson Tides

When a moon turns exactly once for each orbit, so the same face always points at its planet.

Almost every large moon in the Solar System is locked, our own included. Pluto and Charon are each locked to the other.

See Tide, Tidal heatingLesson Tides

A long curved streamer of stars pulled out of a galaxy by the gravity of another.

It forms because the near side of a galaxy is pulled harder than the far side, the same uneven pull that raises tides.

See Galaxy merger, TideLesson When galaxies collide

The stretching of one body by another's gravity, which pulls harder on the near side than on the far side.

The stretch raises a bulge on both sides, which is why most coasts on Earth get two high tides a day.

See Tidal locking, Tidal heating, Roche limitLesson Tides

Any small body whose orbit lies, on average, beyond Neptune's.

See Kuiper belt, Dwarf planet

The path a spacecraft coasts along to get from one orbit to another, most cheaply half of an oval that touches both.

See Ellipse, Orbital period, Gravity assistLesson Leaving and arriving: how spacecraft get about

When a small body crosses the face of a larger one as we see it.

Mercury and Venus can transit the Sun. Most known planets of other stars were found by the tiny dip in starlight as they crossed.

See Occultation, Exoplanet

An asteroid that shares a planet's orbit, keeping about 60° ahead of it or behind.

Jupiter has many thousands. They gather round the Lagrange points called L4 and L5.

See Lagrange point, Asteroid

Light with wavelengths a little shorter than violet, given off by very hot things.

The Earth's ozone layer stops most of it, so it has to be studied from space.

See Electromagnetic spectrum, X-rays

The dark middle of a shadow, where the light source is completely hidden.

The same word is used for the dark centre of a sunspot.

See Penumbra, Solar eclipse, Sunspot

A star whose brightness changes, either because it swells and shrinks or because something passes in front of it.

Mira, a red giant, takes 331 days over each cycle. Delta Cephei takes 5.366 days.

See Cepheid, Binary star, Red giantLesson Variable stars and the distance ladder

A huge region of space with very few galaxies in it.

Voids are the gaps between the threads and walls of the cosmic web.

See Cosmic web, Galaxy clusterLesson The cosmic web

The distance from one crest of a wave to the next.

For light it sets the colour. The eye sees from about 380 nanometres (violet) to about 750 (red).

See Frequency, Electromagnetic spectrumLesson Light is the only messenger

The force of gravity on a mass. It changes from world to world while the mass stays the same.

On the Moon you would weigh about a sixth of what you do on Earth.

See Mass, Gravity

The hot, dense core left behind when a star like the Sun dies: about the size of the Earth, and no longer making energy.

See Red giant, Planetary nebula, Chandrasekhar limit, Black dwarf, DensityLesson White dwarfs

Light of very short wavelength, given off by gas at millions of degrees.

The Sun's corona and the gas falling towards black holes both glow in X-rays.

See Electromagnetic spectrum, Corona

The point in the sky straight above your head.

See Celestial sphere

The band of constellations that the ecliptic runs through, where the Sun, Moon and planets are always found.

The Sun passes through thirteen constellations in a year, not twelve. The extra one is Ophiuchus.

See Ecliptic, Constellation

The strip of sky where the disc of our own galaxy hides the galaxies beyond it.

Dust and crowded stars in the Milky Way block the view, so maps of distant galaxies have an empty band along it.

See Milky Way, Cosmic webLesson The cosmic web