ILLUSTRATED

ENCYCLOPEDIA

SPACE

ISBN 978 1 7418 3768 1

9 781901 323511

The Sun is a star at the centre of ourSolar System. It is one of billionsof stars in the Milky Way Galaxy,

itself one of billions of galaxies in theUniverse. Like all stars, the Sun is anenormous spinning globe of glowing gasthat produces massive amounts ofenergy by converting hydrogen intohelium. To us on Earth, the Sun is ofcrucial importance since no life couldexist without it. The Sun is about 4.6billion years old. It will keep shining foraround another 5 billion years.

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Chromosphere A thin layer of the Sun’satmosphere around 2000 km deep, justabove the photosphere and beneath thecorona. The chromosphere can reachtemperatures of up to 20,000°C.

Convective zone The uppermost innerlayer of the Sun. Hot gas bubbles up tothe surface of the convective zone beforesinking down to be reheated again.

Corona The Sun’s hot outer atmosphere.The corona extends for millions ofkilometres into space. It is visible fromEarth only during a total solar eclipse.

Flare An explosion of hydrogen in theSun’s atmosphere, releasing enormousamounts of energy.

Granules Bubbles in the Sun’sphotosphere. Granules occur where heatrises up from the convective zone.

Heliosphere A magnetic “envelope” thatsurrounds the Sun and extends to theedge of the Solar System. The heliosphereis created by the solar wind.

Infrared radiation A form ofelectromagnetic energy ( 29). Infraredradiation is what we feel as heat. Morethan half of the energy the Sun gives o! isin the form of infrared radiation.

Nuclear fusion The process by which theSun generates energy. Inside the Sun’score, hydrogen atoms fuse together toform atoms of helium. The energyreleased through this process is thesource of the Sun’s light and heat as wellas other forms of radiation.

Photosphere The surface of the Sun, fromwhere the Sun radiates its energy. Thephotosphere is the part of the Sun thatwe can see. The photosphere is onlyabout 500 km thick and, at 5500°C, ismuch “cooler” than the Sun’s core. It is ina state of constant motion, like waterboiling in a kettle.

Plasma Gas that is heated to such a high temperature that its atoms become electrically charged. The Sun’sinner layers and atmosphere are in aplasmic state.

Prominence A flaming arch of gas thatleaps o! the photosphere and out intothe corona. Prominences are held up bythe Sun’s magnetic field ( 6)

Radiation The transmission of heat, lightand other forms of energy. Radiantenergy travels in waves. Di!erent formsof radiation are classified in theelectromagnetic spectrum ( 29).

Radiative zone One of the inner layers ofthe Sun. Energy radiates out through thislayer from the Sun’s core.

Solar core The innermost part of the Sun.The core is a region of high pressure (200billion times that on the Earth’s surface)and intense heat (about 15 million°C). Thisis where the energy that keeps the Sunshining is produced by nuclear fusion.Energy flows out from the core throughthe radiative zone to the convective zone.

Solar eclipse The passage of the Moonbetween the Sun and the Earth, blockingout some or all of the Sun’s light (seediagram opposite).

Solar wind A stream of subatomicparticles ( 28) that flows steadily awayfrom the Sun. This occurs because gasesin the corona have too much energy to beheld in by even the Sun’s stronggravitational field ( 6).

Spicule A narrow, flaming jet of gas thatleaps up to 10,000 km above thephotosphere then falls back.

Sunspot A darker, cooler area thatappears temporarily on the Sun’sphotosphere. Sunspots are places wherelines of magnetic force ( 6) pass throughthe photosphere. They usually occur insmall groups. The dark inside of a sunspotis called the umbra and the lighter outsideis called the penumbra.

About five billion years from now, the hydrogenthat the Sun uses as fuel will start to run out, andthe Sun will balloon into a red giant ( 23),engulfing Mercury, Venus and Earth. Above is anartist’s impression of what the Earth’s surfacemay look like when this happens.

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Prominence

Convectivezone

Radiativezone

CorePhotosphere

Sunspots

Flare Earth Jupiter

(to scale)

Spicules

Prominence

Sunspots

A solar eclipse. By coincidence, the Sun andMoon appear the same size in our sky. When theMoon passes between the Earth and the Sun itmay partially or fully block out the Sun. Duringa total eclipse, the Moon covers the Sun’ssurface entirely and we can see the corona.

The Sun with a segment taken out to reveal itsinternal layers (above)

10 11

Moon

Sun

Earth

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Corona

The Sun grows to the size of a red giant.

Small prominence

Umbra

Penumbra1

23

! The Sun contains more than 99% of all thematter in the Solar System.

! About three-quarters of the Sun is madeup of hydrogen.

! The Sun is so big that approximately1,400,000 globes the size of the Earth couldfit inside it.

! Never look directly at the Sun or look at itthrough binoculars or a telescope. This canblind you or seriously damage your eyesight.

! If the Earth were any closer to the Sun itwould have been too hot for life to develop.If it were any farther away from the Sun itwould be too cold for life.

! It takes about eight minutes for light totravel from the Sun to the Earth.

! Covers 12 major subject areas of astronomy

! More than 200 keywords alphabetically listedand clearly explained

! Fact panels with extra information

! More than 60 detailed illustrations

! Comprehensive index

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First published in 2012 by Orpheus Books Ltd., 6 Church Green, Witney, Oxfordshire, OX28 4AW, England

www.orpheusbooks.com

Copyright ©2012 Orpheus Books Ltd.

Created and produced by Nicholas Harris, Sarah Hartley, Katie Sexton, Ruth Symons and Erica Williams, Orpheus Books Ltd.

Text Ruth Symons

Illustrated by Sebastian Quigley and Gary Hincks

Consultant David Hawksett, astronomer, writer and broadcaster

All rights reserved. No part of this book may be reproduced, stored in aretrieval system, or transmitted in any form or by any means, electronic,

mechanical, photocopying, recording or otherwise, without the priorwritten permission of the copyright owner.

ISBN 978 1 7418 3768 1

Printed and bound in Singapore

Photograph on page 22: NASAPhotograph on page 27: Science Photo Library

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Each double page contains a brief introduction,explaining the general subject, followed by key

words arranged in alphabetical order. To look up aspecific word, turn to the index at the back of thisbook: this will tell you which page to go to. If you wantto learn more about a subject, take a look at the factfile, orfollow the arrows to read related entries.

Jupiter’s four largest moons are called the Galileansafter Galileo who discovered them in 1610.

Saturn rotatesvery quickly.This producesa distinctbulge at itsequator.

Galileo Mission A mission that sent a spaceprobe to orbit Jupiter in 1989. It was thefirst spacecraft to orbit the planet.

Galileo Galilei (1564–1642) Italianastronomer who was the first person to usea telescope. He made many importantobservations and discoveries, including thediscovery of Jupiter’s moons. By observingthat Venus, like the Moon, had phases, heconfirmed that the planets all orbit the Sun.

Ganymede The largest of Jupiter’s moons.Bigger than the planet Mercury, it measures5268 km across and is the largest moon inthe Solar System. Ganymede has an icysurface with dark plains covered in groovedpatterns and craters.

Gas giants The four large planets in theSolar System: Jupiter, Saturn, Uranusand Neptune. They mostly consistof gases with no solidsurface.

Io Jupiter’s third largest moon. Itmeasures 3643 km across. Io is coveredwith active volcanoes and pools of moltenrock. It is the most volcanically activeplace in the Solar System,

Jovian planets Another name for the gasgiants. The word “Jovian” means“belonging to Jupiter”.

Great Red Spot A giant storm in Jupiter’satmosphere. The storm has been ragingfor at least 300 years.

Great White Spot A large storm thatoccurs in Saturn’s atmosphere aboutevery 30 years.

Huygens, Christiaan (1629–1695) Dutchastronomer who discovered the moonTitan and spent much of his careerstudying the rings of Saturn.

Cassini Division The largest of the gapsbetween Saturn’s rings. The gap is about4800 km wide. Gaps between rings arecaused by the gravitational pull ( 6) ofthe planet’s moons.

Cassini-Huygens Mission A project thatsent a spacecraft to study Saturn and itsrings and moons. The Huygens Probelanded on Titan in 2005. Cassini is stillorbiting Saturn and returning data.

Enceladus One of Saturn’s moons. Itmeasures 498 km across. Enceladus’ssurface is mostly made up of water ice.

Europa Jupiter’s fourth largestmoon. It measures 3130 kmacross. Europa has an icysurface, which may have awater oceanbeneath it.

Jupiter is the largest planet in theSolar System. It is more massivethan all the other planets combined.

Jupiter is known as a “gas giant”because it is mostly made up of gas withno solid surface at all. Its neighbour,Saturn, another gas giant, is the secondlargest planet. Both planets aresurrounded by “rings” of rock and dust.Saturn’s rings, broad and bright, reachout more than 420,000 km from theplanet. Jupiter and Saturn both have alarge number of moons.

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Jupiter’s Great Red Spot. The white ovals are aircurrents flowing past the spot.

Belts The darker of the striped cloudbands that surround Jupiter.

Callisto Jupiter’s second largest moon. Itmeasures 4800 km across. For its size, ithas more craters than any other planet ormoon in the Solar System.

Cassini, Giovanni Domenico (1625–1712)Italian astronomer who first observed fourof Saturn’s moons and also discovered thelargest gap between Saturn’s rings.

Mimas One of Saturn’s moons.Mimas has an enormous crater that

measures 130 km across—a diameter onethird that of the whole moon.

