OBSERVING THE   UNIVERSE

What is space?

We live on a small, blue planet called Earth. 
It has a surface of liquid water and rock and 
is surrounded by a blanket of air called the 
atmosphere. Space begins at the top of the 
atmosphere. It is an unimaginably vast, 
silent, and largely empty place, but it has 
many amazing properties. Blacker than black 
In photos taken from space, our planet is 
surrounded by blackness. This shows how empty outer space is. Planets like Earth shine because they reflect light from the Sun. Stars  shine because they produce huge amounts of  energy by burning fuel. Most of space looks  black because there is nothing there to  produce or reflect light.  

THE EDGE OF SPACE

Earth’s atmosphere does not end

suddenly—it gets gradually thinner

and thinner as you travel up from the

ground. Most experts agree that outer

space starts at a height of 60 miles

(100 km). Yet even above this height,

there is a layer of very thin air called

the exosphere. Hydrogen and other

light gases are slowly escaping into

space from this outermost part

of Earth’s atmosphere.

FACT FILE

■ In the US, anyone who flies above an

altitude of 60 miles (100 km) in a rocket or

spacecraft gets a special badge called

“astronaut wings.”

■ Our most common name for people who

travel into space is “astronauts,” meaning

star travelers. The Russians call them

“cosmonauts,” meaning travelers through

the universe, and the Chinese name is

“taikonauts,” from their word for space.

■ A person who stepped out into space

without a protective suit would quickly die.

He or she would be unable to breathe, but

would still have around 10 seconds to

reach safety before losing consciousness.

, THE ATMOSPHERE protects

Earth’s surface from harmful radiationand the full heat of the Sun. At night,

it stops heat from escaping into space.

IN A VACUUMA place without any air or gas is called a 
vacuum. On Earth, air transfers heat from 
one place to another. In space, there is no 
air to distribute heat, so the sunlit side of a 
spacecraft gets very hot, while the other side is in darkness and gets very cold. Spacecraft have  to be tested in a thermal vacuum chamber  before they are launched to make sure that  they can survive these extreme  space temperatures.
Sun u ESCAPING GRAVITY
The space shuttle uses up all the fuel in its two booster rockets just to overcome gravity 
and reach outer space. Getting off the groundIt is difficult to get into space because Earth’s gravity holds everything down. To overcome gravity and go into orbit, a rocket has to reach a speed of 17,500 mph (28,000 km/h), known as escape velocity. This requires a lot of fuel to provide energy. To reach the Moon and planets, spacecraft have to travel at an even higher speed—25,000 mph (40,000 km/h).Our place in spacePlanet Earth is our home and to us it seems a very bigplace. Flying to the other side of the world takes anentire day, and sailing around the world takes manyweeks. Yet in the vastness of the universe, Earth isjust a tiny dot. In fact, an alien flying through our galaxy would probably not even notice  our little planet.
EARTH AND MOONEarth’s nearest neighbor is the Moon, ourplanet’s only natural satellite. The Moon isa lot smaller than Earth. Its diameter isonly about one-quarter the diameter ofEarth, and fifty Moons would fit insideEarth. Although it looks quite close, theMoon is actually about 240,000 miles(384,000 km) away. It takes a mannedspacecraft three days to travel from Earthto the Moon.THE SOLAR SYSTEM
Earth is just one of many objects that orbit the star we call theSun. The Sun’s “family” consists of eight planets, five dwarfplanets, hundreds of moons, millions of comets and asteroids,and lots of gas and dust. All these things together are called thesolar system. The four small planets closest to the Sun are madeof rock, while the four outer planets are a lot larger and mademostly of gases. Earth is the third planet out from the Sun andis just the right temperature to support life. The solar system isbig—the Voyager spacecraft took 12 years to reach Neptune,the outermost planet.
THE LOCAL GROUP
The Milky Way is one of the largest galaxies in a clusterof about 45 galaxies, known as the Local Group. Mostof these galaxies have no particular shape and are muchsmaller than the Milky Way. The two closest galaxies to theMilky Way are called the Large and the Small MagellanicClouds. They lie about 200,000 light-years away and areeasily visible with the naked eye from Earth’s southernhemisphere. The biggest galaxy in the Local Group is theAndromeda Galaxy—a great spiral galaxy, much like theMilky Way. It lies about 3 million light-years away, in the constellation of Andromeda.
THE UNIVERSE
The universe is everything that exists—all the stars, planets,  galaxies, and the space between  them. There are millions of  galaxy clusters in the universe:

in fact, wherever we look with 
telescopes, the sky is full of 
galaxies. And scientists estimate 
that there must be about 10 
thousand billion billion stars in 
the universe—more than the 
number of grains of sand on all 
the beaches on Earth.

