Knowledge Encyclopedia

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Selección de Discursos del Presidente de la República Bolivariana de Venezuela, Hugo Chávez Frías (Tomo 1, Año 1999) Ediciones de la Presidencia de la República Hugo Chávez Frías Year: 2005 Language: spanish File: PDF, 4.72 MB

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Практическая логика Нобель Пресс Светлов В. А. Year: 2013 Language: russian File: PDF, 154.87 MB

ENCYCLOPEDIA

smithsonian

ENCYCLOPEDIA

LONDON, NEW YORK, MELBOURNE,
MUNICH, AND DELHI
US Senior Editor Rebecca Warren
US Editor Kate Johnsen
Senior Editors Shaila Brown, Daniel Mills, Ben Morgan

CONTENTS

Senior Art Editors Vicky Short, Smiljka Surla
Editors Lizzie Munsey, Sam Priddy, Alison Sturgeon
Designers Daniela Boraschi, Tannishtha Chakraborty, Richard Horsford,
Hedi Hunter, Fiona Macdonald

SPACE

Visualizer Peter Laws
Illustrators Peter Bull, Rob Cook, FOREAL™, Mike Garland, Mark Garlick,
Gary Hanna, Jason Harding, Arran Lewis, Maltings Partnership, Medi-Mation,
Peter Minister, Gerson Mora and Anna Luiza Aragão/Maná e.d.i., Moonrunner
Design, Ian Naylor, Alex Pang, Dean Wright and Agatha Gomes
DK Picture Library Emma Shepherd, Rob Nunn
Jacket Designer Laura Brim
Producer, pre-production Francesca Wardell
Producer Alice Sykes
Managing Editors Julie Ferris, Paula Regan
Managing Art Editor Owen Peyton Jones
Publisher Sarah Larter
Art Director Phil Ormerod
Associate Publishing Director Liz Wheeler
Publishing Director Jonathan Metcalf
Contributors Kim Bryan, Robert Dinwiddie, Jolyon Goddard, Ian Graham,
Reg G. Grant, Jacqueline Mitton, Darren Naish, Douglas Palmer,
Philip Parker, Penny Preston, Sally Regan, David Rothery,
Carole Stott, Paul Sutherland, Chris Woodford, John Woodward

First American Edition, 2013
Published in the United States by DK Publishing, 345 Hudson Street
New York, New York 10014
13 14 15 16 17 10 9 8 7 6 5 4 3 2 1
001—187527—Oct/13
Copyright © 2013 Dorling Kindersley Limited
All rights reserved
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publication may be reproduced, stored in a retrieval system, or transmitted
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recording, or otherwise, without the prior written permission of both the
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ISBN: 978-1-4654-1417-5
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THE UNIVERSE
The Big Bang
Galaxies
Star birth
Star death
The Sun
The Solar System
Inner planets
Outer planets
The Moon

10
12
14
16
18
20
22
24
26
28

SPACE EXPLORATION
Astronomy
Mission to the Moon
Exploring the planets

30
32
34
36

THE SMITHSONIAN
Established in 1846, the Smithsonian—the world’s largest museum and
research complex—includes 19 museums and galleries and the National
Zoological Park. The total number of artifacts, works of art, and specimens
in the Smithsonian’s collection is estimated at 137 million. The Smithsonian
is a renowned research center, dedicated to public education, national
service, and scholarship in the arts, sciences, and history.

EARTH

NATURE

PLANET EARTH
Inside the Earth
Earth’s climate

40
42
44

TECTONIC EARTH
Plate tectonics
Volcanoes
Earthquakes

46
48
50
52

EARTH’S RESOURCES
Rocks and minerals

54
56

WEATHER
Hurricanes
The water cycle

58
60
62

SHAPING THE LAND
Caves
Glaciers

64
66
68

EARTH’S OCEANS
The ocean floor

70
72

HOW LIFE BEGAN
Timeline of life
The dinosaurs
Tyrannosaurus rex
How fossils form

76
78
80
82
84

THE LIVING WORLD
Plant life
Green energy

86
88
90

INVERTEBRATES
Insects
Butterfly life cycle

92
94
96

VERTEBRATES
Fish
Great white shark
Amphibians
Frog life cycle
Reptiles
Crocodile
Birds
How birds fly
Mammals
African elephant

98
100
102
104
106
108
110
112
114
116
118

SURVIVAL SECRETS
Habitats
American desert
Amazon rainforest
African savanna
Coral reef
Animal architects
Predators and prey

120
122
124
126
128
130
132
134

HUMAN BODY

SCIENCE

BODY BASICS
Building blocks
The skeleton
Muscle power
The skin

138
140
142
144
146

FUELING THE BODY
From mouth to stomach
The intestines
In the blood
The heart
Fighting germs
Cleaning the blood
Air supply

148
150
152
154
156
158
160
162

IN CONTROL
Nervous system
Brainpower
How vision works
Inside the ear
Taste and smell
Control chemicals

164
166
168
170
172
174
176

LIFE CYCLE
A new life
Life in the womb
Growing up
Genes and DNA

178
180
182
184
186

MATTER
Atoms and molecules
Atom smasher
Solids, liquids, and gases
The elements
Chemical reactions
Material world

190
192
194
196
198
200
202

FORCES
Laws of motion
Engines
Simple machines
Flotation
Magnetism
Gravity
Flight

204
206
208
210
212
214
216
218

ENERGY
Electromagnetic spectrum
Signals from space
Light
Telescopes
Sound
Heat
Electricity
Power network
Radioactivity

220
222
224
226
228
230
232
234
236
238

ELECTRONICS
Digital world
Robotics

240
242
244

HISTORY
THE ANCIENT WORLD
The first humans
The first towns
Early empires
Ancient Egypt
The pharaohs
Ancient Greece
Ancient Athens
The Roman Empire
Roman society

248
250
252
254
256
258
260
262
264
266

THE MEDIEVAL WORLD
Viking raiders
Fortresses
Wars of faith
World religions
The Ottoman Empire
The Silk Road
Samurai warriors

268
270
272
274
276
278
280
282

THE AGE OF DISCOVERY
Voyage to the Americas
Ancient Americas
The Renaissance
Shakespeare’s theater

284
286
288
290
292

Imperial China
Rulers of India
THE MODERN WORLD
The slave trade
The Enlightenment
The American Revolutionary War
The French Revolution
The Industrial Revolution
The Age of Steam
The Civil War
World War I
Trench warfare
World War II
Modern warfare
The Cold War
The 1960s
The 21st century

REFERENCE
GLOSSARY
INDEX
ACKNOWLEDGMENTS

294
296
298
300
302
304
306
308
310
312
314
316
318
320
322
324
326

328
346
350
359

SPACE
When you look into the blackness of the night sky, you are
peering into the fathomless depths of the Universe. Stars,
planets, and galaxies stretch into space, not just farther than
you can see, but farther than you can imagine.

10

THE
UNIVERSE
The Universe is the whole of existence—all of space,
matter, energy, and time. The Universe is so vast that it
seems unimaginable, but we do know that it has been
steadily expanding following its beginning 13.8 billion
years ago in an explosive event called the Big Bang.

UNDERSTANDING
THE UNIVERSE
People used to think of the
Universe as a giant sphere, but
we now know that things are
not so simple. The Universe
probably has no center or outer
edge. Only a fraction of it—the
observable Universe—is visible
to us. The whole Universe may
be vastly bigger than this,
perhaps infinitely so.

CELESTIAL BODIES
The Universe is at least 99.999999999999
percent empty space. Floating in this vast,
dark void are all sorts of different objects,
which astronomers call celestial bodies.
They range from grains of dust to planets,
stars, and galaxies. Our Solar System
includes a star (the Sun) and a large family
of planets and moons that formed from
the same cloud of gas that gave birth to
the Sun. In recent years, planets have
been seen around hundreds of other
stars, showing that our Solar System
may be one of billions in our galaxy.

Asteroid
Rocky lumps left over from
the formation of the Solar
System are called asteroids.
They range in size from
boulders to bodies close to
the size of a dwarf planet.

Looking back in time

Closed
A dense Universe would bend itself into a
closed shape. Traveling in a straight line
would bring you back to your starting point.

Because light takes time to travel,
when we look into space we are looking
back in time. The most distant objects
visible are galaxies photographed by
the Hubble Telescope. We see them as
they were 13 billion years ago. The
Universe extends far beyond these,
but it’s impossible to see objects
much further because their light
hasn’t had time to reach us.
Furthest objects
The light from the faintest galaxies in this
photo from the Hubble Space Telescope
took 13 billion years to reach Earth.

The shape of space
The three dimensions of space
are bent by the force of gravity
from matter in the Universe into
a fourth dimension that we can’t
see. This is hard to visualize, so
scientists use the metaphor of a
two-dimensional rubber sheet to
explain the idea. The mass of the
Universe could bend this rubber
sheet in one of three ways,
depending on how densely
packed with matter the Universe
is. Most scientists now think the
shape of the Universe is flat.

Open
If the Universe isn’t dense enough, it might
stretch into an open shape, making it
infinite in size with no outer edge.

Flat
Just the right amount of matter would give
the Universe a flat shape. This would also
be infinite in size with no outer edge.

What’s the matter?
The elements hydrogen and
helium make up 98 percent of
the matter we can see in the
Universe. But there doesn’t
seem to be enough matter to
account for the way stars and
galaxies are pulled by gravity.
As a result, astronomers think
galaxies contain dark matter,
which we cannot see. There is
also an unknown force making
the Universe expand, known
as dark energy.

27%
dark matter

23%
helium
75%
hydrogen

68%
dark energy
5% matter

2% other
elements

MAKEUP OF THE UNIVERSE

THE SCALE
OF SPACE
The Universe is so vast that
we cannot appreciate its size
without making leaps of scale.
In this series of pictures, each
stage represents a microscopic
speck of the image to its right.
When dealing with the vast
distances in space, miles
aren’t big enough. Instead,
astronomers use the speed of
light as a yardstick. Light is so
fast it can travel around the
Earth 7.5 times in a second.
One light year is the distance
light travels in a year: nearly
6 trillion miles (10 trillion km).

Earth and Moon
Earth is 7,926 miles (12,756 km) wide.
Our nearest neighbor in space—the Moon—
orbits Earth at a distance of 238,855 miles
(384,400 km). If Earth were the size of a
soccer ball, the Moon would be the size of
a cantaloupe about 69 ft (21 meters) away.

Solar System
The Sun’s family of eight planets occupy
a region of space 5.6 billion miles (9 billion
km) wide. If Earth were a soccer ball, it
would take five days to walk across this
part of the Solar System. The nearest star
would be a 58-year walk away.

Stellar neighborhood
The nearest star to the Sun is Proxima
Centauri, which is just over four light years
away. There are around 2,000 stars within
50 light years of the Sun. These make up
our stellar neighborhood, which is a tiny
fraction of the Milky Way galaxy.

11

Comet
Comets are chunks of ice
from the outer reaches of
the Solar System. Some
grow long tails of gas and
dust as they approach the
Sun and are warmed by it.

Moon
Also called a natural
satellite, a moon is a body
that orbits a planet. Earth
has only one moon but
the planet Jupiter has
67, including Io (above).

IS THERE ANYBODY
OUT THERE?
One of the biggest questions in
science is whether life is unique
to Earth or has arisen on other
worlds. And if life has appeared
elsewhere, could intelligent
beings have evolved?
Scientists have set up projects
to watch and listen for signals
from extraterrestrials, and
messages have been sent to
the stars to inform any aliens
out there of our existence.

Dwarf planet
Dwarf planets are larger
than asteroids but smaller
than planets. Like planets,
they are round in shape.
Pluto (above) is the best
known dwarf planet.

Arecibo message
In 1974, scientists used the
giant Arecibo radio telescope
in Puerto Rico to broadcast a
radio message toward the star
cluster M13. The message
contains symbols (right) that
represent human beings, our
base-10 counting system, the
DNA molecule, and the Solar
System. More a publicity stunt
than a serious attempt to
contact aliens, the broadcast
will take 25,000 years to
reach M13, and a reply will
take 25,000 years to return.

Planet
A planet is a large and
nearly spherical object that
orbits a star and has swept
its orbital path clear of
debris. The Solar System
has eight planets.

Stars
These luminous balls of
gas, such as the Sun, shine
by generating their own
nuclear power. Stars come
in a wide range of types,
temperatures, and sizes.

