The Scale of the Universe (Cambridge (CIE) IGCSE Physics)

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

• 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

Extended tier only

• One light year is equal to 9.5 × 10¹² km, or 9.5 × 10¹⁵ m