THE MILKY WAY

The solar system is located in a large spiral-shaped galaxy called

the Milky Way and lies about 30,000 light-years from the center

of the galaxy. The Sun is just one of at least 100 billion stars in

this galaxy. The Milky Way is vast—it measures about 100,000

light-years across. That means that a spaceship traveling at the

speed of light (186,000 miles or 300,000 km per second) would

take 100,000 years to fly from one side of the galaxy to the

other. The Milky Way is so big that the stars in it are usually

a long way apart. The nearest star to our Sun is more than 4light-years away.

Early ideas

Compared with everything else around us, Earth seems incredibly large.

Ancient peoples believed it was the biggest and most important place in the

universe and that everything revolved around it. These ideas only began to

change very slowly after the introduction of the telescope in the early 1600s.

Curving surface

of Earth

Line of sight

d LAND AHOY! As the boat gets closer to the island, the

sailor sees the tops of the mountains first. Then, as the boat

moves over the curve, lower land comes into view.

Ancient peoples watched the Sun, Moon,

and stars very carefully. They saw that

all of them traveled from east to

west across the sky. Clearly, they

were all going around a stationary

Earth. For several thousand years,

almost everyone believed that the Earth

was at the center of the universe. The

main problem with this idea was that

it did not explain the movements of

some of the planets—sometimes Mars

or Jupiter appeared to stand still or

even move backward.

EARTH-CENTERED UNIVERSE

Earth

Sun

FLAT EARTH OR ROUND EARTH?

Stand on the seashore and look at the horizon. It

seems to be flat. For a long time, people thought

that Earth was flat and that if you went too far you

would fall off the edge. However, it was gradually

realized that Earth was round, like a giant ball.

Nature provided several clues:

■ The shadow that the Earth casts on the Moon during a lunar eclipse is curved, not straight.  

■ A sailor traveling due north or south sees stars appear and disappear over the horizon. On a flat Earth, he would always see the same stars.  

■ A ship sailing over the horizon should simply get smaller and smaller if Earth is flat. In fact, the hull disappears first and the top of the sails last.

Telescopes

Telescopes are instruments for looking

at things that are far away. Almost

everything we know about space has

been discovered by looking through

telescopes. Optical telescopes can

capture light from the deepest parts

of space, but are limited by the size

of their mirrors and lenses.

. THE YERKES OBSERVATORY was  

funded by business tycoon Charles T. Yerkes, who had  made his fortune developing  Chicago’s mass-transit system. 

REFRACTING TELESCOPE

The first telescopes were refracting telescopes, which used lenses to bend and focus light. The biggest  refracting telescope is at Yerkes Observatory in Wisconsin. Built in 1897, it is still used for looking at stars  and tracking their  movements through  space.

EVEN BIGGER TELESCOPES

Although reflecting telescopes can be built much

bigger than refracting telescopes, they too will have

problems if the mirror is more than 27 ft (8 m)

across. Astronomers solve this problem by

using a number of smaller mirrors that can

be fitted together to make one big mirror.

Each mirror section is controlled by a

computer that can adjust its position by

less than the width of a human hair.

Giant telescopes

The Hale telescope caused quite a stir when it was completed in 1948.

Equipped with a 16 ft (5 m) mirror, it was the largest and most

powerful telescope ever built. As technology has improved, telescopes

have been built with mirrors up to 33 ft (10 m) across. Even larger

telescopes are now planned, with mirrors of 100 ft (30 m) or more.