Numbers 1 to 10
in binary, reading
left to right

Chemical formula of
DNA (the molecule
that carries the
blueprint of life)

Nebula
A glowing cloud of gas and
dust in space is known as a
nebula. Some nebulae are
clouds of wreckage created
by dying stars. Others give
birth to new stars.

Pioneer plaque
The robotic spacecraft Pioneer 10
and Pioneer 11 visited the planets
Jupiter and Saturn in 1973–74 and
then flew off into deep space. If
aliens ever discover either craft
drifting through interstellar space,
they will find a gold-plated plaque
engraved with a message from Earth.

Hydrogen
atom

Man and
woman in front
of Pioneer craft

Shape of DNA
molecule

SETI
Astronomers involved in the
SETI (search for extraterrestrial
intelligence) project use powerful
radio telescopes to scan the skies
in search of artificial radio signals
broadcast by alien civilizations.
The SETI project has been running
since 1960, but it has so far found
no conclusive evidence of alien
signals, despite some false alarms.

Milky Way galaxy
The Milky Way is a vast cloud of 200 billion
stars. Its shape resembles a pair of fried
eggs held back to back, with a central bulge
surrounded by a flat disk. It measures
100,000 light years across the disk and
2,000 light years deep through the bulge.

Human figure and
population of Earth
in 1974
Earth’s position
in Solar System
RADIO
TELESCOPE

Local Group of galaxies
The Milky Way is just one of perhaps seven
trillion galaxies in the observable Universe.
Galaxies exist in groups called clusters, held
together by gravity. The Milky Way is part
of a cluster known as the Local Group,
which is about 10 million light years wide.

Arecibo Telescope,
which sent message

Supercluster
Clusters of galaxies exist in even larger
groupings called superclusters. We live
in the Virgo Supercluster, which is one
of millions of superclusters in the known
Universe. Between these are immense
empty areas called cosmic voids.

Position of
Sun in Milky
Way galaxy

Plan of Solar
System, with
Pioneer’s flight path

Universe
Superclusters are thought to form a vast
web of filaments riddled with enormous
voids containing no galaxies. The true size
of the Universe is a mystery, and only a
fraction of it is visible to us. The Universe
may even be infinite in size.

12

space

570

THE UNIVERSE

The Big Bang

thousand million million million miles (1 million million million
million kilometers)—the diameter of the observable Universe.

The expanding Universe

About 14 billion years ago, the Universe materialized
out of nothing for unknown reasons. Infinitely smaller
than an atom to begin with, the Universe expanded to
billions of miles across in under a second—an event
called the Big Bang.
Time came into existence when the Universe began, so the
question “What happened before?” has no meaning. Space also
came into existence. The Big Bang was not an explosion of
matter through space—it was an expansion of space itself.
At first the Universe consisted of pure energy, but within
a trillionth of a second some of this energy turned into matter,
forming a vast soup of subatomic particles (particles smaller
than atoms). It took nearly 400,000 years for the particles to
cool down enough to form atoms, and then another 300 million
years before the atoms formed planets, stars, and galaxies.
The expansion that began in the Big Bang continues to this
day, and most scientists think it will carry on forever.

The illustration below does not show the
shape of the Universe, which is unknown.
Instead, it is a timeline that shows how the
Universe has expanded and changed since the
Big Bang. We know the Universe is expanding
because the most distant galaxies are rushing
apart at rapid speeds. By running the clock
backward, astronomers figured out that the
expansion began 13.8 billion years ago
at a single point: the Big Bang.

Rate of
expansion
increases

First galaxies form

Stars form

The Universe began
as something called a singularity: a
point of zero size but infinite density.

9

Atoms form
Protons and
neutrons form
Energy turns
into particles
8

The Universe begins

7

6

5

4

2 Within a tiny fraction of a
second, the Universe balloons in
size from trillions of times smaller
than an atom to the size of a city.
The rate of expansion then slows.
3
2
1

1 The Universe appears out of
nowhere. At the start, the Universe
consists purely of energy and is
infinitely dense and unimaginably
hot—18 billion trillion trillion °F
(10 billion trillion trillion °C).

3 The intense energy of the
newborn Universe creates matter.
At first, the matter is a soup of
particles and antiparticles. These
crash into and cancel each other
out, turning back into energy. But
some of the matter is left over—
this will eventually turn into atoms
and later stars and galaxies.

13
11

Discovery of the Big Bang
The first scientific evidence for the Big Bang was found
in 1929, when astronomers discovered that light from
distant galaxies is reddened. This color change happens
when objects are moving away from us, making
lightwaves stretch out and change color. The more
distant the galaxies are, the faster they are rushing
away. This shows that the whole Universe is expanding.
STATIC UNIVERSE
No change
in starlight

10

EXPANDING UNIVERSE
Light waves
stretched

Big Bang afterglow
More evidence of the Big Bang came in the 1960s,
when astronomers detected faint microwave radiation
coming from every point in the sky. This mysterious
energy is the faded remains of the intense burst of
energy released in the Big Bang.
MICROWAVE MAP
OF WHOLE SKY

Solar System forms
24% helium

Changing elements
At 300 million years, stars
appear. Stars form when great
clouds of gas are pulled into tight
knots by gravity. The pressure and
heat become so intense in the
dense pockets of gas that nuclear
reactions begin, igniting the star.
7

4 The Universe is now about 1
microsecond old and 60 billion miles
(100 billion km) wide. The leftover
particles begin to form protons and
neutrons—the particles that today
make up the nuclei of atoms. But the
Universe is too hot for atoms to
form yet. Light cannot pass through
the sea of particles, so the young
Universe resembles a dense fog.
5 After 379,000 years, the
Universe cools enough for atoms
to form. The Universe is now an
enormous cloud of hydrogen and
helium. Light can pass through
space more easily, and the Universe
becomes transparent.
6 Half a million years after the Big
Bang, matter is spread out almost
evenly in the Universe, but tiny
ripples exist. Working on these
denser patches, gravity begins
pulling the matter into clumps.

8 At 500 million years, the first
galaxies are forming. Galaxies are
enormous clouds of stars, held
together by gravity.
9 Now 5 billion years old, the
Universe consists of vast clusters
of galaxies arranged in threads,
with gigantic voids between them.
The voids get ever bigger as space
continues to expand. At 8 billion
years, the expansion of the
Universe begins to accelerate.
10 Our Solar System forms at 9
billion years. When the Universe is
20 billion years old, the Sun will
expand in size and destroy Earth.
11 The Universe will carry on
expanding forever, becoming
cold and dark everywhere.

For hundreds of millions
of years, the Universe
consisted almost entirely
of hydrogen and helium
—the very simplest
chemical elements.
After stars appeared,
new elements began to
be made in the cores
of dying stars. All the
complex elements in our
bodies were forged in
dying stars this way.

76% hydrogen

EARLY UNIVERSE
75% hydrogen
23% helium
1% oxygen
0.4% carbon
0.4% neon
0.1% iron
0.1% nitrogen
+ traces of other
UNIVERSE TODAY

Big Bounce theory
What caused the Big Bang? We may never know for sure,
but some scientists have suggested that there may have
been lots of big bangs, with the Universe expanding after
each one and then shrinking again. This theory is called
the Big Bounce because the process repeats itself.
Universe expands

Big Bang

Universe shrinks

TIME

14

space

THE UNIVERSE

Galaxies
Our Sun belongs to a giant whirlpool of stars
called the Milky Way. Huge collections of
stars are called galaxies, and like all galaxies
the Milky Way is unimaginably vast.
Galaxies come in many shapes and sizes. Some are
spirals like our own galaxy, but others are fuzzy
balls or shapeless clouds. The smallest have just
a few million stars. The largest contain trillions.
Although they look packed with stars, galaxies
are mostly empty space. If you made a scale model
of the Milky Way with a grain of sand for each star,
the nearest star to the Sun would be 4 miles (6 km)
away. The furthest would be 80,000 miles (130,000
km) away. The stars in a galaxy are held together
by gravity and travel slowly around the galactic
heart. In many galaxies, including ours, a
supermassive black hole lies hidden in the
center. Stars and other material are sucked
into this cosmic plughole by gravity
and disappear forever.

200

23

billion—the approximate number
of stars in the Milky Way galaxy.

—the number of times our Solar
System has orbited the Milky Way.

The Milky Way

Orion Arm
Our Solar System lies in
this small arm. Many of the
stars we see in the night
sky are in the Orion Arm.

If you could look down on the Milky Way galaxy from
above, the view would be like flying over a glittering
city at night. Most of the galaxy’s 200 billion stars are
in the central bulge. Curving around this are two vast
spiral arms and several smaller arms. The Milky Way
is thought to be a barred spiral (see panel), but we
can’t see its shape clearly from Earth since we view
it from the inside. In the night sky, the Milky Way
appears only as a milky band of light.

MILKY WAY OVERHEAD VIEW

U
SC

T

UM

CE

RUS
U
A
NT

10
,00
0

1 Galactic center
This photo from an
infrared (heat-sensitive)
telescope shows stars
and gas clouds packing
the center of the Milky
Way. A supermassive
black hole lies hidden
somewhere in this area.

20
,00

NO

2 Solar System
Our Solar System
is in a minor spiral arm
called the Orion Arm.
We orbit the center of
the galaxy once every
200 million years,
traveling at about 120
miles (200 km) a second.

3 Crab Nebula

Clouds of gas and dust
occur throughout
the Milky Way, especially
in the spiral arms. The
Crab Nebula is a cloud of
wreckage left behind by a
dying star that exploded.

30

40
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00

lig

, 00
0 li

ght-y
ears

ht
yea
rs

MILKY WAY SIDE VIEW

4 Globular cluster
Not all the Milky Way’s
stars are in the main disc
of the galaxy. Many are in
globular clusters—tightly
packed balls of ancient
stars floating above and
below the galaxy in a
spherical region called
the halo.

Main disc containing arms

0 ligh
t-years

RM

AA

RM

AR M

light-years

7

6,000

trillion—the approximate number of
galaxies in the observable Universe.

years—the length of time it would take to count
the Milky Way’s stars at a rate of one a second.

Perseus Arm
This is one of the Milky Way’s two
major spiral arms. We see stars in
this arm when we look toward the
constellation Perseus.

15

Galaxy shapes
Astronomers classify galaxies into just
a few main types, depending on the
shape we observe from Earth.
Circles show
distance from the
center of the galaxy.

3

2

YOU ARE HERE

SAG
IT

Spiral
A central hub of stars
is surrounded by spiral
arms curving out.

Barred spiral
A straight bar runs
across the center,
connecting spiral arms.

Elliptical
More than half of all
galaxies are simple
ball shapes.

Irregular
Galaxies with no clear
shape are classified
as irregular.

TA
R

SA
IU
RM

CROSS SECTION THROUGH
MAIN DISC OF GALAXY
Bulge formed
by spiral arm
Central bar

1

How spiral arms form

P E RS

EU

R
SA

The stars in a galaxy orbit the center, taking
millions of years to make one circuit. Spiral
arms appear where stars pass in and out of
crowded areas, like cars passing temporarily
through a traffic jam. One theory is that
these traffic jams happen because the orbits
of different stars don’t line up neatly.

M

If stars all had neat,
parallel orbits, the
galaxy would have
no spiral arms.

Gas cloud

If stars’ orbits don’t
line up neatly,
crowded zones
form, giving the
galaxy spiral arms.

Colliding galaxies
Dark lane formed
by dust

Sometimes galaxies crash and tear each
other apart. Individual stars don’t collide, but
gas clouds do, and gravity pulls the colliding
galaxies into new shapes.

Scutum-Centaurus Arm
This is the second of the two
main spiral arms. The area
where it joins the central bar
is rich in star-forming clouds.
Central bulge

4 Globular cluster

End of the Milky Way
In 4 billion years our galaxy will collide with the
Andromeda galaxy. This artist’s impression shows
what the sky might look like as they merge.

16

space

Star birth
The clouds that give birth to the stars are cold and dense
and consist mainly of hydrogen gas. The newly formed
stars are huge spinning globes of hot, glowing gas—mainly
hydrogen, with helium and small amounts of other
elements. Much of this material is packed tightly into the
stars’ cores, and it is here that nuclear reactions release
energy in the form of heat and light.

1

Interstellar cloud
Stars are born within
enormous, cold, dense clouds of
gas and dust. The process of star
formation may be triggered if
something disturbs the cloud,
such as a collision with another
cloud or a shockwave from a
supernova explosion.