Pioneer Missions A series of US missionsthat sent space probes to Jupiter, Saturnand Venus in the 1970s. Pioneer 10 laterbecame the first man-made object toleave the boundaries of the Solar System.

Ring A band of dust, rocks, ice or othermaterials surrounding a planet at itsequator. Saturn’s rings are probablyfragments of a moon that was smashed bya passing comet.

Ringlet A very fine planetary ring. Saturn’srings are made up of hundreds orthousands of ringlets.

Titan Saturn’s largest moon. It measures5150 km across, and is the second largestmoon in the Solar System. Titan is the onlymoon in the Solar System known to havea thick atmosphere.

Zones The lighter of the striped cloudbands that surround Jupiter.

EuropaCallisto Ganymede

Io is covered with volcanoes and lava pools.

Saturn as seen from the surface of Titan.

Jupiter’s bright colours of red, orange, yellow and brown, are produced by the elements sulphur and phosphorus.

Io

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From Earth, it ispossible to identify

three of Saturn’srings: an outer

ring (ring A),separated from

two inner rings (B andC) by the Cassini Division.

The Cassini Division

CB

A

SATURN

JUPITER

16 17

Saturn’s rings are made up of billions of blocks ofrock and ice, some bigger than houses.

INTRODUCTIONThis explains thegeneral subjectand provides somebasic knowledge.

KEY WORDS AND ENTRIESKey words are arranged alphabeticallyacross each double page. Each entryprovides a short explanation of whatthe key word means.

PAGE NUMBERPage numbersare easy to findat the side ofthe page.

FACTFILEThe factfile provides extrainformation on the subject. Factsare presented in easy to read bulletpoints.

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BOLD WORDSThese highlightuseful words thatdo not have theirown entry.

ARROWS These arrows show you where to look up otherwords mentioned in the entry. For example, ( 26) tells you to go forward to page 26 and ( 6) tells you to turn back to page 6.

Space probe An unmannedspacecraft guided from Earth. Some

space probes have passed close to,entered orbit around or landed on other

planets and moons.

Telescope An instrument that makesdistant objects appear closer.

Volcano An opening in a planet or moon’ssurface through which hot, molten rockand gases spurt out.

Year The time that it takes an object tocomplete one full orbit of the Sun.

THE PLANETS FORM 1 A shock wave causes a cloudof gas and dust to collapse under its own gravity. 2 The collapsed cloud becomes a swirling disc ofmatter with a bulge at its centre. 3 Small fragmentsof rock clump together in blocks called

planetesimals. 4 The core of the disc becomes a star. The solar

wind ( 9) strips the four innerplanets of their originalatmospheres. 5 The SolarSystem as it is today.

Planetesimal A large lump of rock thatorbited the Sun during the formation ofthe Solar System. The planetesimalscollided with each other, eventuallybuilding up to form the eight planets ofthe Solar System.

Pressure The amount of weight, or force,bearing down on an object.

Revolve To spin in a circle arounda central point or another

object. For example, theEarth revolves on its axisonce per day and revolves

around the Sun once per year.

Eclipse The passage of onecelestial body in front ofanother, totally or partially hidingit from an observer.

Equator An imaginary line around aplanet’s middle, half way between itsnorth and south poles.

Flyby The flight of a space probe veryclose to a planet or moon, from where itcan obtain detailed images.

Gravity The force that attracts all objectsto each other. The larger an object’s mass,or the greater its density, the greater itsgravitational pull. The greater thedistance between objects, the smaller theforce of gravity between them. Gravity isthe force that keeps the planets in orbitaround the Sun.

Heliocentric Centred around the Sun.

Lander A spacecraft designed to land on aplanet, moon or asteroid in order togather information from its surface.

Lava Hot molten, or melted, rock.

Light year The distance a ray of lighttravels in one year. One light year isapproximately 9.46 million million km.

Magnetic fieldThe region surrounding a magnet, anobject which has two ends, calledpoles, and a force of attractionbetween them. The Sun and someplanets, including Earth, havemagnetic fields.

Mantle The rocky layer that lies betweenthe crust and core of a planet.

Mass A measure of the amount of matter ( 28) an object contains

Moon A natural object that orbits a planet.

Orbit The circular or elliptical (oval-shaped)path that one object takes around anotherobject. For example, the Moon orbits theEarth, while the Earth orbits the Sun.

Perihelion The point in an object’s orbitwhen it is closest to the Sun.

Planet A large, round object that orbits theSun. A planet does not share its orbit withany other objects.

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Albedo A measure of how strongly anobject reflects light. A completely dark, non-reflective object has an albedo of 0, whilean object that reflects all light that hits ithas an albedo of 1. The Earth has an albedoof 0.36 and Venus has an albedo of 0.65.

Aphelion The point in an object’s orbitwhen it is farthest away from the Sun.

Astronaut Someone who is trained to travel into space in a spacecraft.

Astronomical Unit (AU) The averagedistance between the Earth and the Sun.One AU is 149,597,871 km.

Atmosphere The envelope of gasessurrounding a planet, moon or star.

Axis An imaginary straight line that runs through the centre of a planet, moon or star.

Celestial body Any natural object locatedoutside of the Earth’s atmosphere.

Conjunction The point when two celestialbodies appear to be in line with each otherin the sky, for example when the Moonappears very close to a planet.

Core The innermost part of a planet,moon or star.

Crust The outer layer of a planet or moon.

Day The time that it takes a moon orplanet to rotate once on its axis.

Density A measure of how compactsomething is. An object is denser thananother if its atoms ( 28) are larger ormore closely packed together.

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The diagram below shows the distances of theplanets from the Sun measured in AstronomicalUnits (AU).

1 Mercury2 Venus3 Earth4 Mars5 Jupiter6 Saturn7 Uranus8 Neptune

Plut

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AU

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! Early culturesbelieved that the Sunand Moon were gods,the Earth was flat and the sky was agreat domesuspended above it.

! In later years astronomers from ancientGreece proved that the Earth was round.Greek astronomer, Aristarchus (310-230 BC),was the first to propose that the planetsorbit the Sun. Most astronomers of his timethought that the Sun, Moon and planetstravelled in circular paths around the Earth.

! Polish priest and astronomer NicolausCopernicus (1473-1543) declared that the Sunlay at the centre of a system of the planetsand only the Moon orbited the Earth.

! German astronomer Johannes Kepler(1571-1630) showed that the planets moved inelliptical orbits.

! Galileo Galilei (1564-1642) was the firstastronomer to use a telescope. By observingthat Venus, like the Moon, had phases ( 13),

he confirmed Copernicus’s theory thatthe planets orbit the Sun.

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Astronomy is the study of space,including planets, stars andgalaxies. We can see some objects

in space with just the naked eye, butmany more, including those that arebillions of light years away, can only bestudied by using a powerful telescope.Much of what we now know about theplanets comes from space probes, whichtravel through space, sendinginformation back to the Earth.

Satellite An object that orbits a largerobject. The Moon is a natural satellite.Artificial satellites are man-made objectsthat have been launched into orbit. Theycan be used for communications,navigation and weather forecasting.

The planets illustrated to scale (above)

The Solar System consists of the Sun and all of the objects that orbit (go round)it. This includes the eight planets, their moons, asteroids, comets andmeteoroids, and vast amounts of gas and dust. The Sun’s mass ( 7), compared

to the rest of the Solar System, gives it the gravitational pull ( 6) that keeps theseobjects in orbit around it. The planets all orbit the Sun in an anti-clockwise direction,following elliptical (oval) paths, rather than circular ones.

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1 Sun2 Mercury3 Venus4 Earth5 Mars6 Jupiter7 Saturn8 Uranus9 Neptune10 Voyager11 Halley’s

Comet12 Pluto13 Pioneer 10

space probe

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ASTEROID BELT

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The Sun is a star at the centre of ourSolar System. It is one of billions of stars in the Milky Way Galaxy,

itself one of billions of galaxies in theUniverse. Like all stars, the Sun is anenormous spinning globe of glowing gasthat produces massive amounts ofenergy by converting hydrogen intohelium. To us on Earth, the Sun is ofcrucial importance since no life couldexist without it. The Sun is about 4.6billion years old. It will keep shining foraround another 5 billion years.

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Chromosphere A thin layer of the Sun’satmosphere around 2000 km deep, justabove the photosphere and beneath thecorona. The chromosphere can reachtemperatures of up to 20,000°C.

Convective zone The uppermost innerlayer of the Sun. Hot gas bubbles up tothe surface of the convective zone beforesinking down to be reheated again.

Corona The Sun’s hot outer atmosphere.The corona extends for millions ofkilometres into space. It is visible fromEarth only during a total solar eclipse.

Flare An explosion of hydrogen in theSun’s atmosphere, releasing enormousamounts of energy.

Granules Bubbles in the Sun’sphotosphere. Granules occur where heatrises up from the convective zone.

Heliosphere A magnetic “envelope” thatsurrounds the Sun and extends to theedge of the Solar System. The heliosphereis created by the solar wind.

Infrared radiation A form ofelectromagnetic energy ( 29). Infraredradiation is what we feel as heat. Morethan half of the energy the Sun gives o! isin the form of infrared radiation.

Nuclear fusion The process by which theSun generates energy. Inside the Sun’score, hydrogen atoms fuse together toform atoms of helium. The energyreleased through this process is thesource of the Sun’s light and heat as wellas other forms of radiation.

