The Scale of the Universe (Cambridge International IGCSE Science)

The Milky Way

The Universe

• The Universe is defined as

A large collection of billions of galaxies

• It is also the name given to the entirety of space

Galaxies

• A galaxy is defined as

A large collection of billions of stars

• Stars are large astronomical objects, such as the Sun

The Milky Way

• The Milky Way is one of many billions of galaxies making up the Universe
• The Sun is one of many billions of stars making up the Milky Way
• Other stars in the Milky Way galaxy are much further away from Earth than the Sun is
• Some of these stars also have planets which orbit them

Hierarchy of the Solar System

The Universe is a large collection of galaxies and a galaxy is a large collection of stars. The Sun is a star at the centre of our Solar System in the Milky Way galaxy

Astronomical distances

Spec point — P6.2.1

• Astronomical distances, such as the distances between stars and galaxies, are extremely large

• To describe these distances, astronomers use a special unit called the light year

• One light-year is defined as:

The distance travelled by light in one year

• The diameter of the Milky Way is approximately 100 000 light-years
• This means that light would take 100 000 years to travel from one side of the Milky Way to the other

Worked Example The centre of our galaxy is 30 000 light years away.

(a) How long does it take light to reach the Earth?

(b) How far away, in light years, is the edge of the Milky Way from Earth?

Answer:

Part (a)

• The centre of our galaxy is 30 000 light years away
• It takes light 30 000 years to reach the Earth from the centre of our galaxy

Part (b)

• The diameter of the Milky Way is 100 000 light years
• The radius of the Milky Way is 50 000 light years
• Therefore, the edge of the Milky Way closest to the Earth is 50 000 – 30 000 = 20 000 light years OR
• The edge of the Milky Way furthest from the Earth is 100 000 – 20 000 = 80 000 light years