Starbirth nebulas
Clouds of gas and dust in space
are called nebulas. Much of the gas
and dust in a nebula is debris from
old stars that exploded when they
ran out of fuel. Over millions of
years, this material is recycled to
make new stars. Starbirth nebulas
are among the most beautiful
objects in space, their colorful
clouds illuminated from within by
the blue light of newborn stars.
Orion Nebula
The Orion Nebula is one of the closest
star-forming regions to Earth. In the
night sky it looks like a fuzzy star in
the sword of Orion. In reality, it is a
vast cloud of gas and dust thousands
of times bigger than the Solar System.

billion years—the age of
the oldest known star.

The largest stars
in the night sky are big enough to
swallow our Sun a billion times.

Stars have been forming throughout the Universe
for most of its life. They take shape in vast clouds
where thousands of stars are born at a time.

Interstellar
gas cloud

13

The brightest stars emit 6 million
times more light than the Sun.

THE UNIVERSE

How new stars form
The star-forming process begins when the cloud becomes
unstable and breaks up into fragments. Gravity pulls the
material in a fragment into an ever-tighter clump, and the
clump slowly forms a sphere as it shrinks. Now a protostar,
this star-to-be keeps on shrinking, its core getting denser and
hotter. Eventually the pressure and temperature are so high
that nuclear reactions begin, and the star starts to shine.

Cloud breaks up
into fragments

2

Fragments form
Now unstable, the cloud breaks
up into fragments of different size
and mass. The most massive and
dense of these fragments are
gradually pulled by their own
gravity into tighter clumps. These
shrinking fragments will eventually
turn into protostars.

Protostar

3

Protostar
A protostar forms. Gravity
pulls material into its core,
where the density, pressure,
and temperature build up. The
more matter the original cloud
fragment contained, the greater
the temperature and pressure
rise as the protostar develops.

Rotating disc

17

Jets of gas

4

Spinning disc
The growing mass at the
center creates a gravitational
pull, drawing ever more gas
and dust inward. A little like
water going down a drain, the
material being pulled in starts
to spin around. Powerful winds
develop, blowing jets of gas
out from the center.

Star clusters
Stars are not formed singly—they
are born in clusters from the same
cloud of material at roughly the
same time. Eventually, the stars of
a cluster will drift apart and exist
alone in space, or with a close
companion or two. Our Sun, like
about half of the stars nearest to
us, is alone. About a third of the
stars in the night sky are in pairs,
bound together by gravity.
Pleiades cluster
A handful of the 5,000 or so stars
that make up the Pleiades cluster
can be seen with the naked eye. In
about 250 million years time, the
stars will have dispersed and the
cluster will no longer exist.

Supergiant

MA
Blue
supergiants

LUMINOSITY

IN

SEQ

UE

NC

E S
TA
R

S

Red giants
Sun
White dwarfs
Dimmer

A star begins to shine when
nuclear reactions in its core
convert hydrogen into helium
and release energy. It is then
called a main sequence star. Not
all main sequence stars are the
same—they differ in size,
temperature, color, brightness,
and the amount of matter they
contain. When stars begin to run
out of fuel and near the end of
their lives, they stop being main
sequence stars and may swell
up and turn into red giants or
shrink to become white dwarfs.

Brighter

Types of star

Hotter

TEMPERATURE

Cooler

Leftover material
may form planets

Star begins to shine

5

A star is born
Squeezed by the force of gravity,
the protostar’s core becomes so hot
and dense that nuclear reactions
occur, and the star begins to shine.
The glowing core produces an outward
pressure that balances the inward pull
of gravity, making the star stable. It is
now a main sequence star.

Classifying stars
The Hertzsprung–Russell
diagram is a famous
graph that astronomers
use to classify stars. The
graph plots brightness
against temperature
and reveals that there
are distinct groupings of
stars, such as red giants
(dying stars) and main
sequence stars (ordinary
stars). Astronomers also
classify stars by color,
which is linked to their
temperature: hot stars
are blue; cooler stars
are orange or red.

6

Planets form
Not all the material from the
gas cloud has been used to make
the star. The leftovers form a
spinning disc of gas and dust
around the star. This debris may
be lost into space, or it may clump
together to form planets, moons,
comets, and asteroids.

18

space

THE UNIVERSE

Supergiant stars can grow to 1 billion
times the volume of our Sun.

Four ways to die
Stars can die in four different ways, all of which are
shown on these pages. Our Sun, a typical star, will
follow the central path, but not yet—it has enough
fuel to keep shining for 5 billion years. When larger
stars die, they turn hydrogen into heavier chemical
elements such as carbon and oxygen, which are
later recycled to form new stars and planets. All
the atoms in your body were created this way.

5

billion tons—the weight of one
teaspoonful of material from
the core of a neutron star.

Stable star
Every young star goes
through a stable phase in
which it shines steadily.

Star death
All stars eventually run out
of fuel and die. Most fade away
quietly, but the most massive stars
self-destruct in a huge explosion
that can outshine an entire galaxy.
Like Earth, stars generate the force of
gravity, which squeezes their hot cores. The
more matter a star has, the greater the force
of gravity and the hotter and denser the core
becomes. The way a star dies depends on how
much matter it contains (its mass) and how
powerfully its core is squeezed by gravity.
Stars make heat and light by the process of
nuclear fusion: hydrogen atoms in the core crash
together to form helium, releasing energy. In small
stars, when hydrogen in the core runs out, the star’s
light slowly fades. But in more massive stars, the core
is so hot and dense that fusion can spread beyond it,
changing the star’s appearance. The most massive stars
are eventually overwhelmed by their own gravity, which
crushes them so violently that they collapse into a
pinprick to create a black hole.

Small stars
Stars with less than half the mass of the Sun fade away
very slowly. Once the hydrogen in the core is used up,
the star begins to feed off hydrogen in its atmosphere.
But it doesn’t generate enough gravity to use other
elements as fuel, so it slowly shrinks to become a
black dwarf. This will take far longer than the
age of the Universe—up to a trillion years.

Medium stars
When a Sunlike star has used up the hydrogen
in its core, nuclear fusion spreads outside the core,
making the star expand into a red giant. The core
collapses until it is hot and dense enough to fuse
helium, but eventually it runs out of helium too.
Finally, it becomes a white dwarf, and its outer
layers spread into space as a cloud of debris.

Massive stars
Stars over eight times more massive than
our Sun end their lives in strange and
violent ways. The heat and pressure inside
the core become so great that nuclear fusion
can not only fuse hydrogen atoms together to
form helium but can fuse helium and larger
atoms to create elements such as carbon or
oxygen. As this takes place, the star swells
into the largest star of all: a supergiant.

Star begins
to shrink

Star expands

Star expands

1

1,600

teaspoonful of material from a red
giant weighs less than a grain of salt.

Light intensity fades
as fuel runs out

Star continues to
shrink and fade

Red giant
Nuclear fusion spreads to
the layer around the core,
heating it up and making
the star expand. Nearby
planets may be swallowed
up by the growing giant.
Outer layer

light years—the distance from
Earth to the nearest black hole.

Light becomes
increasingly dim

19

Black dwarf
Finally, its fuel used up and
its light extinguished, the
star becomes a black dwarf—
an Earth-sized cinder.

Planetary nebula
The star’s outer layers disperse into
space as a glowing cloud of wreckage
—a planetary nebula. The material in
this cloud will eventually be recycled
to form new stars.

White dwarf
All that remains is the dying
core—a white dwarf. This
Earth-sized star will slowly
fade and become a cold,
dead black dwarf.

Core

Neutron star
Up to three times heavier than the
Sun, yet just a few miles wide,
neutron stars are unimaginably
dense, fast-spinning stars.

Red supergiant
The star has grown into a supergiant.
Nuclear fusion carries on inside the core,
forcing atoms together to form heavier and
heavier elements, until the star’s core turns
into iron. When this happens, the core no
longer generates enough outward pressure to
resist the crushing force of gravity, and the
whole star suddenly collapses, causing a
catastrophic explosion—a supernova.

Supernova
The star self-destructs in an explosion brighter
than a billion suns. Its outer layers are blasted
into space, but its massive core continues to
collapse in on itself. What happens next
depends on how massive the core is. A smaller
core becomes a neutron star, but a massive
core never stops collapsing. It shrinks until it’s
billions of times smaller than an atom and
becomes a black hole.

Black hole
The force of gravity close to a black hole
is so intense that nothing can escape
from it—not even light. Anything falling
inside is torn apart by gravity and then
crushed into a point of infinite density.

20

space

THE UNIVERSE

Loop prominence
Gigantic loops of glowing gas
extend high above the Sun’s
surface, anchored to the star’s
tangled magnetic field. Called
loop prominences, these gas
eruptions can last for months.

Spikes of gas
Jets of hot gas rise all the
time from the Sun’s surface,
forming towering spikes
that last just a few minutes
before collapsing. Called
spicules, these formations
can reach thousands of
miles in height. Seen from
above (right), they form
shimmering, hairlike
patterns around a sunspot.

The Sun
The Sun is a nearly perfect sphere of
hot, glowing gas. Its source of power lies
buried deep in the central core, where a
nuclear furnace rages nonstop, turning
matter into pure heat and light.
Slightly bigger than a typical star, the Sun is
large enough by volume to swallow 1.3 million
Earths. It contains 99.8 percent of all the matter
in the Solar System, and the force of gravity
generated by this enormous mass keeps the
planets trapped in orbit around it. Seen from
Earth, the Sun is a life-sustaining source of light
and warmth that shines steadily on us. Closer
views, however, reveal a world of astonishing
violence, its seething surface bursting with vast
eruptions that hurl fiery gases into space.

Core
Inside the hot, dense core,
the process of nuclear fusion
releases energy. Every second,
683 million tons of hydrogen are
fused into helium in the core.

Radiative zone
Outside the core is the
radiative zone, which is
not dense enough for
nuclear fusion to take
place. Energy from the
core seeps very slowly
out through this layer.

Convective zone
In the convective zone, vast
bubbles of hot gas rise to the
surface, cool, and then fall,
transferring energy from the
Sun’s heart to the exterior.

Inside the Sun
Scientists divide the Sun’s interior
into three distinct layers: the core, the
radiative zone, and the convective zone.
All three are made solely of gas, but the
gas gets hotter and denser toward the
center. In the core, the temperature soars
to 27 million °F (15 million °C) and the
gas is 150 times more dense than water.

Photosphere
The Sun’s apparent surface
is called the photosphere.
Energy escapes into space
from here as light.

385

million billion gigawatts—the amount of
energy output from the Sun each second.

SIZE OF EARTH
COMPARED
TO THE SUN

21

Sun statistics
Diameter .......... 865,374 miles (1,393,684 km)
Distance from Earth ................. 93 million miles
(150 million km)
Mass (Earth = 1) ........................................ 333,000
Surface temperature ....... 10,000 °F (5,500 °C)
Core temperature.. 27 million °F (15 million °C)

Energy release
Solar flare
A sudden burst of energy
from the Sun’s surface is
called a solar flare. Flares are
often followed by a coronal
mass ejection (see panel).

It takes only eight minutes for light from the
Sun to reach Earth, but it can take 100,000
years for energy released in the Sun’s core
to travel to the surface and emerge as light.
The journey is slow because the energy is
absorbed and reemitted by trillions of atoms
as it passes through the dense radiative zone.
100,000 years

Sunspots
Cooler, darker patches on the
Sun are called sunspots. The
number of sunspots rises and
falls over an 11-year cycle.

8 minutes

EARTH

SUN

Rotation
Like all objects in space, the Sun rotates.
Unlike the Earth, which rotates as a solid
object, the Sun is a ball of gas and turns at
different speeds in different places. The
equator takes 25 Earth days to rotate once,
but the polar regions take 34 days.
34 days
25 days
24 hours

EARTH

34 days

SUN

Mass ejections
Vast bubbles of superhot gas (plasma), each
with a mass of around 1.1 billion tons, erupt
from the Sun up to three times a day. Called
coronal mass ejections, these bubbles grow
millions of miles wide in a few hours and
then burst, sending a blast of charged
particles hurtling across the Solar System.
The blast waves sometimes collide with the
Earth, lighting up the polar skies with
unusually brilliant auroras.
Grainy surface
The bubbles of hot gas that
rise up inside the Sun make its
surface look grainy. There are
some 4 million granules on the
Sun’s face, each about 600 miles
(1,000 km) wide and lasting for
around eight minutes.