Photosphere The surface of the Sun, fromwhere the Sun radiates its energy. Thephotosphere is the part of the Sun thatwe can see. The photosphere is onlyabout 500 km thick and, at 5500°C, ismuch “cooler” than the Sun’s core. It is ina state of constant motion, like waterboiling in a kettle.

Plasma Gas that is heated to such a high temperature that its atoms become electrically charged. The Sun’sinner layers and atmosphere are in aplasmic state.

Prominence A flaming arch of gas thatleaps o! the photosphere and out intothe corona. Prominences are held up bythe Sun’s magnetic field ( 6)

Radiation The transmission of heat, lightand other forms of energy. Radiantenergy travels in waves. Di!erent formsof radiation are classified in theelectromagnetic spectrum ( 29).

Radiative zone One of the inner layers ofthe Sun. Energy radiates out through thislayer from the Sun’s core.

Solar core The innermost part of the Sun.The core is a region of high pressure (200billion times that on the Earth’s surface)and intense heat (about 15 million°C). Thisis where the energy that keeps the Sunshining is produced by nuclear fusion.Energy flows out from the core throughthe radiative zone to the convective zone.

Solar eclipse The passage of the Moonbetween the Sun and the Earth, blockingout some or all of the Sun’s light (seediagram opposite).

Solar wind A stream of subatomicparticles ( 28) that flows steadily awayfrom the Sun. This occurs because gasesin the corona have too much energy to beheld in by even the Sun’s stronggravitational field ( 6).

Spicule A narrow, flaming jet of gas thatleaps up to 10,000 km above thephotosphere then falls back.

Sunspot A darker, cooler area thatappears temporarily on the Sun’sphotosphere. Sunspots are places wherelines of magnetic force ( 6) pass throughthe photosphere. They usually occur insmall groups. The dark inside of a sunspotis called the umbra and the lighter outsideis called the penumbra.

About five billion years from now, the hydrogenthat the Sun uses as fuel will start to run out, andthe Sun will balloon into a red giant ( 23),engulfing Mercury, Venus and Earth. Above is anartist’s impression of what the Earth’s surfacemay look like when this happens.

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Prominence

Convectivezone

Radiativezone

CorePhotosphere

Sunspots

Flare Earth Jupiter

(to scale)

Spicules

Prominence

Sunspots

A solar eclipse. By coincidence, the Sun andMoon appear the same size in our sky. When theMoon passes between the Earth and the Sun itmay partially or fully block out the Sun. Duringa total eclipse, the Moon covers the Sun’ssurface entirely and we can see the corona.

The Sun with a segment taken out to reveal itsinternal layers (above)

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Moon

Sun

Earth

1 2 3

Corona

The Sun grows to the size of a red giant.

Small prominence

Umbra

Penumbra1

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! The Sun contains more than 99% of all thematter in the Solar System.

! About three-quarters of the Sun is madeup of hydrogen.

! The Sun is so big that approximately1,400,000 globes the size of the Earth couldfit inside it.

! Never look directly at the Sun or look at itthrough binoculars or a telescope. This canblind you or seriously damage your eyesight.

! If the Earth were any closer to the Sun itwould have been too hot for life to develop.If it were any farther away from the Sun itwould be too cold for life.

! It takes about eight minutes for light totravel from the Sun to the Earth.

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The Moon is a ball of rock that travelsaround the Earth. It is the brightestobject in the night sky, although it

does not give out its own light, but reflectslight from the Sun. The Moon is about fourbillion years old. It may have formed whena large object crashed into the young Earthand the resulting debris came together toform our Moon. There is neither air norliquid on the Moon. It is a barren world,made up of craters, mountain ranges, andwide lava plains. The Moon spins once onits axis in exactly the time it takes it toorbit the Earth. This means that we onlyever see one side of the Moon.

A lunar eclipse in progress (above)

Apogee The point in the Moon’s orbitwhen it is farthest away from the Earth.

Apollo 11 The mission that first put men onthe Moon. The Lunar Module of Apollo 11touched down on 20th July 1969. USastronauts Neil Armstrong and Buzz Aldrinwere the first people to walk on the Moon.

Basin A large dip in the surface of theMoon. Basins are shallower than craters.

Crater A saucer-shaped dip on the surfaceof the Moon. Craters, or impact craters areformed when large lumps of rock, calledmeteorites, collide with the Moon. They arealso found on the surface of other moons,asteroids and planets, including Earth.

Crescent Moon A phase of the moon thatoccurs just before and after every NewMoon. A crescent moon appears as asmall curved sliver of light.

Earthshine Sunlight reflected from theEarth and on to the Moon’s surface. Thereflected light makes the dark surface ofthe Moon appear to glow.

Full Moon A phase of the Moon. During aFull Moon, the visible side of the Moon isturned towards the Sun and lit up so that itappears as a full disc of light.

Gibbous Moon A phase of the Moon thatoccurs between Half Moon and Full Moon.The Moon appears as an oval of light.

Half Moon One of the phases of the Moon,sometimes called a Quarter Moon. In a HalfMoon, only half of the visible side of theMoon is lit up.

Luna Programme A Russian space projectthat sent several space probes to theMoon. In 1959, the Luna 2 became the firstman-made object to touch the Moon. Inthe same year, the Luna 3 took the firstphotographs of the far side of the Moon.

Lunar A word that means “about the Moon”.

Lunar eclipse The passage of the Earthbetween the Sun and Moon during whichthe Earth casts a shadow on the Moon.

Lunar month The time it takes the Moon topass from New Moon to New Moon—about 29 and a half days.

Mare A smooth lava plain on the surface ofthe Moon, formed when lava from ancienteruptions turned to solid rock. To theobserver, it appears as a dark patch. Mare isthe Latin word for sea. This is because earlyastronomers believed the plains were seas.

New Moon One of the phases of the Moon.A New Moon occurs when the Moon liesbetween the Sun and the Earth so that theside facing us does not reflect any light.

Perigee The point in the Moon’s orbit whenit is closest to the Earth.

Phase The di!erent appearance of anobject as it orbits the Earth. The lunarphases refer to our changing view of theMoon throughout the month. The Moonappears to change shape because our viewof its sunlit part changes (see opposite).

Regolith A layer of loose dust, soil androck that covers harder layers of rock. Itis found on many moons and asteroids.

Rilles Cracks, or channels, that run acrossthe smooth parts of the Moon, probablycaused by ancient volcanic eruptions.

Selenography The study of the surface ofthe Moon.

Terrae The light areas on the Moon as wecan see it from Earth. These arehighlands of mountains and crater edges.

Waning The period between a Full Moon and a New Moon, when the visible part of the Moon becomes smaller and smaller.

Waxing The period between a New Moonand a Full Moon, when the visible part ofthe Moon becomes larger and larger.

THE PHASES OF THE MOON: The moon only spinsonce as it travels round the Earth, so the same faceremains pointed towards us at all times. When theface pointed towards us is turned away from theSun we cannot see the moon at all. This is called

a New Moon (1). When it is turned towards us wesee a Full Moon (5). In between these phases itpasses through crescent (2), quarter (3) andgibbous (4) phases.

US astronaut Buzz Aldrin standing beside the lunarmodule of Apollo 11. The lunar module was called Eagle.

1

234

5

67

8

1 2 3 4 5 6 7 8

DDiiaammeetteerr:: 3476 kmDDaayy:: 27.3 Earth daysAAvveerraaggee ddiissttaannccee ffrroomm tthhee EEaarrtthh:: 384,600 km SSuurrffaaccee tteemmppeerraattuurree:: -155 °C to +105°CAAttmmoosspphheerree:: practically noneGGrraavviittyy (Earth = 1):: 0.16MMaassss (Earth = 1):: 0.012DDeennssiittyy (water = 1) :: 3.34

! The Moon is just over a quarter the sizeof the Earth. Most other moons are smallerin comparison to their “parent planet”.

! Because the Moon has no atmosphere, ithas no wind or rain. This means thatfootprints, left on the Moon by astronauts,remain there undisturbed.

! A total of twelve men have walked onthe Moon. The first to do so was NeilArmstrong, on 20th July 1969.

THE MOON

12 13

Craters on the surface of the Moon (below)

Mare

Terrae

Basin

Venera Missions A series of Soviet spaceprobes sent to observe Venus. In 1967,Venera 4 was the first space probe toenter another planet’s atmosphere. In1970, Venera 4 was the first space probeto land successfully on the surface ofanother planet and send information back

to the Earth.

Viking Programme Two US spaceprobes that were sent to Mars in1975 to search for evidence of life,past or present.

Wrinkle ridges Enormous cli!s onthe surface of Mercury that were

created as the planet’s surfacecooled down and shrank over time.

Some of Mercury’s “wrinkle ridges” aremore than 4 km high.

Tharsis Bulge A large mound on theMartian equator, where several massivevolcanoes are found, including ArsiaMons, Ascraeus Mons, and Pavonis Mons.

Valles Marineris A huge canyon on Mars.At 200 km wide and 7 km deep it is morethan four times deeper than the GrandCanyon on Earth and as long as the UnitedStates is wide.

Mercury, Venus, Earth and Mars areknown as the “inner planets”because they are the planets

closest to the Sun. They are also called the“terrestrial” planets, meaning “Earth-like”.This is because, like the Earth, they aremostly made up of rocks. Mercury is asmall planet covered with craters. Venus isabout the same size as the Earth, shroudedin clouds of sulphuric acid, beneath whichit is covered with lava plains and volcanoes.Because its cloud cover reflects light fromthe Sun, it is a very bright object in thenight sky. Mars is a reddish-brown planet.Like Earth it has volcanoes, mountains,canyons and polar icecaps.

