Learning outcomes
- Define the light-year as a distance.
- Relate the Sun to the Milky Way galaxy.
- Recall the approximate diameter of the Milky Way.
- Describe galaxies as containing many billions of stars.
- Recognise the hierarchy of astronomical structures.
7.1 Why ordinary units become inconvenient
Kilometres and metres are valid for all distances, but the numbers become extremely large when describing stars and galaxies. Astronomers therefore use the light-year, the distance travelled by light in vacuum in one year.
A light-year is a unit of distance, not time. Its name contains ‘year’ because the definition uses the distance light covers during that time interval.
7.2 Calculating one light-year
Using the speed of light 3.0 × 10^8 m/s and approximately 3.15 × 10^7 s in a year gives one light-year ≈ 9.46 × 10^15 m. This calculation combines ideas from waves, speed and standard form.
The syllabus does not require memorising the metre value, but deriving it helps establish scale. Light from a star 10 light-years away has travelled for 10 years before reaching Earth.
7.3 The Milky Way
The Sun is one star in the Milky Way galaxy. The Milky Way contains many billions of stars, along with gas, dust and planetary systems. Other stars in the Milky Way are much farther from Earth than the Sun is.
The Milky Way is approximately 100 000 light-years in diameter. Therefore, even light takes about 100 000 years to cross the galaxy from one side to the other.
7.4 Galaxies and the Universe
A galaxy is a huge gravitationally bound collection containing many billions of stars. The Universe contains many billions of galaxies. The Milky Way is therefore one member of an unimaginably larger population.
A common hierarchy is: planet → planetary system → galaxy → Universe. The Solar System is not a galaxy, and the Milky Way is not the entire Universe.
7.5 Looking into the past
Because light has a finite speed, observing distant objects means observing earlier events. A galaxy one million light-years away is seen as it was one million years ago.
This principle allows astronomy to investigate the history of the Universe. It also means that distances and times must be interpreted carefully: the light travel time tells us how old the observed image is, not necessarily the object’s present state.
Worked examples
Distance represented by 4.0 light-years
4.0 × 9.46 × 10^15 m ≈ 3.8 × 10^16 m.
Crossing the Milky Way
At 100 000 light-years across, light takes approximately 100 000 years to cross the Milky Way.
Practical focus
Investigation or modelling activity
Construct a logarithmic scale diagram showing Earth, the Solar System, nearby stars, the Milky Way and the observable Universe. A linear scale cannot show all levels meaningfully. Explain why models that place each level at equal spacing are conceptual rather than proportional.
Examination guidance
- Define a light-year using distance travelled by light in vacuum in one year.
- Do not write that a light-year is a time.
- State that the Sun is in the Milky Way.
- Recall the Milky Way diameter as approximately 100 000 light-years.
- Do not confuse the Solar System with the Universe.
Check your understanding
- What is a light-year?
- Which galaxy contains the Sun?
- Approximately how wide is the Milky Way?
- Why do observations of distant galaxies show the past?
Answers
- The distance travelled by light in vacuum in one year.
- The Milky Way.
- Approximately 100 000 light-years.
- Their light takes a long time to reach Earth, so it was emitted long ago.