3:23 PM

6:09 PM

6:25 PM

space

22

THE UNIVERSE

If the Sun were the size of a basketball,
Earth would be a pea 270 ft (83 m) away.

If the Sun were the size of a basketball, Neptune
would be a strawberry 170 miles (270 km) away.

Asteroid
Asteroids are giant space rocks that
drift around the inner Solar System.
Most lie in a belt between Mars and
Jupiter, but some occasionally come
dangerously close to the Earth. The
smallest are the size of houses, while
the largest are big enough to be
classified as dwarf planets. Scientists
think asteroids are leftovers from the
material that formed the planets. All
of them together amount to less than
a twentieth of the Moon’s mass.

Saturn
The second biggest planet
is striking for the dazzling
system of bright rings that
encircle it. It has 62 moons
and dozens more moonlets.

Sun
The Sun is like a vast nuclear
power station that produces
energy by converting hydrogen
into helium. It is the only star
we can study close up.

Earth
Our home planet is the
only place known to
support life, thanks to the
liquid water on its surface.

Mercury
The closest planet to the Sun,
Mercury is also the smallest
planet. Its surface is scarred
by ancient craters.
Mars
Mars is a bitterly
cold, desert world. Like
Earth, it has mountains,
canyons, and icy poles.

Venus
Though similar in size to
Earth, Venus is a hellish
world where any visiting
astronaut would be
crushed and boiled alive.

Mars

Earth

Venus

Sun

Mercury

Orbital distance
The scale bar below shows the relative
distances of the planets from the Sun. The
distance between one planet and the next
increases greatly as we move out through
the Solar System.
Jupiter

Saturn

500 million miles
(805 million km)

1,000 million miles
(1,609 million km)

26

The number of asteroids known to be
more than 124 miles (200 km) wide.

Comet
These small, icy bodies can
develop spectacular tails of
gas and dust when they
approach the Sun.

Neptune
The most distant planet, Neptune
is a blue giant with 13 known
moons. It takes Neptune nearly
164 years to orbit the Sun once.

Kuiper Belt
Thousands, if not millions, of
small icy bodies occupy the
region beyond the planets.
The Kuiper Belt is home to
dwarf planet Pluto and is a
likely source of comets.

Uranus
Blue giant Uranus orbits
the Sun tipped over on its
side, perhaps because of a
crash with a smaller planet.
It has 27 moons.

The Solar System
The force of gravity generated by the Sun’s vast mass
keeps a family of planets and other bodies trapped in
orbit around it. Together, the Sun and all these bodies
make up our Solar System.

Jupiter
The largest planet in the Solar System,
Jupiter is more massive than all the
other planets put together. It has its
own family too, with at least 67
moons, some as big as planets.

The Solar System
The Solar System’s planets form two groups.
There are four small, inner planets made of
rock and metal, and four giant, outer planets
made of gas and liquid. Between the two is
a belt of rocky bodies called asteroids, and
beyond the planets is a zone of icy bodies
including dwarf planets and comets. Even
farther out is a vast, spherical cloud of more
comets—the Oort Cloud. The Solar System has
no certain outer boundary.

A slow cannonball
falls to the ground.

Orbits
Every major body in the Solar System orbits
the Sun counterclockwise. The planets are on
near-circular orbits in the same plane as the
disc of gas and dust from which they formed.
Many smaller objects, such as dwarf planets
Pluto and Eris, have stretched orbits tilted to
this plane. Comets arrive from all directions.

Our Sun formed from a great cloud of dust and gas around 4.6
billion years ago. Vast amounts of matter were drawn in by the
developing star, but not all of it was fully absorbed. A tiny
fraction of leftover material—a mere 0.14 percent of the Solar
System’s mass—formed a disc of gas and dust encircling the
newborn star. Over millions of years, the grains of dust in this
disc clumped together, growing into ever larger bodies until
they grew to the size of planets, pulled into spheres by their
own gravity. In the inner Solar System, where the Sun’s heat
was too intense for gases to condense, planets formed from
rock and metal. In the outer Solar System, gases condensed
to form much bigger planets.
Today the Solar System has eight planets, more than
100 moons, an unknown number of dwarf planets, and
countless millions of comets and asteroids.

A very fast cannonball
escapes Earth’s gravity.

At just the right
speed, a cannonball
keeps falling but
never lands.

Dwarf planets
Dwarf planets are round in shape
but smaller than true planets, and
their gravity is not strong enough
to sweep their region of space
clear of smaller debris. The most
famous dwarf planet is Pluto,
which was classified as a true
planet until 2006.
Ceres

Uranus
Neptune
Pluto
Saturn
Sun
Eris

Haumea

How orbits work
English scientist Isaac Newton was the first person to
realize why moons and planets travel in orbits: because
they are trapped by gravity. To explain his theory, he
drew a giant cannon firing cannonballs off Earth. If a
cannonball moved fast enough, the curve of its path as
it fell back would be gentler than the curve of Earth’s
shape, and it would never land—it would stay in orbit.

Makemake
Earth
Pluto
Eris

SIZE COMPARED TO EARTH

Uranus

1,500 million miles
(2,414 million km)

Neptune

2,000 million miles
(3,219 million km)

2,500 million miles
(4,023 million km)

24

space

Mercury’s iron core accounts for
two-thirds of the planet’s mass.

THE UNIVERSE

Inner planets

This view of Venus’s
surface was made using
radar to see through the
planet’s thick clouds

Maat Mons is
Venus’s largest
volcano

Mercury, Venus, Earth, and Mars are the Solar
System’s inner planets. On the face of it, they
are worlds apart—but underneath the surface,
it is a different story.
The inner planets all formed from the same material
about 4.6 billion years ago. All are a mix of rock and
metal, with interiors that are roughly divided
into layers. The heavier metals are concentrated
toward the center, while the lighter rock is on top.
Each of these planets was bombarded by
asteroids and comets early in the Solar System’s
history, and each has been affected by volcanic
activity too. Mercury’s heavily cratered face still
bears the scars of the early bombardment, but
the surfaces of the other three worlds have
changed over time.
Smooth plains
formed from lava
flows in distant past

Brightest craters
are the youngest

MERCURY

VENUS

In the Sun’s glare

Mercury profile

Lava land

Venus profile

Mercury is the smallest of the
Solar System’s planets and lies
closest to the Sun. It is a lifeless
world that has hardly changed
in 3 billion years. The planet’s
entire surface is pitted with
craters formed when asteroids
crashed into it while Mercury
was young. The craters range
from small, bowl-shaped ones
to the huge Caloris Basin, which
is nearly one-third the width
of the planet.
Mercury orbits the Sun more
quickly than any other planet,
but it rotates slowly: for every
two orbits, it spins around just
three times. So a “day” on
Mercury (sunrise to sunrise)
takes 176 Earth days. Such long
days and nights, coupled with
a very thin atmosphere, give
Mercury the greatest surface
temperature range of all the
planets. In the daytime, the
surface is hot enough to melt
lead, but at night it’s cold
enough to liquefy air.

Diameter .... 3,032 miles (4,879 km)
Average surface
temperature .................333°F (167°C)
One spin on axis .... 58.6 Earth days
One orbit of Sun......... 88 Earth days
Number of moons ..............................0

Venus is sometimes described as
Earth’s twin because it’s almost the
same size as our planet and has a
similar internal structure. But the
two worlds are very different.
Any astronaut who tried to
walk on Venus would be killed in
seconds. The surface is as hot as
the inside of a pizza oven, and the
crushing air pressure is 90 times
greater than that on Earth.
Venus’s deadly surface is hidden
from our view by thick cloud
cover, but orbiting spacecraft have
used radar to see through the
gloom, and landers have touched
down to take photos. Venus is a
world of volcanoes, many thought
to be active, and its surface is
littered with broken rock from
solidified lava. It is permanently
overcast, with a sickly yellowish
light filtering through the cloud.
Venus spins more slowly
than any other planet. It also
spins in the opposite direction
(clockwise) to every planet
apart from Uranus.

Diameter ..7,521 miles (12,104 km)
Average surface
temperature .................867°F (464°C)
One spin on axis ..... 243 Earth days
One orbit of Sun...224.7 Earth days
Number of moons ..............................0

Hot spots
The colors on this heat map
of Mercury show the planet’s surface
temperature. The red region, which is
on the equator, faces the Sun and is
hottest. Next warmest are yellow
areas, then green. The planet’s polar
regions (blue) are coolest.

Greenhouse effect
Venus is hot because of a process called
the greenhouse effect. The Sun’s heat
passes through the atmosphere and
warms the ground, which then reemits
warmth. The reemitted warmth is
trapped by the atmosphere, much as
glass traps heat in a greenhouse.

Sunlight warms
the ground

Heat from ground
is trapped by
atmosphere
Atmosphere is
about 50 miles
(80 km) deep

2,500

miles (4,000 km)—the length of
Mars’s Valles Marineris canyon.

Mars’s moons Phobos and
Deimos were once asteroids.

Liquid water
covers most of
Earth’s surface

14

miles (22 km)—the height of the
Olympus Mons volcano on Mars.

Ice cap at
north pole

25

Like Earth, Mars
has a crust made
of solid rock

Like all the inner
planets, Mars has
a core made of
red-hot iron

The reddish color
comes from iron oxide
(rust) in the soil
EARTH

MARS

Living world

Earth profile

The red planet

Mars profile

Third out from the Sun, Earth is
the largest of the inner planets. It’s
the only planet with liquid water
flowing freely on the surface, and
it’s the only planet in the Universe
known to sustain life.
Earth’s surface consists of vast
oceans (71 percent), continents of
land, and two polar ice caps—all
supported by a thin, rocky crust.
The crust is broken into seven
huge segments and many smaller
ones. Called tectonic plates, these
giant slabs of rock creep slowly
over Earth’s surface, pushed by
churning movements in the softer,
hot rock that fills most of Earth’s
interior. As tectonic plates move,
they bump into each other and
grind past one another, generating
immense forces that thrust up
mountain ranges, unleash volcanic
eruptions, and trigger earthquakes.
These powerful forces continually
change Earth’s appearance, as do
the actions of wind and water—
and the planet’s 7 billion
human inhabitants.

Diameter ..7,926 miles (12,756 km)
Average surface
temperature ......................59°F (15°C)
One spin on axis .............. 23.9 hours
One orbit of Sun...............365.3 days
Number of moons ..............................1

The second smallest planet in the
Solar System, Mars is half the size
of Earth. It’s sometimes called the
red planet because of its rusty
coloring. A vast canyon called
Valles Marineris stretches a
quarter of the way around this
frozen desert world. It formed long
ago when the crust of the young
planet split open. Elsewhere are
dusty plains strewn with boulders
and giant, extinct volcanoes,
including Olympus Mons—the
Solar System’s largest volcano.

Diameter .... 4,220 miles (6,792 km)
Average surface
temperature ..................–81°F (–63°C)
One spin on axis .............. 24.6 hours
One orbit of Sun...... 687 Earth days
Number of moons ..............................2

NORTH POLE
23°.5

Axis

Northern
hemisphere
Equator
Southern
hemisphere
SOUTH POLE

Tilted planet
Earth spins around once a day, but
it isn’t perfectly upright. Its axis—
the imaginary line from pole to pole
around which it spins—is tilted by
23.5°. So as Earth travels around the
Sun, one hemisphere and then the
other is tilted toward the Sun. This
is what causes the seasons.

Rocky floodplain
Mars hasn’t always been a desert. Dry
river beds show that water flowed
here long ago. Floods swept rocks
across the land and dumped them on
floodplains like the one below. Mars
may even have been warm and wet
enough for life to flourish.

space

26

1,300

THE UNIVERSE

The number of times Earth could fit
inside Jupiter’s volume.

Size of Earth
relative to
planets

1665

The year Jupiter’s Great Red
Spot was first seen.

Gas atmosphere
Jupiter’s swirling outer atmosphere
is 620 miles (1,000 km) thick and
consists mainly of hydrogen gas.
Liquid layer
The outer atmosphere merges
gradually into a deep layer of
liquid hydrogen and helium.

Blue color caused
by methane in
atmosphere

Great Red Spot
This giant storm is bigger
than Earth and has been
raging for over 300 years.

JUPITER

Solid core
Jupiter’s rocky core
is hotter than the
surface of the Sun.