DDiiaammeetteerr:: 12,105 kmDDaayy:: 243 Earth daysYYeeaarr:: 225 Earth daysAAvveerraaggee ddiissttaannccee ffrroomm tthhee SSuunn::108 million kmSSuurrffaaccee tteemmppeerraattuurree:: 490°CAAttmmoosspphheerree:: carbon dioxide and tracesof nitrogenGGrraavviittyy (Earth = 1):: 0.9NNuummbbeerr ooff mmoooonnss:: none MMaassss (Earth = 1):: 0.81DDeennssiittyy (water = 1):: 5.2NNaammee:: the Roman goddess of love

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VVEENNUUSS

MMEERRCCUURRYYDDiiaammeetteerr:: 4880 kmDDaayy:: 58.6 Earth daysYYeeaarr:: 88 Earth daysAAvveerraaggee ddiissttaannccee ffrroomm tthhee SSuunn:: 58 million kmSSuurrffaaccee tteemmppeerraattuurree:: -180 °C to +430°CAAttmmoosspphheerree:: traces of heliumGGrraavviittyy (Earth = 1):: 0.38NNuummbbeerr ooff mmoooonnss:: none MMaassss (Earth = 1):: 0.055DDeennssiittyy (water = 1):: 5.4NNaammee:: the Roman messenger of the gods

MMEERRCCUURRYY,,VVEENNUUSS &&MMAARRSS

Beta Regio A canyon (a deep narrowvalley) on Venus that is nearly 1 km deep.

Caloris Basin The largest crater on Mercury.It measures 1500 km across and is one ofthe largest craters in the Solar System.

Deimos The smaller of Mars’s two tinymoons. It measures 16 km across at itswidest. Astronomers believe that it mayonce have been an asteroid.

Dust storm A storm created when strongwinds lift up loose dust and blow it intoclouds. Dust storms on Mars cansometimes cover the entire planet.

Greenhouse e!ect Warming caused whenheat from the Sun becomes trapped by aplanet’s atmosphere. The intensetemperatures on the surface of Venus arethe result of heat being trapped by theplanet’s thick atmosphere.

Iron oxide Otherwise known as rust, ironoxide is a reddish-brown chemical thatgives Mars its distinctive colour.

Ishtar Terra A region of mountains onVenus that covers an area the size ofAustralia. The highest of these mountains isslightly taller than Mount Everest on Earth.

Magellan Probe A US space probe sent toVenus in 1989. The probe used radar tobuild up pictures of the planet. Radardetects objects by sending out radio wavesand receiving their “echoes”.

Mariner Programme A US project that sentspace probes to observe Mercury, Venusand Mars between 1964 and 1973.

Martian polar caps Two large areas offrozen carbon dioxide and water that coverthe land at Mars’s north and south poles.

Maxwell Montes A mountain range onVenus. It contains the highest summit onthe planet.

Messenger A US space probe launched in2004 to observe Mercury.

Olympus Mons An enormous volcano onMars. At 27 km high and 700 km wide, it ismore than twice as high as Mount Evereston Earth, and its base covers an area ofland larger than England.

Pathfinder Probe A US space probe thatwas sent to Mars in 2007. The probedispatched the small Sojourner Rover,which was the first wheeled vehicle totravel across the Martian surface.

Phobos The larger of Mars’s two tinymoons. It measures 26 km across at itswidest. Astronomers believe that it mayonce have been an asteroid. Phobos willprobably collide with Mars in about 50million years.

Pioneer Venus Project A US probe that waslaunched in 1978 to carry out experimentson Venus’s atmosphere.

Sulphuric acid A very corrosive acid thatmakes other substances disintegrate.Venus is covered with thick clouds ofsulphuric acid.

MERCURY

Borealis Basin A large flat dip in the surfaceof Mars. The Borealis Basin may have beencreated by an impact with an asteroid.

A number of valleys and channels have been carvedinto the Martian plains. These are thought to havebeen formed by rivers or seas that no longer exist.

The space probe Magellan, launchedin 1989, used radar to build up apicture of the entire globe of Venus.

DeimosOn the left we can see Venus as it appears fromspace, shrouded in thick cloud. On the right, is whatthe surface of Venus looks like beneath the clouds.

Maat Mons (below) is the highest volcano on Venus.

Mercury is covered in craters, made by meteoroids. These would normally burn up as theypass through a planet’s atmosphere, but Mercury’satmosphere is too thin for this to occur.

DDiiaammeetteerr:: 6797 kmDDaayy:: 24.6 Earth daysYYeeaarr:: 687 Earth daysAAvveerraaggee ddiissttaannccee ffrroomm tthhee SSuunn:: 228 million kmSSuurrffaaccee tteemmppeerraattuurree:: -120 °C to +25°CAAttmmoosspphheerree:: carbon dioxide and nitrogenGGrraavviittyy (Earth = 1):: 0.4NNuummbbeerr ooff mmoooonnss:: 2 MMaassss (Earth = 1):: 0.11DDeennssiittyy (water = 1):: 3.9NNaammee:: the Roman god of war

VENUS

MARS

14 15

Phobos

Jupiter’s four largest moons are called the Galileansafter Galileo who discovered them in 1610.

Saturn rotatesvery quickly.This producesa distinctbulge at itsequator.

Galileo Mission A mission that sent a spaceprobe to orbit Jupiter in 1989. It was thefirst spacecraft to orbit the planet.

Galileo Galilei (1564–1642) Italianastronomer who was the first person to usea telescope. He made many importantobservations and discoveries, including thediscovery of Jupiter’s moons. By observingthat Venus, like the Moon, had phases, heconfirmed that the planets all orbit the Sun.

Ganymede The largest of Jupiter’s moons.Bigger than the planet Mercury, it measures5268 km across and is the largest moon inthe Solar System. Ganymede has an icysurface with dark plains covered in groovedpatterns and craters.

Gas giants The four large planets in theSolar System: Jupiter, Saturn, Uranusand Neptune. They mostly consistof gases with no solidsurface.

Io Jupiter’s third largest moon. Itmeasures 3643 km across. Io is coveredwith active volcanoes and pools of moltenrock. It is the most volcanically activeplace in the Solar System,

Jovian planets Another name for the gasgiants. The word “Jovian” means“belonging to Jupiter”.

Great Red Spot A giant storm in Jupiter’satmosphere. The storm has been ragingfor at least 300 years.

Great White Spot A large storm thatoccurs in Saturn’s atmosphere aboutevery 30 years.

Huygens, Christiaan (1629–1695) Dutchastronomer who discovered the moonTitan and spent much of his careerstudying the rings of Saturn.

Cassini Division The largest of the gapsbetween Saturn’s rings. The gap is about4800 km wide. Gaps between rings arecaused by the gravitational pull ( 6) ofthe planet’s moons.

Cassini-Huygens Mission A project thatsent a spacecraft to study Saturn and itsrings and moons. The Huygens Probelanded on Titan in 2005. Cassini is stillorbiting Saturn and returning data.

Enceladus One of Saturn’s moons. Itmeasures 498 km across. Enceladus’ssurface is mostly made up of water ice.

Europa Jupiter’s fourth largestmoon. It measures 3130 kmacross. Europa has an icysurface, which may have awater oceanbeneath it.

Jupiter is the largest planet in theSolar System. It is more massivethan all the other planets combined.

Jupiter is known as a “gas giant”because it is mostly made up of gas withno solid surface at all. Its neighbour,Saturn, another gas giant, is the secondlargest planet. Both planets aresurrounded by “rings” of rock and dust.Saturn’s rings, broad and bright, reachout more than 420,000 km from theplanet. Jupiter and Saturn both have alarge number of moons.

JJUUPPIITTEERR &&SSAATTUURRNN

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JJUUPPIITTEERR

Jupiter’s Great Red Spot. The white ovals are aircurrents flowing past the spot.

Belts The darker of the striped cloudbands that surround Jupiter.

Callisto Jupiter’s second largest moon. Itmeasures 4800 km across. For its size, ithas more craters than any other planet ormoon in the Solar System.

Cassini, Giovanni Domenico (1625–1712)Italian astronomer who first observed fourof Saturn’s moons and also discovered thelargest gap between Saturn’s rings.

Mimas One of Saturn’s moons.Mimas has an enormous crater that

measures 130 km across—a diameter onethird that of the whole moon.

Pioneer Missions A series of US missionsthat sent space probes to Jupiter, Saturnand Venus in the 1970s. Pioneer 10 laterbecame the first man-made object toleave the boundaries of the Solar System.

Ring A band of dust, rocks, ice or othermaterials surrounding a planet at itsequator. Saturn’s rings are probablyfragments of a moon that was smashed bya passing comet.

Ringlet A very fine planetary ring. Saturn’srings are made up of hundreds orthousands of ringlets.

Titan Saturn’s largest moon. It measures5150 km across, and is the second largestmoon in the Solar System. Titan is the onlymoon in the Solar System known to havea thick atmosphere.

Zones The lighter of the striped cloudbands that surround Jupiter.

EuropaCallisto Ganymede

Io is covered with volcanoes and lava pools.

Saturn as seen from the surface of Titan.

Jupiter’s bright colours of red, orange, yellow and brown, are produced by the elements sulphur and phosphorus.