Keck Telescopes

■ Size of primary mirror 16 ft (5 m)

■ Location Palomar Mountain, California

■ Altitude 5,580 ft (1,700 m)

Even today, more than 60 years after

it was built, the Hale telescope is the

second-largest telescope using mirrors

made of a single piece of glass. Mirrors

much larger than this tend to sag  under their  own weight,  distorting  the image  received. 

Hale Telescope

■ Size of primary mirror 33 ft (10 m)

■ Location Mauna Kea, Hawaii

■ Altitude 13,600 ft (4,145 m)

Until 2009, the twin Keck telescopes

were the world’s largest optical

telescopes. The Keck II telescope

overcomes the distorting effects of

the atmosphere by using a mirror that

changes shape 2,000 times per second.

Size of primary mirror 26 ft (8 m)

■ Location North: Mauna Kea, Hawaii.

South: Cerro Pacho, Chile

■ Altitude North: 13,822 ft (4,213 m).

South: 8,930 ft (2,722 m)

Gemini telescopes

The twin Gemini telescopes are located on each side of  the equator. Between them,  they can see almost every part  of both the northern and southern skies. The two  telescopes are linked through a special high-speed  internet connection.

■ Size of primary mirror 27 ft (8.2 m)

■ Location Mount Paranal, Chile

■ Altitude 8,645 ft (2,635 m)

Infrared astronomy

We are all familiar with the colors of the rainbow—red, orange, yellow, green, blue, indigo, and violet. These colors are part of what is known as the visible spectrum. Beyond the red end of the spectrum Is infrared light, which we call heat. Although we cannot see infrared Light, we can detect it using special telescopes, which reveal things usually hidden by clouds of dust.

SPITZER SPACE TELESCOPE

Infrared light from space is almost

completely absorbed by Earth’s

atmosphere, so infrared telescopes are

placed on high mountains, on aircraft,

or on satellites. NASA’s Spitzer Space

Telescope is one of the most powerful

infrared observatories. Spitzer took

18 hours and over 11,000 exposures to

compose this image of the Andromeda

Galaxy (below).

American engineer Karl Jansky was the

first to discover radio waves coming from

space, using a homemade antenna in 1931.

Today, scientists use radio waves to learn

about all kinds of objects in space and

have even attempted to contact alien life.

Arecibo

The largest single radio telescope in the world is Arecibo, on the Caribbean island of Puerto  Rico. The telescope measures 1,000 ft  (305 m) across and its dish is built into a dip  in the hillside, with the radio receiver suspended  450 ft (137 m) above like a giant steel spider.  

Although Arecibo’s dish doesn’t move, its location near the equator means it can see a wide region of the sky. 

RADIO ASTRONOMY

Radio astronomy is the study of objects in space that produce radio waves. Radio waves are like waves of light but are beyond the visible end of the spectrum.  

Invisible radio waves are detected by radio telescopes and can then be converted into images for us to see.

Invisible rays

Ultraviolet (UV) light, X-rays, and gamma rays are types of electromagnetic radiation emitted by extremely hot objects. They are invisible and most are absorbed by Earth’s atmosphere, so the best way to view them is with telescopes  on high-flying balloons, rockets, or spacecraft.

Flying highAlthough only in the air for six days, this helium balloon, part of a project called Sunrise, helped astronomers to get a unique look in UV light at how the Sun’s  magnetic fields form. It lifted  a large solar telescope 23 miles ( 37 km) into the sky, high above  the obscuring effects of Earth’s atmosphere.

INTEGRAL

The INTEGRAL space observatory is equipped with highly sensitive detectors that can view objects in X-rays, gamma rays, and  visible light all at the same time. Sent into  space in 2002, it circles Earth every three  days on the lookout for explosive GRBs,  supernova explosions, and black holes.

SDO

The Solar Dynamics Observatory (SDO) studies the Sun at many  different wavelengths, particularly  those at the extreme end of UV.  Scientists use the data that it collects from its continuous observations to learn more about how solar activity affects life on Earth.

Jupiter calling EarthThe first radio signals from a distant planet were detected from Jupiter in 1955. Since 
then, all of the giant gas planets have been shown to produce radio waves. Radio 
signals can also be bounced off the rocky planets and asteroids.