Liquid metal layer
Deep inside Jupiter, intense pressure
turns hydrogen into a liquid metal.

NEPTUNE

King of the planets

Jupiter profile

Blue planet

Neptune profile

Mighty Jupiter is the fifth planet
from the Sun and the largest in
the Solar System—so big, in fact,
that it’s 2.5 times more massive
than all the other planets put
together. Its strong gravitational
pull greatly affects the orbits of
other bodies in the Solar System.
Jupiter’s fast rate of spin has
stretched its surface clouds into
bands, with spots (storms) and
ripples where neighboring
bands swirl together.
Several craft have visited
Jupiter, including Galileo, which
orbited from 1995 to 2003.

Diameter ......................... 88,846 miles
(142,984 km)
Average surface
temperature .............–186°F (–121°C)
One spin on axis ................. 9.9 hours
One orbit of Sun ......11.9 Earth years
Number of moons ........................... 67

Neptune, the eighth and furthest
of the planets from the Sun, was
discovered in 1846. Astronomers
had noticed Uranus wasn’t
following its expected path—there
seemed to be an unseen body,
perhaps an undiscovered planet,
pulling on it. Two mathematicians—
John Couch Adams in England and
Urbain Le Verrier in France—
calculated where in the sky the
undiscovered planet must be.
Within days, Neptune was spotted
from an observatory in Germany.
Neptune is slightly smaller
than Uranus and looks bluer
because its atmosphere contains
more methane. It has a deep, fluid
mantle that is hot and dense and
contains water, ammonia, and
methane. Neptune also has a
barely visible system of rings. Its
biggest moon, Triton, resembles
Pluto and was likely captured by
Neptune’s gravity in an encounter
billions of years ago.

Diameter ........................ 30,775 miles
(49,528 km)
Average surface
temperature .............–330°F (–201°C)
One spin on axis ...............16.1 hours
One orbit of Sun 163.7 Earth years
Number of moons ........................... 13

Metis

The Jupiter system
Like a king surrounded by his courtiers,
Jupiter is circled by a great number
of moons. The inner moons, including
the four largest, are shown below.
Ganymede, the largest, is bigger than
the planet Mercury. Most of Jupiter’s
other moons are probably asteroids
captured by the planet’s gravity.

Thebe
Io
Ganymede
Europa

Adrastea

Amalthea

Callisto

Fastest known winds
When Voyager 2 flew past Neptune in
1989, it photographed white clouds
blown into streaks by winds of up to
1,300 mph (2,100 kph)—the fastest
sustained winds in the Solar System.
This violent weather is thought to be
powered by heat from inside Neptune
since the planet is too far from the
Sun to absorb much of its warmth.

43,000

°F (24,000 °C)—the estimated
temperature of Jupiter’s core.

Cloud bands
Saturn’s clouds form
bands around the planet,
like those on Jupiter.

Outer atmosphere
Although it looks calm,
fierce winds whip through
Saturn’s atmosphere at
1,800 kph (1,120 mph).

150

The number of moons and moonlets
observed orbiting Saturn.

27

Outer planets
Four gigantic planets dominate the outer Solar System.
Very different from the rocky, inner planets, these
strange worlds are huge globes of gas and liquid,
with no solid surface and hundreds of moons.
After the Sun first formed, its heat drove gases out of the inner
Solar System, leaving behind heavier compounds such as rock and
metal. The rock and metal formed the solid inner planets, while
the gases formed the outer planets. Astronomers
call the outer planets gas giants, though they
consist mostly of liquid and they have
solid cores. These four worlds
have much in common. All have
numerous moons, a deep, stormy
atmosphere, and a set of rings made
of flecks of rock or ice.
On a roll
Uranus leans so far to
one side that it appears
to roll along as it travels
around the Sun.

Gaps
Gaps in the rings are areas
swept clear of ice by the
gravity of Saturn’s moons.

Main rings
The rings consist of billions of sparkling
fragments of ice, varying in size from
microscopic to as big as a house.

SATURN

Rings around Uranus
Thirteen rings are known
to circle the planet.

URANUS

Lord of the rings

Saturn profile

Topsy-turvy world

Uranus profile

The second-largest planet and the
sixth farthest from the Sun, Saturn
shines like a bright yellow star. Even
a small telescope will reveal its
most famous feature: a magnificent
ring system. Despite Saturn’s size,
it is only half as dense as Jupiter.
Its clouds form less obvious bands
than Jupiter’s, but fierce storms
blow up every 30 years or so,
creating giant white spots.
Saturn’s largest moon, Titan,
has a dense atmosphere and a
rocky surface with seas of liquid
methane. The Cassini spacecraft
has been orbiting Saturn since
2004. It released a probe, Huygens,
that landed on Titan in 2005.

Diameter ........................ 74,898 miles
(120,536 km)
Average surface
temperature ........... –292°F (–180°C)
One spin on axis ...............10.7 hours
One orbit of Sun.... 29.5 Earth years
Number of moons ........................... 62

Uranus, the seventh planet from
the Sun, was unknown to ancient
astronomers, even though it is
just visible with the naked eye
in perfectly clear and dark skies.
It was discovered by musician
William Herschel from his back
garden in Bath, England, in 1781.
Uranus is similar to Neptune but
has a paler blue, almost featureless
face. It is the coldest of all the
planets and generates very little
heat from within. It orbits on its
side—perhaps because it was
knocked over by a collision
with another planet early in its
history. Its extreme tilt gives it
very long seasons.
Uranus has a faint set of rings,
which were discovered in 1977.
The planet’s moons are all named
after characters in works by
William Shakespeare or the
English poet Alexander Pope.

Diameter ................... 36,763 miles
(51,118 km)
Average surface
temperature ........–315°F (–193°C)
One spin on axis ..........17.2 hours
One orbit of Sun...84 Earth years
Number of moons ...................... 27

Ring system
Saturn’s main rings are 220,000
miles (360,000 km) wide, yet they
are only 30 ft (10 m) thick. A scale
model of the rings made with a
sheet of paper would be 2 miles
(3 km) wide. Beyond the main rings
are hazy outer rings, photographed
by Cassini while the Sun was
behind Saturn (below).

Outer atmosphere,
the cloud layer
Atmosphere
(hydrogen, helium,
and methane gases)
Core of
silicate rock
Hot, liquid
mantle

Ice giant
Uranus’s pale blue color is due to the
methane in its atmosphere. Water and
ammonia have also been detected in
the clouds. The planet contains less
hydrogen and helium than Jupiter and
Saturn but has more rock and water.

space

28

THE UNIVERSE

Moon profile
Diameter ........................ 2,159 miles (3,474 km)
Average surface temperature .....–63°F (–53°C)
Length of lunar day ...................... 27 Earth days
Time to orbit Earth ....................... 27 Earth days
Gravity (Earth = 1) ...........................................0.17

How the Moon formed
Scientists think the Moon formed as a result
of a collision between Earth and a planet
4.5 billion years ago. The debris was pulled
together by gravity and became the Moon.

Impact
A planet smashes into
Earth and blasts molten
rock into space.

Moon formation
A disc of debris forms.
The particles slowly
join to form a Moon.

18

days—the time it would take to fly to
the Moon at the speed of a jumbo jet.

The Moon
The Moon is Earth’s closest neighbor in space
and looms larger than any other object in the
night sky. Its cratered surface may be cold and
lifeless, but deep inside the Moon is a gigantic
ball of white-hot iron.
Earth and Moon have existed together in space
ever since the Moon formed as the result of a
cosmic collision. It orbits around our planet, keeping
the same face toward us at all times. As we gaze
on its sunlit surface, we look at a landscape that has
barely changed since 3.5 billion years ago. Back then,
the young Moon was bombarded by asteroids.
For millions of years they blasted out surface
material and formed craters. The largest of these
were then flooded with volcanic lava, creating
dark, flat plains that look like seas.
1

SEA OF SHOWERS
(MARE IMBRIUM)

Lunar maria
Dark, flat areas known as
maria, or seas, are huge
plains of solidified lava.

Phases of the Moon
As the Moon orbits the Earth, a changing
amount of its face is bathed in sunlight.
The different shapes we see are the Moon’s
phases. One cycle of phases lasts 29.5 days.

NEW MOON

WAXING CRESCENT

FIRST QUARTER

WAXING GIBBOUS

FULL MOON

WANING GIBBOUS

LAST QUARTER

WANING CRESCENT

NEW MOON

The far side
The Moon keeps the same face toward the
Earth all the time. The face we never see—the
far side—can only be viewed by spacecraft.
Its crust is thicker and more heavily cratered
than the near side. Elevation (height) maps
reveal high and low areas of land.
Highlands

Gray surface
The Moon’s surface is covered
in a layer of fine gray dust
an inch or so deep.

Lunar craters
Craters exist all over the Moon.
They range from small bowl-shaped
hollows a few miles wide to the vast
South Pole–Aitken Basin, which is
1,600 miles (2,500 km) in diameter.
Many craters, like Eratosthenes
(above), have central hills that formed
as the ground rebounded after the
asteroid struck.

Man on the Moon
Astronauts landed on the Moon six times
during NASA’s Apollo program. They found
a world of gray, dusty plains and rolling hills
under an inky black sky. Below, an astronaut
heads back to his rover vehicle parked near
Camelot Crater (left), where he had been
collecting samples. The large boulders were
flung out of the crater when it formed.

Lunar layers
Like Earth, the Moon is made of different
layers that separated out long ago, when
its whole interior was molten. Lightweight
minerals rose to the top, and heavier
metals sank to the center. The outermost
layer is a thin crust of rock like the rock
on Earth. Under this is the mantle—a deep
layer of rock that gets hotter toward the
center. The bottom part of the mantle is
partly molten. In the Moon’s center is an
iron core heated to about 2,600°F (1,400°C)
by energy from radioactive elements.
Scientists think the outer core is molten
but the inner core is squeezed solid by
the pressure of the rock around it.

Height of surface
High

Low

South Pole–Aitken Basin

2

Every year, the Moon drifts 1.48 in
(3.78 cm) further away from Earth.

12

people have walked
on the Moon.

Highlands
All the Moon’s hills and
mountains are the rims
of craters or central
peaks in craters.

29

3

2
SEA OF SERENITY
(MARE SERENITATIS)

3

SEA OF TRANQUILITY
(MARE TRANQUILLITATIS)

The mantle gets
hotter toward
the center.

Hadley Rille
A deep gorge named Hadley Rille cuts
through flat plains at the edge of the
Moon’s Sea of Showers, winding for
more than 60 miles (100 km). How it
formed is a mystery, but it might be
an ancient lava channel. In July 1971,
Apollo astronauts drove their rover to
the edge of Hadley Rille to take
photographs and study it.

Inner core
A hot, 300-mile- (480-km-)
wide ball of solid iron
forms the inner core.

1

Outer core
The outer part of the
Moon’s iron core is
molten or partially
molten iron.

Inner mantle
Heat from radioactive
elements has partially
melted the inner mantle.

Mantle
The mantle is mainly
solid rock and is rich in
silicate minerals, which
are common on Earth.

Crust
Made of granitelike
rock, the Moon’s crust
is about 30 miles (48
km) thick on the near
side and 46 miles
(74 km) thick on
the far side.

Far side
The Moon’s far side,
which is not visible
from Earth, is covered
in craters and has no
large maria (seas).

Craters in craters
On many parts of the Moon
the craters overlap each
other, the newer ones lying
on top of older craters.

30

SPACE
EXPLORATION

Robot explorers

EXPLORING
THE PLANETS

Stars and planets have fascinated people since
ancient times, but it wasn’t until the 20th century
that exploring space became possible. In recent
decades we have sent astronauts to the Moon,
robotic spacecraft to the outer reaches of the Solar
System, and used huge telescopes to peer across
the vastness of the Universe.

Robotic spacecraft can visit places too
far or dangerous for human beings.
Launched into space by rocket, they
travel vast distances across space and
may take years to reach their target.
There are various types of spacecraft,
each suited to a particular mission.

The planets are too far for
manned missions, so robotic
spacecraft are sent instead. The
first to visit another planet was
Mariner 2, a US craft that flew
past Venus in 1962. Since then,
and despite a number of early
failures, hundreds of spacecraft
have visited the Solar System’s
planets, moons, asteroids, and
comets. Most spacecraft either
fly past or orbit their target,
but some also release landers
that touch down on the surface.