Io

DDiiaammeetteerr:: 120,514 kmDDaayy:: 10.2 Earth daysYYeeaarr:: 29.5 Earth yearsAAvveerraaggee ddiissttaannccee ffrroomm tthhee SSuunn:: 1427 million kmSSuurrffaaccee tteemmppeerraattuurree:: -180 °CAAttmmoosspphheerree:: hydrogen and heliumGGrraavviittyy (Earth = 1):: 11.16NNuummbbeerr ooff mmoooonnss:: 62MMaassss (Earth = 1):: 95.2DDeennssiittyy (water = 1):: 0.7NNaammee:: the Roman god of farming

DDiiaammeetteerr:: 143,884 kmDDaayy:: 9.8 Earth daysYYeeaarr:: 11.8 Earth yearsAAvveerraaggee ddiissttaannccee ffrroomm tthhee SSuunn:: 778 million kmSSuurrffaaccee tteemmppeerraattuurree:: -150 °C AAttmmoosspphheerree:: hydrogen and heliumGGrraavviittyy (Earth = 1):: 2.64NNuummbbeerr ooff mmoooonnss:: 63MMaassss (Earth = 1):: 318DDeennssiittyy (water = 1):: 1.3NNaammee:: the Roman king of the gods

From Earth, it ispossible to identify

three of Saturn’srings: an outer

ring (ring A),separated from

two inner rings (B andC) by the Cassini Division.

The Cassini Division

CB

A

SATURN

JUPITER

16 17

Saturn’s rings are made up of billions of blocks ofrock and ice, some bigger than houses.

The spaceprobe Voyager

Ariel The brightestof Uranus’s moons.

Ariel measures 1160 kmacross. The rarity of craters

on Ariel suggests thatvolcanic eruptions, and movements of themoon’s crust, may have erased them.

Cryovolcano An ice volcano that forms oncold moons and other objects far out inspace. Triton is covered in cryovolcanoes.Rather than molten rock, they eruptnitrogen gas.

Galle, Johann (1812-1910) Germanastronomer who discovered Neptune in1846. He was the first person to viewNeptune and recognize it as a planet.

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1

4

UURRAANNUUSS

Titania The largest of Uranus’s moons,made up of ice and rock.

Triton The largest of Neptune’s moonsand the only major moon in the SolarSystem to orbit its “parent planet” in theopposite direction to the planet’s rotation. At -238°C, it is the coldest known object in the Solar System.

Umbriel One of Uranus’s five large moons.It is covered in large craters. One of these,the Wunda crater has a ring of brightmaterial at its base. Scientists do not yetunderstand what this material is.

Voyager Programme A US space missionthat sent space probes to Jupiter, Saturn,Uranus and Neptune in the 1980s. Theysent back data and images of the planetsand their moons and rings. The spaceprobe Voyager carries an audio-visual disc.Should aliens come across it, they wouldhear the sounds of whales, a baby cryingand greetings in 55 languages.

UURRAANNUUSS &&NNEEPPTTUUNNEE At

mos

phere

Water andammonia

Core

Proteus The second largest of Neptune’smoons. It measures about 400 km. Itreflects very little light, so it appears verydark against the night sky.

“Scooter” A giant, fast-moving storm on the planet Neptune.

Shepherd satellite A moon that orbits aplanet close to its rings. Shepherdsatellites exert a gravitational pull ( 6) ona planet’s rings, holding them together ina narrow band. Uranus’s moons Opheliaand Cordelia are shepherd satellites.

Great Dark Spot A huge storm systemthat was photographed on Neptune in1989. Winds in the spot were recorded tobe moving at 2400 km/h—the fastestwinds ever recorded in the Solar System.In 1994, the spot was discovered to havevanished and been replaced with anothersmaller storm.

Herschel, William (1738-1822) An amateurBritish astronomer who discoveredUranus in 1781. He was also the firstastronomer to realize that the SolarSystem is, itself, travelling through space.

Le Verrier, Urbain (1811-1877) Frenchastronomer who predicted the locationand existence of Neptune, independentlyof John Couch Adams.

Methane A chemical found in layers of iceand liquid on Neptune and Uranus.Uranus’s upper layer of clouds is made upof methane gas. This layer absorbs redlight from the Sun, so that it only reflectsblue light. It is this that gives Uranus itsblue colour.

Miranda The smallest of Uranus’s majormoons, measuring 470 km across.

Miranda has a jumbled-up surface,covered with deep grooves,

canyons and mountain ranges.Astronomers think the unusualsurface was formed by themoon being blasted apart and

reassembled again (see left).

Astronomers believe that Uranus’s moonMiranda may once have been smooth (1) before itwas blasted apart by a meteoroid (2). Gravity held the shatteredfragments together (3) and kept them from drifting into space.The pieces eventually reassembled into a lumpy ball of mixed-uppieces (4), giving Miranda its jagged surface.

2

3

The volcanoes on Triton are plumes of nitrogen gasthat burst through cracks in the moon’s icy surface.

Nereid One of Neptune’s moons. Itmeasures 340 km across and has a veryunusual orbit. It takes it as close as 1.4million km from Neptune and then as faras 9.6 million km away from it.

Oberon The second largest of Uranus’smoons. It measures 1522 km across and isthe farthest major moon from the planet.It is covered in deep craters.

Triton (not to scale)

NEPTUNE

Adams, John Couch (1819-1892) Britishastronomer who predicted the existenceand position of Neptune. Without knowingit, Adams made his predictions at almostexactly the same time as Le Verrier.

Ammonia One of the most commonchemicals found on Neptune and Uranus. A layer of ice and liquid water, ammoniaand methane lies between the rocky coreand outer atmosphere of both planets.

DDiiaammeetteerr:: 51,118 kmDDaayy:: 17.2 Earth daysYYeeaarr:: 84 Earth yearsAAvveerraaggee ddiissttaannccee ffrroomm tthhee SSuunn:: 2869 million kmSSuurrffaaccee tteemmppeerraattuurree:: -210 °CAAttmmoosspphheerree:: hydrogen, helium and methaneGGrraavviittyy (Earth = 1):: 1.17NNuummbbeerr ooff mmoooonnss:: 27MMaassss (Earth = 1):: 14.5DDeennssiittyy (water = 1):: 1.3NNaammee:: the Greek god of the sky

DDiiaammeetteerr:: 49,557 kmDDaayy:: 16.1 Earth daysYYeeaarr:: 164.8 Earth yearsAAvveerraaggee ddiissttaannccee ffrroomm tthhee SSuunn:: 4496 million kmSSuurrffaaccee tteemmppeerraattuurree:: -220 °CAAttmmoosspphheerree:: hydrogen, helium and methaneGravity (Earth = 1): 1.2NNuummbbeerr ooff mmoooonnss:: 13MMaassss (Earth = 1):: 17.14DDeennssiittyy (water = 1):: 1.77NNaammee:: the Roman god of the sea

Uranus and Neptune are both gasgiants ( 16). Like the others, theyare surrounded by rings ( 17)

and have a large number of moons.Uranus is a featureless blue globe ofgas. Uranus orbits the Sun at a veryunusual angle. It is tilted at 98° from thevertical, meaning that it orbits the Sunalmost on its side. Neptune is a brightblue globe, streaked by fast-movingclouds and the occasional storm. Uranusand Neptune are the most featureless ofall the planets in the Solar System.

URANUS

18 19

Uranus’s internal structure is verysimilar to that of Neptune’s.

Halley, Edmond (1656-1742) Englishastronomer who was the firstperson to realize that comets areobjects that orbit the Sun.

Kuiper Belt A region of the Solar Systembeyond the planet Neptune. It mainlyconsists of icy bodies, including the dwarfplanet Pluto. The Kuiper Belt is thought tobe 20 times as wide as the Asteroid Belt.

Meteor A streak of light in the sky thatoccurs when a meteoroid enters theEarth’s atmosphere. Meteors aresometimes called “shooting stars”.

Meteorite A meteoroid that lands on thesurface of a planet or moon.

ASTEROIDS

Dwarf planet A relatively large, roundobject that orbits the Sun. Dwarf planetsshare their orbit with other objects of asimilar size to themselves.

CCOOMMEETTSS &&AASSTTEERROOIIDDSS

Diagram of the Oort Cloud

Kuiper Belt

Oort Cloud

SolarSystem

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Eris The largest dwarf planet in the SolarSystem. Eris measures 3000 km across.Located far beyond Pluto, it is one of themost distant objects in the Solar System.

Giotto Probe A European space probethat flew by Halley’s Comet in 1986,sending back pictures and sampling thegases and dusts given o! by the comet.

Halley’s Comet A very bright comet thatappears in our skies every 76 years.

Trojan An asteroid that shares the sameorbital path as Jupiter.

Tunguska fireball The name given to thehuge explosion in the Tunguska region ofSiberia, Russia on 30th June 1908. TheTunguska fireball may have been a cometexploding at an altitude of about 6 km.

! The name “comet” comes from theGreek kometes meaning “long-haired”because a comet’s tail makes it look like astar with hair.

! Throughout history, the unexpectedarrival of comets in the sky has often beenseen as an sign, or omen, of change. Comets

were thought to bring famines, and somepeople thought that when a comet

appeared a king would die.

! Edmond Halley believed thatcomets recorded in 1531 and 1607were earlier sightings of a comet

he himself observed in 1682.Although he did not live to witness the

event, Halley’s Comet, as it is now called,appeared in 1758, as Halley had predicted.

! The term “dwarf planet” was onlycreated in 2006 when the discovery ofobjects larger than Pluto led astronomers too!cially reclassify the term “planet”.

! In July 1994, drawn in by gravity, thecomet Shoemaker-Levy collided withJupiter, creating massive fireballs on impact.

