366Science Alive for VELS Level 5

Introducing the universehe universe is so big we can barely imagine its size. The outer edges of the

universe are probably further away than we can see, even with the most

powerful telescopes. The universe includes everything that

we know, and things we are yet to learn about.

T

Many scientists believe that, in the beginning,

everything in the universe was squashed into a space

much smaller than a pinhead. They believe that the

small space burst apart in a Big Bang about 13 billion

years ago. After the Big Bang, everything started to

move apart. Since then, the universe has been getting

bigger and bigger.

Astronomers are scientists who study the universe.

They believe that light takes 12–15 billion years to reach

the ‘edge’ of the universe after leaving Earth. The

‘edge’ is the part of the universe that is currently visible.

But scientists don’t actually know what exists past this,

or if this really is the edge of the universe. Scientists are

also studying the movement of objects in the universe,

in the hope of discovering the shape of the universe.

Astronomers believe that the universe could be one of these shapes. At the moment, scientists favour the flat and saddle-shaped models.

Light-years apartOne of the units that scientists use to measure

distances in the universe is called the light-year. A light-year is the distance that light travels in one

year. That’s a long way considering that light travels

300 000 kilometres in just one second.

In one second, light travels 300 000 kilometres.

In one minute, light travels (300 000 × 60)

kilometres.

In one hour, light travels (300 000 × 60 × 60)

kilometres.

In one day, light travels (300 000 × 60 × 60 × 24)

kilometres.

In one year, light travels (300 000 × 60 × 60 × 24

× 365) kilometres.

Can you work out how far that is?

 

 

 

 

36716. The universe

✓ le

arn

ing I CAN:

describe what is meant by a

light-year

understand the need for large

units, like light-years

define, in simple terms, what is

meant by the universe.

Looking back in timeBesides our Sun, the closest stars to

the Earth are the three stars of

Alpha Centauri. They are about

4.3 light-years away. When we look at

these stars, we see the light that the

stars give off. But the light we see has

taken a long time to reach us. So,

what you see is how the stars looked

nearly four and a half years ago!

The Earth’s place in the universeAbout 10 billion years ago, the

galaxy we live in was formed. At

this time, about 100 billion other

galaxies formed as well.

Four and a half billion years ago

our Sun formed. It is one of about

200 billion stars in our galaxy.

Over the next billion years, our

planets formed into what we call a

solar system.

The Earth is one planet in our

solar system. Our solar system is one

of 200 billion solar systems in our

galaxy. And our galaxy is one of

100 billion galaxies. The Earth is the

one planet, in this huge universe,

that has life. Or are there others?

Note: A billion is a thousand

million. It is easier to say a billion

than write it out fully. If you were

to write it out fully, a billion would

be 1 000 000 000.

REMEMBER

1. How old do astronomers

believe the universe is?

2. How far can light travel in

one second?

3. Explain what is meant by a

light-year.

4. How close to the Earth, in

terms of light-years, is:

(a) the closest group of

stars (after the Sun)?

(b) the brightest star (after

the Sun)?

5. How long does it take for

light from Alpha Centauri to

reach us?

THINK

6. Explain why looking at stars

is like looking back in time.

7. Why is the light-year used

instead of kilometres to

measure distances in space?

8. Why do you believe that

you cannot see stars (apart

from our Sun) during the

day?

DESIGN AND CREATE

9. Draw a time line showing

events from the creation of

the universe up until the

creation of the Earth.

Are close stars

brighter?The brightness of a star

does not necessarily tell

us how far away a star

is. The closest star to

our solar system,

Proxima Centauri,

was not discovered

until modern

telescopes were

invented. It is so dim

that it cannot be

viewed with the

naked eye or even a

basic telescope.

The brightest star

in the sky is Sirius.

It is almost twice

the distance of

Proxima

Centauri, at

8.6 light-years

from Earth.

 

 

 

 

368

Science Alive for VELS Level 5

Galaxiesn very dark places, away from city lights, a hazy white band can be seen across the sky. Greek legend explains that the hazy band of light was formed from milk

spilled by the baby Hercules. The hazy band is actually part of the Milky Way Galaxy that we live in. The word galaxy comes from the Greek word gala, meaning milk.

I

What’s a galaxy?A galaxy is made up of a huge number of stars and dust, held together by gravity. A hundred years ago, astronomers believed that the Milky Way Galaxy was the only galaxy in the universe. Thanks to developments in optical telescopes and radio telescopes, astonomers have now detected about 100 billion other galaxies. With the help of the Hubble

Space Telescope that orbits our Earth, astronomers have been able to locate galaxies that are more than one billion light-years away!

An artist’s impression of the Milky Way

Galactic shapesHow can we see the Milky Way Galaxy if we are in the Milky Way Galaxy? The answer lies in its shape.

The Milky Way is shaped into a spiral, with ‘arms’ coming from its centre. The Earth is located on one side of the galaxy, on a spiral arm called the Orion arm. When looking at the Milky Way in the night sky, you are actually seeing the stars in the central part of the spiral.

American astronomer Edwin Hubble recognised that galaxies could be grouped according to their shapes. Galaxies like ours are called spiral galaxies. Elliptical and irregular galaxies are two other types.

Direction

of rotation

Globular

clusterPossible black hole

in centre

Spiral arms

Areas of glowing pink, blue

and green gas are nebulae

where new stars form

Approximate location

of our solar system on

the Orion arm of the

Milky Way

 

 

 

36916. The universe

Spiral galaxies

Spiral galaxies

rotate. They have a

bright bulging middle

with two or more

curved arms of stars

spiralling out from

the centre. The middle

parts of spiral galaxies

spin faster than the edges.

They contain stars as well as

dust and gas. The older red

stars are found closer to the centre

and the younger blue stars are located on the outer

arms of the spiral. The Andromeda Galaxy is a spiral

galaxy. At 2.2 million light-years away, it is the most

distant object visible from Earth with the naked eye.

REMEMBER

1. (a) Name three different types of galaxies.(b) Give an example of each type of galaxy.

2. Which galaxies are the closest to our own galaxy?3. What is the most distant object visible from Earth

with the naked eye?4. What is the Orion arm?

THINK

5. How is it possible that we can see the Milky Way in the night sky, even though we are in the Milky Way Galaxy?

INVESTIGATE

6. Find out where the band of stars called the Milky Way will be in the sky tonight.

7. How big is the Milky Way Galaxy?

IMAGINE

8. What would we see in the night sky if our solar system was in the central bulge of the Milky Way Galaxy?

✓ le

arn

ing I CAN:

describe what is meant by a galaxyunderstand that Earth is part of the Milky Way Galaxylist three different types of galaxiesgive an example of each type of galaxy.

Barred spiral galaxies

These are a type of spiral galaxy. However, the

central disk is replaced by a bar-shaped

middle. In barred spiral galaxies,

arms spiral out from either

end of the bar.

Elliptical galaxies

Elliptical galaxies are

oval or egg-shaped

galaxies. They contain

masses of old red stars

with little gas or dust.

Unlike spiral galaxies,

the stars in elliptical

galaxies move around

in every direction.

Elliptical galaxies, such

as M87, have grown to

an enormous size by

pulling in other

galaxies.

Irregular galaxies

These have no definite shape. Irregular galaxies tend

to have very hot, new stars mixed in with lots of dust

and gas. The Magellanic

Clouds are two small,

irregular galaxies that

look like two fuzzy

clouds near the

constellation called

the Southern Cross.

They are the closest

galaxies to our own

Milky Way Galaxy.

The gravitational

pull between all

three galaxies is so

strong that eventually

they will all become part of

the Milky Way Galaxy.