FLYBY SPACECRAFT
Some spacecraft observe a target as they
fly past. NASA’s famous Voyager 1 and
Voyager 2 flew past several planets.
ORBITER
An orbiter flies around a planet repeatedly,
giving it plenty of time to study its target.
Orbiters have visited the Moon and all the
planets except Uranus and Neptune.

OBSERVING THE SKIES

Capturing light

LAUNCH VEHICLES

For centuries, astronomers have observed the heavens
with their eyes alone or used simple telescopes that
magnify the view. But the visible light we see is just one
part of a much bigger spectrum of electromagnetic rays
that reaches Earth from space. Stars and other objects
also emit invisible radio waves, X-rays, infrared, and
ultraviolet rays. Modern telescopes can see all of these,
and each type of radiation reveals something different.

Telescopes come in many
different styles and designs, but
basically all do the same thing:
collect electromagnetic radiation
from space and focus it to create
an image. Earth’s atmosphere can
block or blur the image, so some
telescopes are located on high
mountaintops or even launched
into space.

Space is only 60 miles (100 km) above
the Earth’s surface and takes less than 10
minutes to reach in a rocket. Although the
journey is short, it takes tremendous power
to escape the pull of Earth’s gravity. Launch
vehicles are built to make the journey only
one time, and most of their weight is fuel.

Mapping the stars
Because Earth is surrounded
by space, when we look
at the night sky it seems
as though all the stars are
pinned to the inside of a giant
sphere. Astronomers call this
the celestial sphere and use it
to map the positions of stars
and planets. Vertical and
horizontal lines are used to
divide the celestial sphere
into a grid, just like the grid
of longitude and latitude lines
used to map Earth’s surface.

Celestial
north pole
This is the
point directly
above Earth’s
North Pole.

Celestial
south pole
This is the
point directly
above Earth’s
South Pole.

X-ray telescopes
These telescopes
capture high energy
rays from extremely
hot objects. X-ray
telescopes only
work in space.

Declination lines
These split the sky into
north–south segments.

Right ascension lines
These divide the sky
into east–west
segments.

Saturn V, which sent astronauts
to the Moon, was the largest rocket
ever built. Its Soviet rival, the N1,
was launched four times but each
attempt ended in disaster.

SATURN V (US)—364 ft (111 m) tall

Ultraviolet telescopes
Astronomers
use ultraviolet
telescopes to
examine radiation
from the Sun, stars,
and galaxies.

X-RAY

N1 (USSR)—344 ft (105 m) tall

Optical telescopes
Using large lenses
or mirrors, optical
telescopes gather
faint visible light and
can see much further
than the human eye.

ULTRAVIOLET

DELTA IV HEAVY (US)—236 ft (72 m) tall

Infrared telescopes
These instruments,
some of which are
sent into space, detect
the heat from objects
such as clouds of
gas and dust.

VISIBLE

LONG MARCH 2F (CHINA)—203 ft (62 m) tall

Radio telescopes
Huge, curved dishes
are used to focus
radio waves given
out by sources such
as galaxies, pulsars,
and black holes.

INFRARED

ARIANE 4 (EUROPE)—193 ft (59 m) tall

RADIO

World’s largest rockets

Launch sites
Many countries have spaceflight launch sites.
Sites closer to the equator can launch heavier
cargo, because rockets there are given a boost
by the speed of Earth’s spin.

Baikonur, Kazakhstan
Cape Canaveral,
FL

Celestial equator
This imaginary line
over Earth’s equator
divides the sky into
north and south
hemispheres.
MAJOR LAUNCH SITES

Xichang,
China

31

12

Most visited
This chart shows
the number of
missions to the
major bodies in
the Solar System.

SATELLITES

LIVING IN SPACE

About 1,000 operational satellites orbit the Earth,
carrying out tasks such as beaming TV signals around
the world, gathering data for weather forecasters, and
spying for the military. Many thousands more pieces of
space junk—old satellites, discarded rocket parts, and
debris from collisions—also circle our planet. The growing
cloud of space debris is a hazard to spacecraft.

Astronauts must adapt to a
zero-gravity environment when
living in space. Although floating
weightlessly can be fun, it can
also cause medical problems.
Space stations are cramped places
with few luxuries. Astronauts eat
ready-made meals that are either
freeze-dried or served in
pouches. All water is recycled,
including the water vapor
from human breath. Astronauts
clean themselves with special
shampoos and soaps that don’t
need water, and they use space
toilets that suck away waste
rather than flushing with water.

Effects on the body
Over 500,000 objects, including satellites and space junk, orbit Earth.

Satellite orbits
Some satellites are a few hundred miles above Earth’s surface,
but others are much further. Some of the highest ones, such as
weather, TV, and phone satellites, have geostationary orbits,
which means they stay over a fixed point on Earth. Satellites
with lower orbits change position all the time.

When the human body spends a long
time in space, it changes. Without
gravity pulling on the spine, the
body gets about 2 in (5 cm) taller.
Body fluids that flow downward on
Earth build up in the head. This gives
astronauts swollen faces and blocked
noses, making food seem tasteless.
When astronauts come back to Earth,
the return of full gravity can make
them feel extremely weak.

30
20
10

MERCURY

Pioneer spacecraft
Pioneers 10 and 11 made flybys of
Jupiter and Saturn. They are now heading
out of the Solar System into deep space.

40

years and 43 days –
the time it took the
spacecraft Voyager 2
to reach Neptune from Earth.

SUN

Generator

PLUTO

50

Asteroid detector

NEPTUNE

60

URANUS

70

JUPITER

80

SATURN

PENETRATOR
A penetrator is designed to
hit its target at high speed
and bury itself. In 2005,
Deep Impact penetrated
the surface of a comet.

In little more than 50 years, around 200 spacecraft have left
Earth’s orbit and headed off to explore the Solar System. More
than half the missions have been to Earth’s nearest neighbors
in space: the Moon and the planets Mars and Venus.

ASTEROIDS

Communication
dish

LANDER
Some craft can touch down
on the surface of another
world. In 1976, Viking 1
became the first craft to
successfully land on Mars.

Solar System missions

MARS

Infrared sensor

ROVER
A rover is a robotic lander
with wheels that can drive
around. Rovers sent to Mars
have studied its rocks for
signs of ancient life.

MOON

Ultraviolet
sensor

ATMOSPHERIC PROBE
This type of craft enters
a planet’s atmosphere.
The Galileo probe dove
into Jupiter’s stormy
atmosphere in 2005.

VENUS

Magnetometer

0

Brain and balance
Without gravity, the inner
ear’s balance system no
longer works, which can
make astronauts feel sick.

Heart
Blood flows more
easily, so the heart
doesn’t need to
work as hard.

Muscles
Movement is easy when
you’re weightless, and
muscles waste away if
not used. Workouts in
the onboard gym help
to slow this process.
Bones
Bones weaken and
become less dense.
Regular exercise is
essential to keep
them strong.

ORBITAL PERIOD
20 HOURS

Geostationary orbit
(communications
satellites)

15 HOURS
10 HOURS
Medium-Earth orbit
(GPS satellites)
5 HOURS

Low-Earth orbit
(International
Space Station)

Space stations
A space station is a crewed satellite—
a kind of orbiting laboratory in which
astronauts and scientists live and
work. The USSR launched the first
station, Salyut 1, in 1971. The US
soon followed with Skylab, in 1973.
Russia’s Mir, in use from 1986 to
2001, was the most successful station
until the US, Russia, and more than
10 other countries joined forces to
build the International Space Station,
in orbit since 1998. China’s own
space station prototype, Tiangong-1,
was launched in 2011.

SKYLAB (US)

SALYUT 1 (USSR)
MIR (USSR)
TIANGONG-1
(CHINA)

INTERNATIONAL
SPACE STATION

d
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I
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Pollux
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S
NI R Pr
CAINO
M
luster
hive C
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CAN

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SOUTH

A
DR
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A
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S
NU

AR I
ES

AX

Mi
ra

IX
N
OE
PH

CE
TU
S

SC
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Measuring brightness
The size of the white spots
on the chart show how bright
the stars are. Astronomers call this
magnitude and measure it on a scale
that runs backward—the smaller the
number, the brighter the star.

-1

0

Ecliptic

RN
FO

Hamal

D
en

Star magnitudes

6
M3
Elnath

3000 bce
150 ce
1543

s

cy
on
O

LEPUS Arneb

CAELUM

n
et
ara
el
Aldeb
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22
u
s
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trix
4
4
Bella
ORION
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A
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Alnlnilam Min
itak
M42
l
Rige

N
OC
ER
OS

Siriu
s

Adh
ara

1610

H

or
iz
on

MACAN
JO IS
R

PU
PP
IS

COLUMBA

Orion the hunter
One of the best-known and brightest
constellations is Orion the hunter,
which is visible the world over.
Orion includes the red giant star
Betelgeuse and the blue-white
supergiant Rigel—two of the
brightest stars in the night sky.

H
M3
8

ya
de
s
Alcy
TA
o ne
UR Ple
US iades

IUM

Today, space telescopes
such as Hubble, which
was launched in 1990,
give us breathtaking
views of distant
objects in space,
including the furthest
galaxies ever seen.

PY
XI
S

VE
LA
G
OLO

Modern astronomy

H

HOR

Isaac Newton
English scientist Isaac
Newton worked out the
laws of gravity—the
force that makes objects
fall to the ground. He
discovered that gravity
keeps the Moon in orbit
around Earth and keeps
the planets in orbit
around the Sun.

Canopus

DORADO

1687

Colored circles on the star
chart show the night sky
you can see from the
locations highlighted
in color above.

PICTOR

Betelgeuse

Rigel

1990

LATITUDE
60°N
40°N
20°N

M
RETICULU

Italian scientist Galileo
Galilei built a telescope
and used it to study the
night sky. He saw spots
on the Sun, mountains
on the Moon, and four
moons orbiting the
planet Jupiter.

Ancient stargazers saw patterns
in the stars and named groups of
stars after mythical beings and
animals. These star patterns,
called constellations, look little
like the objects they are meant
to represent, but we still use the
old names. Today, astronomers
divide the whole sky into 88
segments, each one named after
the constellation within it. Star
charts like the one here show
which constellations are visible
at a particular time and place.
This chart shows the stars you
can see at midnight in January
from the northern hemisphere.

A

Galileo Galilei

The sky at night

CA
RIN

Earth
Polish astronomer
Nicolaus Copernicus
proposed that the Sun,
not Earth, is the center
of the Solar System. It
was a shocking idea
since it meant Earth
must be flying through
space, spinning around.

Copernicus

Professional astronomers investigate not only stars but everything to do
with space—from the meteors that burn up spectacularly as shooting stars in
Earth’s atmosphere and the planets of the Solar System to distant galaxies
billions of light years away. Astronomy makes a rewarding hobby too, and
many amateur stargazers enjoy observing the night sky with backyard
telescopes or binoculars. Whenever astronomers observe the sky, they are
looking back in time. This is because light takes such a long time to reach us
from distant objects in space. We see the Moon as it was one and a quarter
seconds ago and the stars as they were hundreds of years ago.

Regulu

The Greek astronomer
Ptolemy cataloged
1,022 stars in 48
constellations. He
believed that Earth was
the center of the Solar
System and Universe,
orbited by the Sun,
Moon, planets, and stars.

People have been looking up at the night sky and marveling at
its beauty and mystery for thousands of years. Today, a whole
branch of science—astronomy—is devoted to studying stars.

IA
TL
AN

Ptolemy

Many monuments built
by ancient peoples, such
as Stonehenge in the UK,
align with the Sun. These
monuments may have
been used as calendars
so that farmers knew
when to sow crops.

Astronomy

n
izo

Many ancient cultures followed the
Sun and stars in order to keep track of
the time of year, and by Ancient Greek
times, astronomers had already worked
out that Earth is round. Today, powerful
telescopes allow us to peer so far into
space that we can look back in time
almost to the birth of the Universe.

trillion miles (40 trillion km)—the distance
to the nearest star, Proxima Centauri.

ANS
SEXT

A brief history of astronomy

Astronomical
calendars

25

SPACE EXPLORATION

r
Ho

space

0ºN
n2
izo
Hor

32

1

PISCES

eb

Ka
ito
s
LP
TOR

2

3

4

5

15

The number of stars
visible to the naked eye.