! The largest known meteorite was foundin Namibia in 1920. It measures 2.7 m longand 2.4 m wide.

The Solar System is full of lumps ofdust, rock and ice. These minorobjects include comets, asteroids

and dwarf planets. Comets are lumps ofdust frozen together by ice. Asteroidsare rocky objects that orbit the Sun,mostly in a band between Mars andJupiter. Dwarf planets are large, roundobjects that orbit the Sun. Together, all

of these minor objects only make up atiny fraction of all the mass ( 7) in

the Solar System.Asteroid Belt The region between Marsand Jupiter where most of the knownasteroids in the Solar System lie.

Bolide An unusually bright meteor thatexplodes in the sky.

Centaurs A group of small rocky objectsthat orbit the Sun between the planetsJupiter and Neptune.

Ceres The smallest dwarf planet in theSolar System and the only one in theasteroid belt. Ceres measures about 950km across.

Charon Pluto’s largest moon. It measuresabout 1200 km across.

Coma The cloud of dust and gas thatcomes o! a comet as it nears the Sun. The

Sun melts the frozen gases and water thatmake up the comet’s nucleus. The solar

wind ( 11) then “blows” the gasesback into a pair of tails.

Comet A lump of dust and ice thatorbits the Sun. On nearing the

Sun a comet develops two tailsof gas and dust. Comets

measure a few kilometresacross. Their tails stretch

behind them for hundredsof millions of kilometres.

As a comet approaches the Sun, the frozen waterand gases in its nucleus melt and trail behind it.

A comet has two tails: a straight gastail and a broader, curved dust tail.

Meteoroid A small lump of rock and dustthat hurtles across the Solar System. It wasonce part of an asteroid or comet.

Meteor shower An event that occurs whena large number of meteors enter theEarth’s atmosphere in a short time.

NEOs (Near-Earth Objects) Any objectwhose orbit brings it close to the Earth.

Oort Cloud A cloud of comets surroundingthe Solar System beyond Pluto.Astronomers believe that the Oort Cloudmay be the place where comets comefrom. It lies one fifth of the way betweenour Solar System and the nearest star,

Proxima Centauri ( 23).

Pluto Previously thought to be theninth planet, Pluto is now classified

as a dwarf planet. It measures 2274 km across and is the largestobject in the Kuiper Belt.

Stardust Probe A US space probethat captured dust grains, or

“stardust”, floating space, as wellas from a comet.

Tombaugh, Clyde (1906-1997) Americanastronomer who discovered Pluto in 1930.

It is feared that one day an asteroid’s orbitmay bring it into contact with the Earth. Animpact could cause climate change sufficientto wipe out many life-forms.

PLUTO

21Gas tail

Dust tail

Nucleus

Asteroid A rockybody that orbits theSun. Asteroids range insize from tiny specks toblocks about 1000 km indiameter. Most asteroids aremade of rock, some are made ofmetal and some are made of rock andmetal. They are sometimes referred to asplanetoids or minor planets.

20

TTHHEE LLIIFFEE CCYYCCLLEEOOFF AA SSTTAARR1 A star begins its life as a dense mass of gasand dust called a protostar.

2 The core becomes so hot that nuclearreactions start deep inside it. Gas and dustare blown away, although some remain in adisc surrounding the new star.

3 Planets may form here.

4 The star is now a main sequence star.

5 When the fuel it uses to produce energyruns out, the core collapses and the

star swells into a red giant.

6 A massive star will become asupergiant that will blastapart in a mighty explosioncalled a supernova.

7 It ends its days as aneutron star or a

black hole.

A less massivestar becomes a

red giant. At theend of its life, its

outer layers puffaway into space (a

planetary nebula) leavingbehind a white dwarf.

Proxima Centauri The second nearest starto the Earth (after the Sun). It is about 4.2light years ( 6) away.

Pulsar A rapidly spinning neutron star thatgives out large amounts of energy.

Red dwarf A small, cool star in the long-lasting, middle-period of its life.

Red giant A large, old star with a lowtemperature and a reddish orange colour.

Sirius The brightest star in the night sky. Sirius appears in the constellation CanisMajor ( 24).

Starburst A region within a galaxy wherelots of stars are born in a very short time.

THE LIFE CYCLEOF A STAR

Supergiant A large, bright star that is nearthe end of its life cycle.

Supernova A massive explosion of asupergiant at the end of its life cycle. A supernova happens when a massive starruns out of hydrogen and collapses underits own gravity. It leaves behind a cloud ofgas and dust or, in some cases, a neutronstar. The most massive supernovas leavebehind a black hole.

Variable star A star that varies inbrightness and size over time.

White dwarf A small, hot, bright star in thefinal stage of its life cycle.

Yellow dwarf A small star in the middle-period of its life. Our Sun is a yellow dwarf.

TheHorseheadNebula

Gas torn from stars by a nearby black hole circlesbefore plummeting inside the black hole.

Even though they are invisible, scientists can tellwhere black holes are by observing their e!ects.

Black hole A region of space from whichnothing can escape. A black hole issometimes left behind when a supergiantstar blasts itself apart in a supernova.Black holes cannot be seen because theytrap light, but they can be detected byobserving the e!ects of their gravity ( 6).

Brown dwarf A small, cool globe of gas. Abrown dwarf is a kind of “failed” star thatis too small to produce heat and light.

Blue giant A massive, hot star that wasonce a larger, cooler red giant. Bluesupergiants are rare and have short lives.They often end in a supernova.

Betelgeuse A red supergiant in theconstellation of Orion ( 24). Betelgeuse is800 times the size of our Sun and is one ofthe largest, most luminous stars known.

Binary star A system of two stars that orbit each other.

Black dwarf The last stage in the life of amedium-sized star. A black dwarf is whatremains of a white dwarf that has cooleddown and shrunk after using the last of its energy.

Above: The Crab Nebula

Left: A planetary nebula

SSTTAARRSS

Stars are giant spinning balls of hotgases. They produce tremendousamounts of energy in the form of

heat and light, which they radiate ( 11)

across space. Stars vary enormously insize and in the amount of light that theygive off. Some are smaller than ourplanet, whilst some are many hundredsof times the size of our local star, the Sun.Like people, stars have a life cycle, andthough their lives are billions of yearslong, they do not last forever. As a rule,the larger a star is, the shorter its life willbe. Our Sun is a typical star of averagesize and brightness.

Parsec The unit used to measure parallax.One parsec is about 3.26 light years ( 6).

Planetary nebula A cloud of gas and dust.A planetary nebula is the remnants of amedium-sized star that has shed its outerlayers in the final stages of its life cycle.

Polaris A bright star, sometimes called the North Star, around which the starsappear to revolve. It lies in line with theEarth’s North Pole and hardly moves inour skies. This makes it useful forlocating due North.

Protostar The early stage in a star’s lifewhen it is a cloud of gas and dust heldtogether by gravity, but is not yet hotenough to generate its own energy.

Colour index The measure of a star’scolour. It tells scientists how hot a star is.Hotter stars shine blue or white and coolerstars shine red or orange.

Crab Nebula An expanding cloud of gas leftbehind after a large star exploded in asupernova almost 1000 years ago.

Globular cluster A ball-shaped group ofhundreds or thousands of stars, heldtogether by gravity.

Horsehead Nebula A gigantic cloud of dustand gas in the shape of a horse’s head. TheHorsehead Nebula is one of the clouds inour galaxy where stars start to form.

Luminosity The amount of light and energygiven o! by a star.

Magnitude A measure of how bright anobject is, where bright objects have low ornegative numbers, and dim objects havehigh numbers. Apparent magnitudemeasures how bright an object appears asit is seen from Earth. Absolute magnitudemeasures how bright an object actually is.

Main sequence The stable middle-period ina star’s life, when it makes energy byconverting hydrogen into helium by way ofnuclear fusion ( 10). The Sun is a mainsequence star.

Nebula A cloud of gas and dust wherestars are formed.

Neutron star The dense remains of a largestar that has collapsed under its owngravity during a supernova.

Nova A star that ejects some of its gases,suddenly becoming much brighter.

Open cluster A group of several thousandstars, loosely held together by gravity.

Parallax An apparent change in theposition of an object (eg. a star) resultingfrom a change in the position of theobserver (eg. on Earth).

The scale illustration shows the sizes of a red giantand two blue giants. Our Sun, a yellow dwarf, is justa tiny yellow dot by comparison.

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2

3

4

5

6

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Aconstellation is a group of starsthat form a pattern. Years ago,early astronomers grouped the

stars into these patterns, imaginingtheir shapes to look like gods, heroesand sacred beasts from popular legends.The 88 constellations that existtoday include 48 thatwere known to theancient Greeks.

Astronomers use constellations as away of mapping the sky. They referto them to show where stars, galaxiesand other objects in the heavens are located.

