KS3 · Physics

Stars, galaxies and the universe

One star lights up the whole sky. The others are faint specks you can barely see. Same kind of thing, so why such a huge difference?

Step outwards from home

From Earth to the whole universe

A map of where things are, not a scale drawing. Tap each band to take one more step away from Earth.

↑ Home: Earth

↓ Further out

Zone 1 of 6

The Sun and the Solar System

Start at home. The Sun is a star, and it's the nearest star to Earth, sitting at the centre of the Solar System. Its light takes eight minutes to get here, so we say Earth is eight light-minutes from the Sun.

  • The nearest star to Earth
  • At the centre of the Solar System
  • Its light takes 8 minutes to reach us

Each level sits inside the next: the Solar System is inside the Milky Way, and the Milky Way is one of billions of galaxies in the universe. The Sun only looks special because it's the star that happens to be close.

Watch out: The stars you see at night are not little lights inside the Solar System. Each one is a star in its own right, a very long way away.

Time or distance?

What does a light-year measure?

Does each unit measure an amount of time, or a distance?

Still to sort

Measures time (0)

How long something takes.

Measures distance (0)

How far it is from one place to another.

Where the line is: Put 'light-' in front of a time and it becomes a distance: how far light travels in that time.

5 of 5 still to sort.

You've just met light-minutes and light-years. Before we go on, decide what kind of thing each unit measures.

Watch out: Having the word 'year' in its name doesn't make a light-year a time.

Back to our own star

So what is the Sun, really?

On a clear day, the Sun floods the sky with light and warmth.

What makes the Sun so hot and bright? Pick the idea closest to what you think right now.
How sure are you?

Why the stars look so tiny

?

Reason it through

The Sun and the night-sky stars are all stars. So why is the Sun a huge, dazzling disc while the others are faint little dots?

Link 1 of 3

First link · your turn

They're the same kind of thing. What's the big difference between the Sun and the others?

2
Locked — reveal the link above first
3
Locked — reveal the link above first

Don't let this catch you out

You've just learned that further away means dimmer. Does that work backwards too?

Two stars sit side by side in the night sky. Star A looks bright and Star B looks dim. What can you say for sure about Star B?

Not all stars are the same

Put the star colours in order

Surface temperature of the star

1 · Coolest surface5 · Hottest surface

  1. Yellow star

  2. Blue star

  3. Red star

  4. White star

  5. Orange star

Watch out: In everyday life red often means 'hot'. For stars, red means the coolest surface.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

So are the stars you see at night. Start at home and step outwards, one level at a time, to see where everything fits.

What you need to know

  • The Sun is a star: an extremely hot sphere of gas that gives out its own light. Planets don't give out their own light.
  • A star's gas isn't burning, because there's no oxygen in space. Nuclear reactions inside stars make them so hot.
  • The night-sky stars look smaller and dimmer than the Sun because they're much further away. They aren't in the Solar System and don't orbit the Sun.
  • A light-year is a unit of distance: how far light travels in one year.
  • Stars group into galaxies of billions of stars. Ours is the Milky Way, and it's one of billions of galaxies in the universe.

The big picture

The Sun is a star: an extremely hot sphere of gas, heated by nuclear reactions, that gives out its own light. It only looks so different from the night-sky stars because it's so much closer to us. Stars live in galaxies of billions of stars. Ours is the Milky Way, one of billions of galaxies in the universe. Distances in space are measured in light-years: the distance light travels in one year.

Key points

1Earth is eight light-minutes from the Sun. The next nearest star, Proxima Centauri, is about four light-years away.
2Further away means a star fills less of our view (looks smaller) and less of its light reaches us (looks dimmer). Its colour looks the same.
3Dimmer doesn't always mean further: a star's real brightness matters too.
4Star colour shows surface temperature: red (coolest), orange, yellow, white, blue (hottest).
5The Milky Way is a spinning spiral disc. We're inside it, so on a dark night it looks like a band of light.
6Other galaxies look like single stars to the eye; a powerful telescope shows their structure.

Worked example

Problem

In the night sky, Sam spots two faint points of light that look exactly alike. An astronomer tells Sam that one is a star and the other is a whole galaxy. Sam says: "They look the same, so they must be the same kind of thing, and about the same distance away." Explain what Sam has got wrong.

⚠ Watch out

Thinking a light-year is a length of time. It's a distance: how far light travels in one year. When a star is 'about four light-years away', that tells you how far it is. It also means its light takes about four years to reach us.

🧠

Memory hook

A light-YEAR tells you how FAR. It's the ruler, not the clock: the year is only how long the light spends travelling.

✓

Check yourself

Cover the page: why can't the Sun be burning, and what really makes it so hot? Then say why a dim-looking star isn't always further away.

Flashcards

(16)
What is the Sun?
A star: an extremely hot sphere of gas that gives out its own light.
Stars or planets: which give out their own light?
Stars. Planets don't give out their own light.
Why can't a star's gas be burning?
Burning needs oxygen, and there's no oxygen in space.
What causes the high temperature of a star?
Nuclear reactions inside the star.
Why does a more distant star look smaller?
It takes up less of our field of view.
Why does a more distant star look dimmer?
Its light spreads out more as it travels, so less reaches our eyes.
Does distance change the colour a star looks?
No. Distance changes how big and bright it looks, not its colour.
Is a dimmer star always further away?
No. Its real brightness and its distance both decide how bright it looks.
What does a star's colour tell you?
Its surface temperature: red is coolest, then orange, yellow, white, and blue is hottest.
What is a light-year?
The distance light travels in one year, so it's a unit of distance, not time.
How many light-minutes is Earth from the Sun?
Eight: sunlight takes eight minutes to reach us.
Which star is nearest to Earth after the Sun, and how far away is it?
Proxima Centauri, about four light-years away.
Do the stars we see at night orbit the Sun?
No. They're stars of their own, far outside the Solar System.
What is a galaxy?
A group of billions of stars.
Why does the Milky Way look like a band of light?
It's our own galaxy, a spinning spiral disc, and we see it from inside.
What is the universe?
The whole of space and everything in it.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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How this lesson was checked. This KS3 Physicslesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 29 September 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.