33

How telescopes work
The invention of the telescope
revolutionized astronomy. A
telescope collects more light
from an object than a human
eye can. It uses this light to
form a magnified image.
There are two basic types
of telescopes: refracting and
reflecting. The refracting
telescope has a large convex
(outward-curving) lens that
gathers and focuses the light.
The reflecting telescope uses
a curved mirror instead.

Eyepiece

Lens

Eyepiece lens
magnifies
image

Focal point

Light from star

Light from star

Smaller mirror

Light rays
converge

Focal point

Refracting telescope
A convex lens bends light
entering the telescope
to focus it, forming an
image. At the other end
of the telescope, a smaller
lens called the eyepiece
magnifies the image.

Reflecting telescope
A concave (inward-curving)
main mirror reflects light on
to a smaller, flat mirror. The
resulting image is magnified
by an eyepiece lens.
Main mirror

EAST

The dotted white line running
down the chart is called the
ecliptic and shows the path of
the Sun. The planets always
stay close to the ecliptic.

NA
RO
CO

A

r
lco

Horizon

Professional astronomers don’t just use visible
light to see the night sky. Their telescopes can
also create images from wavelengths of light
that our eyes cannot see, such as X-rays, radio
waves, and infrared rays. The images below
all show Kepler’s Supernova—the wreckage
left by a giant star that exploded in 1604.
X-ray image
This image of Kepler’s
Supernova is from the
orbiting Chandra X-ray
Observatory. It shows
a cloud of incredibly
hot gas that emits
high-energy X-rays.

Infrared image
Taken by the Spitzer
Space Telescope, this
infrared image shows
dust clouds that were
heated by a shock wave
from the exploding star.

Cy
gn

us

Ri
ft

CY
GN
US

DRACO

Horizon

Horizo

20ºN

Ca
ph

LA
CE
RT
A

De
ne
b

Polaris

PE
IA

CA
ME L
OPARDALIS

URSA
MINOR

Cas

X

AU
R I GA
AS
SIO

Al Delt
de
a
ram Cep
C
hei EPHEUS
in

LY

N

fa
k

Seeing the invisible

Visible light image
Very little of the object
can be seen in visible
light, even in this image
from the Hubble Space
Telescope. The bright
areas are clumps of gas.

Alb
ire
o

Be

h
e
bh oug
Du
Pl

M

U Me
M RS rak
AJ A
OR
S

North
star

BO

ar
iz

r
Co

C

LE
MINO
OR

ap
ella

Eps
ilon
Vega Lyrae HERCULES
LYRA

Algieba

A
ED
ROM
A ND
z
t
a
Alpher

Pointers

ºN
n 40

LEO

ar
ol
i
C
V AN
E
NA ES
TIC
I
C

EU
RS
h
ac
Alm

M
LU
NGU
T RI A
h
c
Mira

ab
Koch
an
Thub

ola

PE

E
ÖT
BO
IS
AL
RE

Deneb

ir
M
Algol

S

C
BEOMA
RE
NI C
ES

C
Epsilon
Aurigae

The Great Bear
Ursa Major is named after a bear.
The seven bright stars running
from its tail form a famous group
of stars known as the Big Dipper or
Plow. The last two of these point to
the North Star, which is always due
north. In ancient times, sailors used
this star to find the way.

NORTH

Haedi

8

2,000

million trillion miles (24 million trillion km)—the distance to the
Andromeda galaxy, the furthest object visible to the naked eye.

S
ASU
PEG
Markab

On a clear, dark night you
can see the Milky Way—
the galaxy to which our
Solar System belongs.

WEST

GALAXY

GLOBULAR CLUSTER

OPEN CLUSTER

Star clusters are
clouds of stars.

Cassiopeia
This constellation is named after a
vain queen in Greek mythology. It’s
very easy to spot in northern skies
as it looks like a funky letter W.

Combined image
Combining all three sources produces a complete
image: a shell of supernova debris expanding
into space at 1,240 miles (2,000 km) per second.

34

space

SPACE EXPLORATION

1

million people traveled to Florida to watch
the launch of Apollo 11 on July 16 1969.

The Saturn V rocket was taller than
the Statue of Liberty in New York City.

Fuel tanks
Tanks within the Service
Module supplied fuel to
the main engine.

Escape rocket
(for emergencies
during launch)
Command Module
Astronauts stayed in
here during launch.

Astronauts
The crew of three stayed in
the Command Module for
most of their journey to
and from the Moon.

Service Module
This module powered
the Apollo spacecraft.

Lunar Module
The Lunar Module
was housed in an
aluminum cone.
Instrument unit

Third stage
This stage reached
low-Earth orbit and
then put Apollo on
course for the Moon.

Single third-stage
engine

Interstage adapter
Covering the thirdstage engine, this
section linked the
rocket’s second
and third stages.

Second stage
The second stage
held a tank of
liquid hydrogen
fuel and a tank
of liquid oxygen.

Second-stage
engines

Interstage adapter
This section linked
the rocket’s first two
stages and also
covered the secondstage engines.

First stage fuel
The first stage had a
tank of kerosene fuel
and a tank of liquid
oxygen to burn it.
The five engines
burned 16 tons of
fuel per second
during the launch.

Five first-stage
engines

Engine nozzle
Nozzle for the
main engine, which
propelled the Apollo
craft through space.

Service Module
This module provided lifesupport systems and power for
the crew, and housed the
Apollo craft’s main engine.

Mission
to the Moon
Humans have set foot on only one world
beyond Earth: the Moon. Just 27 daredevil
astronauts have traveled there, of whom
12 walked on its cratered, lifeless surface.
Eight space missions visited the Moon between
1968 and 1972 as part of NASA’s Apollo program.
Each mission carried three American astronauts
inside an Apollo spacecraft, which was launched
by a Saturn V rocket. Apollo 8 tested the craft as
it orbited the Moon. Then, in a dress rehearsal prior
to landing, Apollo 10 flew close to the lunar surface.
The first of the six missions that successfully landed
on the Moon was Apollo 11 in 1969. Astronauts Neil
Armstrong and Buzz Aldrin touched down on the
surface in July of that year. As Armstrong took the
first historic step, he said, “That’s one small step for
man, one giant leap for mankind.”

Thrusters
Small thrusters made fine
adjustments to the Apollo
spacecraft’s movements.

Command Module
The Command Module was the only part
of the Apollo craft to return to Earth. Its
conical shape helped it withstand the
heat of reentry into Earth’s atmosphere.

Apollo spacecraft
The Apollo spacecraft had three parts: the
Command, Service, and Lunar Modules. These
were all linked together for the 250,000-mile
(400,000-km) trip to the Moon. Once there,
the Lunar Module took two astronauts down
to the Moon’s surface, while the third crew
member remained in lunar orbit in the
combined Command and Service Module (CSM).
The top half of the Lunar Module, known as the
ascent stage, later returned the two astronauts
to the CSM for the journey back to Earth.

Saturn V rocket
The Apollo astronauts were blasted into space
inside the nose cone of the largest rocket ever
built: Saturn V. Standing nearly 364 ft (111 m) tall,
the Saturn V was as tall as a 30-story building.
This giant launch vehicle consisted of three
rockets in one. The first two parts, or stages, lifted
the Apollo craft into space, and the third stage set
the spacecraft on course for the Moon.

21

hours—the length of time
Apollo 11 astronauts Neil
Armstrong and Buzz Aldrin
spent on the Moon. Apollo 17 astronauts
spent three days on the lunar surface.

300

The total number of hours that
astronauts have spent on the Moon.

There and back
Each of the six Apollo missions
that landed men on the Moon
took the same route, taking off
from Florida, and ending with
the astronauts splashing down
in the Pacific Ocean.
Saturn V rocket carrying
Apollo craft blasts off and
positions craft in Earth’s orbit.
2 The rocket’s third stage and
Apollo craft leave Earth’s orbit
and head toward the Moon.
3 Combined Command and
Service Module (CSM) separates
from the rocket.
4 CSM turns and docks with
1

Docking tunnel
Astronauts used this tunnel to
move between the Command
and Lunar Modules.

842

Beef, potatoes, and grape juice—the first meal
eaten by the Apollo 11 astronauts in space.

lb (382 kg) of lunar rock and soil were
brought back by the Apollo astronauts.

The trip from the Earth
to the Moon took
about three days.

11
10
1

35

Landing sites
The Apollo landing sites
were on the side of the
Moon that faces Earth.

12
5

2

7
3

4

Lunar Module. Third rocket
stage is now discarded.
5 Apollo craft adjusts its
course to go into lunar orbit.
6 Lunar Module transports two
astronauts to lunar surface.
7 Third crew member continues
to orbit the Moon in CSM.
8 Ascent stage of Lunar Module
takes astronauts back to CSM,

8

9

6

after which it is discarded.
9 CSM adjusts its course and
heads back to Earth.
10 Service Module is jettisoned.
11 Command Module enters
Earth’s atmosphere.
12 Command Module makes a
parachute landing in the sea.

Ascent stage
The ascent stage of the Lunar
Module was the astronauts’ home
while they explored the Moon.

Legs and pads
Flexible legs with wide pads
on the bottom cushioned the
Module’s landing and kept it
stable on the surface.

Descent stage
This bottom half of the Lunar
Module acted as the launch
platform when the top half blasted
off back into space. The descent
stage stayed on the Moon.

Hatch
The astronauts
climbed through a
hatch to go outside.

Descent engine
This engine was used to slow
down the Lunar Module’s
descent during landing.

Gas tanks
The larger tank
contained helium;
oxygen was held in the
adjacent smaller tank.

Leg ladder
Astronauts used the
ladder to climb down
to the lunar surface.

Fuel tank
This tank contained fuel
for the Lunar Module’s
descent engine.

Man on the Moon
The Lunar Module was the only part
of the Apollo craft to reach the Moon’s
surface. Preprogrammed controls
maneuvered it into position above the
landing site, then an astronaut steered
the craft to touchdown. Scientific
equipment, a TV camera, and tools and
storage boxes for rock collecting were
all stored in the bottom half.

Sensing probe
Probes on the legs touched
the ground first during
landing and sent signals
to shut down the engine.

space

36

SPACE EXPLORATION

4

spacecraft have visited
the planet Saturn.

Cassini-Huygens spacecraft
Path to the planets
The paths of spacecraft are often carefully planned to
take them close to one or more planets on the way to
their final destination. Using the pull of gravity of each
planet boosts their speed and saves fuel. Cassini-Huygens
flew past Venus, Earth, and Jupiter on its way to Saturn.
6. Arrival at
Saturn

2. First Venus fly-by

Cassini-Huygens is the largest spacecraft
to visit another planet. It was launched in
1997 and arrived at Saturn in 2004. It had
two parts: the Cassini orbiter, designed to
orbit Saturn until 2017, and a probe called
Huygens, which touched down on Saturn’s
large moon Titan. The main aim of the mission
was to discover more about Titan—the only
world in the Solar System other than Earth
that has a dense, nitrogen-rich atmosphere.

3. Second Venus fly-by
Jupiter’s orbit

Boom
A 36-foot (11meter) boom carries
instruments to measure
Saturn’s magnetic field.
The long boom reduces
interference from other
instruments on Cassini.

CASSINI

1. Launch
5. Jupiter fly-by
4. Earth fly-by

Landmark missions
Since the first spacecraft to visit a planet was launched in
1962, about 200 craft have explored the Solar System.
Some of the most famous missions are shown here.
Descent capsule

Venera 7
The first craft to touch down
on another planet, Venera 7
landed on Venus in 1970. It
lasted 23 minutes before the
searing heat destroyed it.

Solar panel

Lunokhod 1
Russian-built Lunokhod 1 was
the first lunar rover. It landed
on the Moon in 1970 and spent
322 days exploring, traveling
a total of 6.5 miles (10.5 km).

Radio dish
Spectrometer

Voyager 1
Launched in 1977 and still
operational, Voyager 1 is
the furthest manmade object
from Earth. It visited Jupiter
in 1979 and Saturn in 1980.

Rovers
A rover is a robotic
vehicle built to explore
the surface of a planet or
moon. Four rovers have
landed successfully on
Mars. They receive radio
commands from Earth
but find their way
around and carry out
tasks independently.
Curiosity lands
The Curiosity rover was
lowered on to Mars in 2012
by a rocket-powered craft.

Sojourner rover
The first rover to explore
another planet was Sojourner.
It reached Mars in 1997 and
spent 12 weeks studying the
soil and taking photos.
Small radio antenna
This is one of two small
radio dishes that serve as
backups in case the main
dish breaks down.