ANTLIA

APUS

AQUARIUS

AQUILA

ARA

CANIS MAJOR

CAPRICORNUS

CARINA

CENTAURUS

CETUS

CIRCINUS

COLUMBA

CORONAAUSTRALIS

CORVUS

CRATER

CRUX

DORADO

ERIDANUS

FORNAX

GRUS

HOROLOGIUM

HYDRA

HYDRUS

INDUS

LEPUS

LIBRA

LUPUS

MENSA

MICROSCOPIUM

MONOCEROS

MUSCAOPHIUCHUS

ORION

PAVO

PHOENIX

PICTOR

PISCES

PISCISAUSTRINUS

PUPPIS

PYXIS

RETICULUM

SAGITTARIUS

SCORPIUS

SCULPTOR

SCUTUM

SERPENSCAUDA

SEXTANS

TRIANGULUMAUSTRALE

TUCANA

VELA

VIRGO

VOLANS

Sirius

Canopus

Large Magellanic Cloud

Alpha Centauri

Rigel

Achernar

Betelgeuse

Procyon

Antares

TRIANGULUM

ARIES

AQUILA

AURIGA

BOÖTES

CAMELO-PARDALIS

CANCERCANESVENATICI

CANISMINOR

CASSIOPEIA

CEPHEUS

COMABERENICES

CORONABOREALIS

CYGNUS

DELPHINUS

DRACO

EQUULEUS

GEMINI

HERCULES

LACERTA

LEO

LEO MINOR

LYNX

LYRA

PISCES

Galaxy

AQUARIUS

PEGASUS

OPHIUCHUS

ORION

TAURUS

CETUS

MONOCEROS

HYDRA

SEXTANSVIRGO

SERPENSCAUDA

SERPENSCAPUT

SCUTUM

URSA MAJOR

VULPECULA

Arcturus

Vega Capella

Procyon

Altair

Betelgeuse

Merak

Dubhe

Aldebaran

Pleiades

Polaris

CCOONNSSTTEELLLLAATTIIOONNSS

These charts show the stars andconstellations that can be seen from thenorthern hemisphere (left hand page) and

southern hemisphere (right hand page). Turn thebook around so that the present month is at thebottom. At 10.00 pm face south if you are in thenorthern hemisphere, or north if you are in thesouthern hemisphere. On a clear night you shouldbe able to locate many of the stars andconstellations on the chart.

STARS OF THENORTHERN SKIES

STARS OF THESOUTHERN SKIES

Orion

Ursa Major (the Great Bear)

Scorpius (the scorpion)

Andromeda

2524

ANDROMEDA

OCTANS

URSAMINOR

Agalaxy is a gigantic, rotatingcollection of gas, dust, andbillions of stars, all held together

by gravity ( 6). Some galaxies onlycontain a few million stars, while somecontain over a trillion. Our own galaxy,the Milky Way, is a vast spiral of about200 billion stars measuring 100,000 lightyears across. There are billions moregalaxies, of various shapes and sizes, allmaking up what we call the Universe.

Active galaxy A galaxy that gives o!exceptional amounts of energy for its size.The energy is produced by a large blackhole ( 22) at the galaxy’s centre. This iscalled an active galactic nucleus. There areseveral di!erent types of active galaxies.These include: seyferts, blazars, quasarsand radio galaxies. Some astronomersbelieve that these are all the same thingbut seen from di!erent angles.

Andromeda Galaxy A spiral galaxy found inthe constellation Andromeda ( 24). It isthe closest spiral galaxy to our own and amember of the Local Group of galaxies. It isone of the few galaxies that can be seenfrom Earth without using a telescope.

GGAALLAAXXIIEESS

Barred spiral A spiral galaxy with along, thin, bar-shaped centre. About two-thirds of all spiral galaxies are barred.

Bulge A large ball of tightly packed starsfound at the centre of spiral and elliptical galaxies.

Cluster A group of galaxies held togetherby gravity.

Dwarf galaxy Anygalaxy that measures less than30,000 light years across.

Elliptical galaxy A round galaxy, shaped likea ball. Elliptical galaxies contain very oldstars and little gas or dust.

Filament A long “string” of galaxysuperclusters stretching across space.These are linked together like a net madeup of strings and knots.

Galactic arms The bright regions of a spiralgalaxy that branch out of its centre.Galactic arms are full of bright, young stars.

Galactic halo The round region surroundingthe centre of a spiral galaxy. It containsdark matter ( 28) and clusters of stars.

Hubble, Edwin (1889-1953) Americanastronomer who proved the existence ofother galaxies than our own. He alsoproved that the Universe is expanding.

Hubble Space Telescope Atelescope that orbits the Earth as a

satellite. It is una!ected by weather orpollution, so it can receive pictures that aremuch clearer than those taken with Earth-based telescopes. It has photographedgalaxies as far as 10 billion light years away.

Irregular galaxy A galaxy with no obviousshape. Irregular galaxies contain lots ofvery young stars.

Lenticular galaxy A flat, disc-shaped galaxythat looks like a spiral galaxy without arms.Lenticular galaxies are made up of old starsnear the end of their life cycle.

Local Group A cluster of more than 30galaxies including the Milky Way.

Magellanic Clouds A pair of galaxies thatorbit the Milky Way. The Large MagellanicCloud and Small Magellanic Cloud are a pairof irregular galaxies that can both be seenwithout the aid of a telescope.

Milky Way Galaxy The galaxy thatcontains our Sun and Solar System. TheMilky Way is a flattened spiral of starsclustered together in space. It is namedafter the misty band of stars we can seeon a clear night—actually our side-on viewof the galactic disc and its spiral arms.

Nucleus The centre of a galaxy.

Quasar A very bright area surrounding ablack hole ( 22) at the centre of an activegalaxy. Its brilliant light comes from a discof hot gas and dust spiralling into theblack hole. One quasar gives o! moreenergy than 100 galaxies combined.

Ring galaxy A galaxy shaped like a hoopsurrounding a ball, with a large area ofempty space between most of the galaxyand its nucleus. Their shapes can be dueto collisions with other galaxies.

Spiral galaxy A spiral-shaped galaxy.Spiral galaxies have a round bulge at their centre, surrounded by a large flatdisc, with arms that spiral out from thecentre. Their arms contain younger starsand their bulges contains older stars. Our own galaxy, the Milky Way, isa spiral galaxy.

Starburst galaxy A galaxy in which anexceptionally large number of stars arecreated. Starburst galaxies are thought toresult when two galaxies collide with eachother, bringing together dense clouds ofgas which collapse, creating stars.

Supercluster A group of clusters ofgalaxies. Superclusters tend to be stretchedinto long “strings”, called filaments, thatstretch across the Universe.

Virgo Cluster A cluster of galaxies withinthe constellation Virgo ( 24). It is a hugeball of more than a thousand billion stars.

THE MILKY WAY GALAXY

Different types of galaxies (below)

The Hubble Space Telescope

! Galaxiesrotate aroundtheir centres.

! The MilkyWay Galaxy is solarge that, evenrotating at aspeed of 250km/s, it takesmore than 200million years tocomplete one full circle.

! All galaxies are moving through spaceand rushing away from each other.

! There are about 100,000 billion galaxiesin the Universe.

! The Milky Way contains at least 200billion stars and measures 100,000 light years across.

! The word “galaxy” comes from theGreek word galaxias meaning “milky”. The ancient Greeks called the Milky Way the Kiklos Galaxias, meaning “Milky Circle”.In Greek myth, the Milky Way was a splash of milk spilt across the heavens bythe goddess Hera.

FF AA CC TT FF II LL EE26 27

Edwin Hubble

1 Elliptical 2 Spiral3 Barred

spiral4 Irregular

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3

4

Right: The Milky Way as seen fromthe side. It looks like a pair of friedeggs stuck together back-to-back.The “yolks” form the centralbulge or nucleus and the “whites”

form the spiral-shaped discsurrounding it.

1 Nucleus2 Crux-Centaurus

Arm3 Perseus Arm4 Orion Arm5 Approximate

position of the Sun6 Sagittarius Arm

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2

3

4

5 6

! The commonest elements in the Universeare hydrogen, helium and oxygen.

! About three-quarters of all matter in theUniverse is hydrogen.

! After the Big Bang, it took about 300,000years for the first atoms—atoms of hydrogen gasand helium gas—tocome together.

! TheobservableUniverse is at least 92 billion light yearswide. It probablycontains more than 100 billion galaxies.

Big Bang The explosion in which theUniverse is thought to have been formed.During this explosion, all matter, energy,space and time were created.

Big Crunch The theory that all matter in the Universe will eventually collapse to asingle point, bringing about the end of the Universe.

Cosmic microwave background radiationEnergy, or radiation ( 10), that comes fromall directions in space. It is thought to beenergy left over from the Big Bang.

Cosmic inflation A theory that theUniverse expanded to trillions of

times its size in the firsttrillionth of a second of its

existence, beforecontinuing to expand ata slower rate.

Red shift Light travels in waves.Red light has a longer wavelength

(the distance between wave crests)than other colours. When a light

source moves away from you, itswavelength stretches and it appears morered. This is red shift. All galaxies otherthan our own show red shift. Thisindicates that they are moving away fromus and that the Universe is expanding.

Void A region of space that is completelyempty of matter, even dark matter.

Dark matter The invisible matter that isbelieved to hold the Universe together.Scientists think that dark matter makes up90% of the Universe. Dark matter cannotbe seen, but it can be detected byobserving the e!ects of its gravity onother objects.

Cosmology Thestudy of theUniverse. Aperson whostudies theUniverse is called acosmologist.

Cosmos Another word forUniverse. It comes from theGreek word kosmos, meaning“ordered world”.

Critical density The density that all of thematter in the Universe would need tohave in order for the Universe to stopexpanding. At exactly the critical densitythe Universe will stop expanding and staythe same size. If the Universe has lessthan the critical density then it willcontinue to expand forever. If theUniverse has more than the criticaldensity then it will eventually be collapsedby its own gravity.

Dark energy A force that could potentially overcome the gravitational pull of other objects. Dark energy is stilljust a theory, meaning that scientists think it may exist, but have yet to prove it.

