The Universe

Galaxies are made up of billions of stars
The Universe is made up of many different galaxies
The Sun is one of the billions of stars in a galaxy called 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 - these are called planetary systems

The Milky Way Galaxy

Our Place in Space, downloadable IGCSE & GCSE Physics revision notes

Our solar system is just one out of potentially billions in our galactic neighbourhood, the Milky Way. There are estimated to be more than 100 billion galaxies in the entire universe

Astronomical Distances

Astronomical distances, such as the distances between stars and galaxies, are so large that physicists use special units to measure them called astronomical units and light years

One astronomical unit (AU) is defined as:

The mean distance between the Earth and the Sun

An astronomical unit (1 AU) is equal to 150 million km, or 1.5 × 10¹¹ m

One light-year is defined as:

The distance travelled by light in one year

The speed of light is the universal speed limit, nothing can travel faster than the speed of light
But compared to large astronomical distances, light travels relatively slowly
The diameter of the Milky Way is approximately 100 000 light-years
- This means that light would take 100 000 years to travel across it

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

Worked example

Some common distances used by astronomers are:

1 astronomical unit (AU) is the distance between the Earth and the Sun
1 light second is the distance travelled by light in 1 second = 300 000 km
1 light year is the distance travelled by light in 1 year = 63 240 AU

(a)

The Sun is 500 light seconds away from Earth.

Use the equation

d i s t a n c e = s p e e d \times t i m e

to calculate this distance in million km.

(b)

The radius of Neptune’s orbit is 30 AU.

Calculate the radius of its orbit in million km.

(c)

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

Calculate this distance in million km.

Answer:

(a)

Step 1: Write down the known quantities:

Light travels 300 000 km each second, so speed = 300 000 km/s
It takes light 500 s to reach the Earth from the Sun, so time = 500 s

Step 2: Use the equation to calculate distance:

$d i s t a n c e = s p e e d \times t i m e$

distance = 300 000 km/s × 500 s

distance = 150 000 000 km

Step 3: State the final answer in million km

distance = 150 million km

(b)

Step 1: Write down the known quantities:

The radius of Neptune's orbit is 30 AU
1 AU is the distance between the Earth and the Sun, so, 1 AU = 150 million km

Step 2: Calculate the radius in million km:

radius = 30 × 150 million km

radius = 4500 million km

(c)

Step 1: Write down the known quantities:

The centre of our galaxy is 30 000 light years away
1 light year = 63 240 AU
1 AU = 150 million km

Step 2: Calculate the distance in million km:

distance = 30 000 × 63 240 × 150 million km

distance = 2.8 × 10¹¹ million km

Examiner Tip

You don't need to learn the conversion factors for metres into AU and light years as you will always be given them in the exam. However, you must get plenty of practice using them!

Remember that the light year is a unit of distance, however, it also represents the time it takes light to travel that distance. For example, the Milky Way galaxy is about 100 000 light years in diameter, which means it takes light 100 000 years to travel from one side to the other. Be aware of this when you see questions such as 'How long does light take to travel from X to Y' as this tends to trick many students!

The Universe (WJEC GCSE Physics: Combined Science)

Revision Note

The Universe

The Milky Way Galaxy

Astronomical Distances

Worked example

Examiner Tip

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Author: Katie M