Lesson focus: Night-sky objects differ in light source, distance and apparent motion.
Distinguish starA huge ball of very hot gas that gives out its own light, like the Sun., planetA large object that travels around a star, like the Earth around the Sun. and moons using evidenceObservations or measurements used to show whether an idea is right or wrong. rather than apparent brightnessHow bright a star looks from Earth, which depends on how much light it gives out and how far away it is. alone.
Build on earlier learning
Forces at a distanceUse light paths as evidence alongside non-contact interactions in space.
You will be successful when you can…
- State stars emit light.
- State planets/moons reflect light.
- Distinguish apparent and intrinsic brightness.
- Use repeated positional evidence.
Words that carry the science
- star
- Luminous sphere of hot plasma producing energy and light.
- planet
- Large body orbiting a star and reflecting its light.
- apparent brightness
- Brightness observed from Earth.
- luminosity
- Total light power emitted by an object.
- constellation
- Apparent pattern of stars seen from Earth.
Trace light before classifying the object
01Luminous or reflecting
starA huge ball of very hot gas that gives out its own light, like the Sun. generate and emit light; planetA large object that travels around a star, like the Earth around the Sun. and moons are visible mainly by reflected sunlight.
02Brightness is not identity
A nearby reflective planet can appear brighter than a much more luminous but distant star. Atmosphere and viewing angle also matter.
Stars make their own light. Planets do not: we see them because they reflectionWhen light or sound bounces off a surface, like light from a mirror. light from the Sun. A planet can still look brighter than a star because it is much closer to us. Things that are further away look dimmer.
03Motion evidence
Planets slowly change position relative to background stars over repeated observations; a single snapshot is insufficient.
The night sky turns
Looking north from the UK, the stars seem to turn anticlockwise around the Pole Star (Polaris) about 15° every hour. It is the Earth that turns, once a day, from west to east.
The seven bright stars joined up are the Plough; the W shape is Cassiopeia. Lines are drawn for the shapes only.
Controls
Real star positions, drawn for the UK (latitude 52°N) on a spring evening. Polaris is above the Earth’s axis, so it hardly moves.
Classify a bright point
Object X is bright and shifts relative to constellationA pattern of stars that we see in the night sky, like the Plough. over several weeks.
- Brightness alone is inconclusive.
- Relative shift suggests Solar System motion.
- If spectrum/light is reflected sunlight, planet is supported.
- Conclude planet is better supported than star, with stated evidence.
How to make the reasoning visible: Use multiple observations and cautious language.
Observation chart
Treat all brightness scales as relative.
| Object | Light evidence | Position evidence | Classification |
|---|---|---|---|
| A | Produces stellar spectrum | Fixed pattern | Star |
| B | Reflected sunlight | Shifts over weeks | Planet |
| C | Reflected sunlight | Moves around planet | Moon |
| D | Brief streak | Rapid track | Meteor observation |
Why can B look brighter than A?
Why repeat observations?
Use the chart. How can you tell object B is a planet and not a star?
Check your understanding
Answer from memory, then check against the suggested answer.
What is star?
What is planet?
What is apparent brightness?
What is a constellation?
Does a planet make its own light?
Why is the Moon bright but not a star?
How can two equally luminous stars look different in brightness?
Why does the Sun look much brighter than other stars, even though some stars are more luminous?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A planet can look brighter in the night sky than a star. Answer the parts to explain why.
Where does a star’s light come from?
(1)Where does a planet’s light come from?
(1)How far away is the planet, compared with the star?
(1)How does distance change how bright something looks?
(1)Total for this question: 4 marks
Command word: Say how the light is made, then why it looks bright.