The Universe is made up of everythingwe know, from the tiniest grain ofsand to the most gigantic star. It

even includes empty space. Nearly all thematter in the Universe is contained ingalaxies, which are grouped into giantclouds called superclusters. These arespread round the Universe like a net. The“holes” between matter are empty spaces,called voids. Scientists think that theUniverse began in an incredible explosionabout 13.7 billion years ago. A number oftheories have been put forward about thefuture of the Universe.

TTHHEE UUNNIIVVEERRSSEE

Accelerating Universe A theory that theUniverse is expanding at an increasingspeed. This may be caused by a force calleddark energy, which could overcome thegravitational force ( 6) that pulls galaxiestowards one another.

Atom A basic building block of matter. Itconsists of a nucleus made up of subatomicparticles called protons and neutrons,surrounded by a number of electrons. All ofthese particles were created in the firstseconds of the Big Bang.

1

2

3

Hubble’s Law Edwin Hubble’s theory thatthe farther away a galaxy is from the Earth, the faster it is moving away from the Earth. The observation that galaxies are moving away from each other is usedas evidence of the Big Bang andAccelerating Universe theories.

Matter The “stu!” that everything in theUniverse is made of. Anything that hasmass and takes up space is made of matter.Every object, substance and material in theUniverse is made of matter.

Multiverse Theory The theory that theremay be other universes than our own,sometimes called “parallel universes”.

Observable Universe The parts of theUniverse that we can observe from Earth(with telescopes). We can see these distantregions because the light they emit has hadtime to reach us since the Big Bang.

Some scientists think that allmatter in the Universe will

eventually collide in a “BigCrunch”. Invisible dark matterin the Universe may exert

su"cient gravity to halt itsexpansion and cause the galaxies

to compress together.

28 29

FF AA CC TT FF II LL EE

It is possible that the Universe will carry onexpanding forever. In this sequence, the

Universe expands rapidly, with all thegalaxies moving away from one

another as the Universe inflateslike a balloon.

Einstein, Albert (1879-1955) German-bornphysicist who is famous for his theoriesabout space, time, matter, gravity andlight. He is most famous for his theories ofrelativity. In 1921, he was awarded a NobelPrize for Physics.

Electromagnetic Spectrum A classificationof di!erent types of radiation (energythat travels in waves). Radio waves,microwaves, light and X-rays are all formsof electromagnetic radiation. They alltravel at the same speed, but havedi!erent wavelengths—the distancebetween one wave crest and the next—and emit di!erent amounts of energy.

The Universe is made up of trillions of galaxies,grouped into clusters of galaxies, which are,themselves, grouped into superclusters. 1 Universe; 2 Galaxy cluster;3 Stars in a galaxy.

Albert Einstein

AAAbsolute magnitude 22Accelerating Universe 28-29Active galactic nucleus 26Active galaxy 26Adams, John Couch 18Albedo 6Ammonia 18Andromeda 24-25, 26Aphelion 6Apogee 12Apollo 11 12Apparent magnitude 22Ariel 18Arsia Mons 15Artificial satellites 7Ascraeus Mons 15Asteroid Belt 20-21Asteroids 6, 13-15, 20-21Astronaut 6, 12-13Astronomical unit (AU) 6Astronomy 6Atmosphere 6, 13-15, 17, 21Atom 6, 28Axis 6, 12

BBBarred spiral 26Basin 12-13Belts 16Beta Regio A canyon 14Betelgeuse 22Big Bang 28-29Big Crunch 28Binary star 22Blazars 26Black dwarf 22Black hole 22-23, 26-27Blue giant 22-23

Bolide 20Borealis Basin 14Brown dwarf 22Bulge 26

CCCallisto 16Caloris Basin 14Canis Major 23-24Canyon 14-15Cassini Division 17Cassini, Giovanni

Domenico 16Celestial body 6Centaurs 20Ceres 20Charon 20Chromosphere 10Cluster 26Colour index 22Coma 20Comet 17, 20-21Constellations 23-25Convective zone 10Cordelia 19Core 6Corona 10-11Cosmic inflation 28Cosmic microwave

background radiation28

Cosmologist 28Cosmology 28Cosmos 28Crab Nebula 22-23Crater 12-14, 16-19Crescent moon 12Critical density 28Crust 6Cryovolcano 18

DDDark energy 28Dark matter 26, 29Day 6Deimos 14-15Density 6Dust storm 14Dwarf galaxy 26Dwarf planet 20-21

EEEarth 6-7, 9-15, 21, 23,

26-27, 29Earthshine 12Eclipse 6Einstein, Albert 29Electromagnetic

Spectrum 11, 29Electrons 28Elliptical galaxy 26Equator 6Eris 20

FFFilament 26Flare 10Flyby 6Full moon 12-13

GGGalactic arms 26Galactic halo 26Galaxy 4, 22-24, 26-29Galle, Johann 18Gallileo Galilei 7, 16Ganymede 16Gas giants 16-18Gibbous moon 12Giotto Probe 20Globular cluster 22Granules 10Gravitational force 28Gravitational pull 6, 8,

16, 19Gravity 6, 18, 20, 22-23,

26, 29Great Dark Spot 18Great Red Spot 16-17Great White Spot 17Greenhouse e!ect 14

HHHalf moon 12Halley, Edmond 21Halley’s Comet 9, 20Heliocentric 6Heliosphere 10

Herschel, William 18Horsehead Nebula 22Hubble Space Telescope

27Hubble, Edwin 26-27, 29Hubble’s Law 29Huygens, Christiaan 17

IIImpact craters 12Infrared radiation 10Io 16-17Iron oxide 14Irregular galaxy 26-27Ishtar Terra 14

JJKKJovian planets 17Jupiter 6, 9, 10, 16-17,

20-21Kuiper Belt 20-21

LLLander 6Lava 6, 12-14Le Verrier, Urbain 18Lenticular galaxy 27Light year 6, 23, 26, 29Local Group 26-27Luminosity 22Luna 2 12Luna 3 12Luna programme 12Lunar 12Lunar eclipse 13Lunar month 13

MMMagellan Probe 14Magellanic Clouds 27Magnetic field 7, 10Magnitude 22Main sequence 22Mantle 7Mare 13Mariner Programme 14Mars 6, 9, 14-15, 20Martian polar caps 14

Mass 7, 8, 20, 29Matter 7, 29Maxwell Montes 14Mercury 6, 9, 11, 14-16Messenger 14Meteor 20-21Meteor Shower 21Meteorite 21Meteoroid 18, 21Methane 18Milky Way Galaxy 26-27Mimas 17Minor planets 20Miranda 18Moon 6-7, 10, 12-13, 16Multiverse Theory 29

NNNatural satellite 7Nebula 23NEOs (Near-Earth

Objects) 21Neptune 7, 9, 18-19, 21Nereid 19Neutron star 23Neutrons 28New Moon 12-13North Star 23Nova 23Nuclear fusion 10, 22Nucleus 20, 26-28

OOOberon 19Observable Universe 29Olympus Mons 15 Oort cloud 20-21Open cluster 23Ophelia 19Orbit 7Orion 22, 24-25, 26

PPParallax 23Parallel universes 29Pathfinder Probe 15Pavonis Mons 15Penumbra 11Perigee 13

Perihelion 7Phase 7, 12-13, 16Phobos 15Photosphere 10-11Pioneer 10 space probe 9Pioneer Missions 17Pioneer Venus Project 15Planet 4, 6-8, 13-21Planetary nebula 23Planetesimal 7Planetoid 20Plasma 10Pluto 7, 9, 20-21 Polaris 23Poles 6-7, 14Pressure 7Prominence 10-11Prominence 11Proteus 19Protons 28Protostar 23Proxima Centauri 21, 23Pulsar 23

QQRRQuarter moon 12Quasar 26-27Radiation 10-11, 28-29Radiative zone 11Radio galaxies 26Red dwarf 23Red giant 11, 22-23Red shift 29Regolith 13Relativity 29Revolve 7Rilles 13Ring 17Ring galaxy 27Ringlet 17Rings 17-19

SSSatellite 4, 7, 27Saturn 6, 9, 16-17“Scooter” 19Selenography 13Seyferts 26Shepherd satellite 19“Shooting stars” 21

30 31

Sirius 23Sojourner Rover 15Solar core 11Solar eclipse 10-11Solar System 7, 8, 10, 14,

16-18, 20-21, 27Solar wind 7, 10-11, 20Space probe 7Spicule 11Spiral galaxy 26-27Star 6, 10, 21-25, 27Starburst 23Starburst galaxy 27Stardust Probe 21Subatomic particles 11, 28Sulphuric Acid 15Sun 4, 6-12, 14, 18, 20-23,

26-27Sunspot 10-11Supercluster 26-29Supergiant 22-23Supernova 22-23

TTTelescope 4, 7, 16,

26-27, 29Terrae 13Tharsis Bulge 15Titan 17Titania 19Tombaugh, Clyde 21Triton 19Trojan 21Tunguska fireball 21

UUUmbra 11Umbriel 19Universe 4, 10, 26, 28-29Uranus 6, 9, 18-19

VVValles Marineris 15Variable star 23Venera Missions 15Venus 6, 9, 11, 14-17Viking Programme 15Virgo 24, 27

Virgo Cluster 27Void 28-29Volcano 7, 14-15, 17, 18-19Voyager 9, 19Voyager Programme 19

WWWaning 13Waxing 13White dwarf 22-23Wrinkle ridges 15Wunda crater 19

YYZZYear 7Yellow dwarf 23Zones 17

IINNDDEEXX

Page numbers inbold refer to mainentries.