CASSINI ORBITER (GREY)

HUYGENS
LANDER
(RED)

Large radio dish
The large radio dish
communicates with Earth.
It is also used to map
Titan, which it does by
bouncing radio waves
off the surface.

98,346

mph (158,273 kph)—the top
recorded speed of Cassini-Huygens.

53

spacecraft have attempted
to reach the planet Mars.

37

While manned spacecraft have ventured
no further than the Earth’s Moon, robotic
craft have visited all the planets in the Solar
System—and more than 100 moons.

Plutonium power
supply (one of three)

Main engine

Robotic spacecraft can visit places that would prove
lethal to astronauts, such as the scalding surface of
Venus or the deadly radiation belts around Jupiter.
Packed with scientific instruments, telescopes, and
cameras, they carry out dozens of experiments
during their missions and capture thousands of
images, which are sent back to Earth by radio.

Helium tank
Helium gas from this tank
pushes fuel from the fuel
tanks to the engine.
Fuel tanks
The fuel tanks carry two
different liquids that burst
into flame when mixed.
Cosmic dust analyzer
This device measures the
size and speed of cosmic
dust particles in space.

Front case of
Huygens probe

Fuel for
thrusters
Back cover of
Huygens probe

Inner body case of
Huygens probe

Experiment platform
Huygens carried a range
of scientific instruments
to study conditions on
Saturn’s moon Titan.

Plasma spectrometer
This device measures
charged particles
trapped by Saturn’s
powerful magnetic field.

missions to Mars
have ended in failure.

Exploring
the planets

Thruster
Cassini has four
thrusters. These small
engines adjust the craft’s
flight path precisely.

Radio antennas
The three long antennas of
the radio and plasma wave
science (RPWS) instrument
detect radio waves generated
by Saturn’s outer atmosphere.

27

Heat shield
Without a heat shield, the Huygens
probe would have burned up like a
shooting star when it entered the
atmosphere of Saturn’s moon Titan.
The shield was made of silica fiber
tiles able to withstand temperatures
up to 2,700°F (1,500°C).

Triple parachute
Packed under Huygens’s back cover were
three parachutes that opened in turn to slow
the lander’s descent onto Titan. Huygens
discovered a world of freezing, orange-brown
plains littered with pebbles of ice.

EARTH
Oceans of water, an oxygen-rich atmosphere, and the
existence of life make Earth a unique planet. Its surface
is continually changing as plates slowly shift and the
relentless force of erosion reshapes the land.

40

Earth is the only place in the
Universe known to support life.
It is thought that life developed
after water began to collect on
the Earth’s surface. Eventually,
tiny life forms evolved that
could survive on water, sunlight,
and chemicals in the water.
These microbes added oxygen
to the atmosphere—an essential
step for the development of
plants and animals.

Earth’s interior has layers.
Scientists discovered this
by studying the paths by
which earthquake waves
pass through the planet.

Earth’s atmosphere

What’s in a layer?

The atmosphere of Earth
is made up of several
different gases.

Earth’s crust and mantle are mostly
made of minerals called silicates,
which are a combination of silicon
dioxide and metal oxides. The mantle
is rich in magnesium-containing
silicates, while the two different
types of crust have less magnesium
and more aluminum and calcium. The
core is dominated by metallic iron.
No part of it has ever been brought
to the surface, but its composition
has been worked out by scientific
methods such as studying
earthquake waves.

0.9% ARGON

0.1% OTHER

21%
OXYGEN

78%
NITROGEN

NITROGEN – 78%
A gas that can be fixed in the soil as
well as loose in the atmosphere. Plants
need nitrogen from the soil to survive.
OXYGEN – 21%
Essential for animals to breathe,
oxygen was absent until microbes
evolved that could use sunlight to
turn carbon dioxide and water into
carbohydrates, releasing oxygen.
ARGON – 0.9%
An inert gas (one that doesn’t
react with other substances).
OTHER – 0.1%
These include carbon dioxide
(C0₂), which was once abundant, but
is now mostly incorporated into
materials such as limestone rock.

JOINED TOGETHER, FORMING
AN ENORMOUS SUPERCONTINENT

KNOWN AS PANGAEA.

Thickness

3.7–56 miles (6–90 km)
1,790 miles (2,880 km)
1,400 miles (2,255 km)
755 miles (1,215 km)

CRUST
Different types of crust
make up Earth’s surface
and its ocean floor. The
crust under the surface
is thicker and contains
more rock types.

OUTER CORE
The only liquid layer,
the outer core is mainly
iron but also contains
some nickel and small
amounts of other
substances.

MANTLE
This rocky layer is
denser than the crust.
It is mostly solid,
although it can very
slowly deform and flow.

INNER CORE
This is solid, and is
mostly iron with some
nickel. Its temperature
is very hot—about
9,900°F (5,400°C).

Key
Silicon dioxide
Aluminum oxide
Iron and iron oxides
Calcium oxide
Magnesium oxide
Nickel
Other

The oceans
Earth’s surface and atmosphere
contain the equivalent of 333 million
miles³ (1.39 billion km³) of water.
There are regions of deep ocean
as well as shallow seas that cover
areas around the edges of the
continents—these are called
continental shelves. Earth’s surface
has not always been as dominated
by liquid water. In the past, during
ice ages when the polar ice caps
were much thicker and more
extensive, so much water became
locked up in them that sea level was
at least 400 ft (120 m) lower than it
is today, exposing the continental
shelves as dry land.

25%
LAND

75%
WATER
Water world
Almost three-quarters
of Earth’s surface is
water. Over 97 percent
of Earth’s water is found
in the oceans.

CORE

Inside our planet

EARTH’S CONTINENTS ALL

MANTLE

UNIQUE PLANET

250 MILLION YEARS AGO,

OCEANIC CRUST

Earth formed about 4.5 billion years ago, but it was a
very different place then. Its surface was a hot inferno
of mostly molten rock, with little or no liquid water
and no oxygen in the atmosphere. Since then Earth
has developed oceans, continents, an oxygen-rich
atmosphere—and life.

Look at a map and it will show you the position of the continents, but
in fact our world is always changing. Earth’s surface is split up into
large slabs called tectonic plates. The plates steadily shift around,
carrying continents and oceans with them. When they collide, new
mountain ranges are pushed up. Afterward, over millions of years,
wind, water, and ice gradually wear the mountains down.

CONTINENTAL CRUST

PLANET
EARTH

THE CHANGING EARTH

41

Continental drift

Plate movement

Over millions of years, tectonic plates have
moved, shifting around the continents on Earth’s
surface. Chunks of continents split away and push
North America,
Europe, and parts of
Asia are one landmass

India moves
north

into each other, creating new land masses
and moving the oceans in a process called
“continental drift.”

Australia is joined
to Antarctica

South America
separates from Africa

Australia moves into
the Pacific Ocean

The continents get rearranged because
they are carried along as parts of
moving plates. This process has been
going on for billions of years, and is
thought to be caused by slow, heatdriven movements in Earth’s mantle.

0

200 MILLION YEARS AGO
The supercontinent
Pangaea has just begun
to break into two main
landmasses.

130 MILLION YEARS AGO
India has escaped from the
southern landmass, and is
slowing moving north,
toward Asia.

70 MILLION YEARS AGO
South America has split
from Africa, while in the
north, North America is
splitting from Europe.

TODAY
Australia has separated
from Antarctica, and India
has collided with Asia,
forming the Himalayas.

LOOKING AT EARTH

A spinning planet

Magnetic Earth

Our planet is far from smooth—
its continents and ocean floors
are scarred and pitted with
marks caused by movement
of plates. Earth’s place in space
also affects its shape, as
constantly spinning makes it
bulge out around the middle
so it is not a perfect sphere.
Spinning also creates a magnetic
safety field around the planet.

Earth’s gravity would pull it into
the shape of a sphere, but its
rotation makes it bulge slightly.
This means its diameter at the
equator is 25 miles (41 km) more
than the distance between its poles.

Because Earth’s outer core is liquid,
the planet’s rotation stirs it into
motion. This motion causes electric
currents to develop in the liquid
iron itself. Any pattern of electric
currents creates a magnetic field,
and in Earth’s case, the field is
similar to what would be produced
by a large bar magnet inside the
planet. The field protects Earth
from damage by harmful, energetic
particles that come from the Sun.

MOUNTAINS

Direction
of rotation

ONE-FIFTH

Not quite round
At the moment, scientists think
that Earth’s equatorial bulge is
growing at a rate of 0.3 in (7 mm)
every 10 years.

MAKE UP ABOUT

OF THE EARTH’S

LANDSCAPE.

Bulge at
middle

The magnetic field
The magnetic poles do not coincide
exactly with the geographic
(rotational) poles, and they gradually
change position over time.

Earth’s surface
The solid surface of the Earth ranges
from about 35,750 ft (10,900 m) below
sea level in the Challenger Deep (part of
the Pacific’s Mariana trench) to 29,029 ft
(8,848 m) above sea level at the summit
of Everest, which may be rising at about
0.16 in (4 mm) per year. The surface of
most land areas is less than 1,650 ft
(500 m) above sea level.

Elevation
Over 13,125 ft (4,000 m)
6,500–13,125 ft (2,000–4,000 m)
3,300–6,500 ft (1,000–2,000 m)
1,600–3,300 ft (500–1,000 m)
800–1,600 ft (250–500 m)
300–800 ft (100–250 m)
0-300 ft (0–100 m)

Sea depth
0–800 ft (0–250 m)
800–6,500 ft (250–2,000 m)
6,500–13,000 ft (2,000–4,000 m)
Below 13,000 ft (4,000 m)

Mountains and trenches
Earth’s solid surface is far from flat. This map shows its elevations and
depths—from the highest mountain peaks to the deepest ocean trenches.

1

2

Yearly shift
Plates typically move at a rate of about
1 in (2.5 cm) in a year. That’s about as fast
as your fingernails grow. Some move
faster—up to 4 in (10 cm) a year.

Geographic
North Pole

Magnetic
south pole

Magnetic
north pole

Magnetic
force

Geographic
South Pole

42

earth

4.5

PLANET EARTH

Inside the Earth

Continental crust
Thicker and less
dense than the
oceanic crust.

We can’t explore much of the Earth—our deepest mines
only travel about a mile into the crust. However, there
are scientific ways to find out what it is like inside.
Geologists are able to study rocks from all depths within the
Earth’s crust, because collisions between continents push up
rock that used to be below the surface, forming mountains.
In some areas, collisions have even unearthed vast swathes
of the mantle. Volcanoes also sometimes erupt lumps of
rock from the mantle. Under the mantle is the core, which
has never been seen at the surface. However, scientists
have used the waves from earthquakes to work out
that the core is split into two layers—a liquid outer
core and a solid inner core.

Volcanoes in Hawaii
The Hawaiian islands in the mid-Pacific have
been built by volcanic eruptions. The rock that
formed them was pushed to the Earth’s surface
by hot rock moving upward in the mantle.
Rifting
Two tectonic plates pull away
from each other, and new
land is created between them.

Oceanic crust
Thinner and
denser than the
continental crust.

9,900°F

(5,400°C) – the approximate
temperature of Earth’s inner core.

Layered Earth
Earth is made up of many rocky layers. The
top layer is the crust. Below that, uniform and
slightly denser rock forms the mantle. The crust
and the top of the mantle form a single rigid
layer together, which is called the lithosphere.
This is broken into sections called tectonic plates.
Below the lithosphere is the asthenosphere. Only
tiny parts of the asthenosphere are liquid, but it
is soft enough to move, pushing around the
plates above. Under the mantle lies the core.
The outer core is a liquid mix of iron and sulfur,
while the inner core is solid iron and nickel.

billion years—the
approximate age of Earth.

Subduction
When two plates
meet, one can be
pushed underneath
the other.

Pacific Ocean

There are four main layers inside the Earth. From the outside
in, they are the crust, mantle, outer core, and inner core.

Lithosphere
The rigid outer shell,
made of the crust and the
top layer of the mantle.

43

The atmosphere

Mantle
A solid layer that
is Earth’s thickest.

Outer core
Molten iron and
sulfur. Currents in
this liquid generate
Earth’s magnetic field.

Earth’s atmosphere is made up of gases, which are
held in place by gravity. There is no clear boundary to
the outer edge of the atmosphere—it just fades into
space. Ou