This mini-lesson walks you through the whole of Edexcel Topic 7 — Astronomy (Physics only): gravity & orbits, our Solar System and the Milky Way, the life cycle of stars, and the evidence for the Big Bang — red-shift and the cosmic microwave background.
Make a guess: why doesn't the Moon fall to Earth?
Mission 1Why does the Moon never fall?
Mission 2What holds a star up, and for how long?
Mission 3Are we at the centre of the universe?
Work through each screen, answer the questions as you go (they test your reasoning, plus one ordering game) and collect ⭐ stars. Press Start when you're ready.
Gravity & weight in space
Weight depends on where you are
Every body in space has a gravitational field around it. Its strength, g, is the gravitational force on each kilogram of mass:
W = m × gweight (N) = mass (kg) × gravitational field strength (N/kg)
Your mass (kg of matter) never changes, but your weight does — because g is different on every world. The value of g at the surface depends on the body's mass and radius: the Moon has far less mass than Earth, so its field is much weaker.
Same mass, different weight — because g is smaller on the Moon.Quick check
On the Moon
?An astronaut travels from Earth (g ≈ 10 N/kg) to the Moon (g ≈ 1.6 N/kg). Which statement is correct?
Our Solar System & the Milky Way
What's out there
Our Solar System is held together by the gravity of the Sun — our nearest star. Orbiting it are:
Eight planets and their natural satellites (moons, such as our Moon);
Dwarf planets (e.g. Pluto);
Asteroids — rocky lumps, mostly between Mars and Jupiter;
Comets — icy bodies on long, very elongated orbits.
Order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune (not to scale).
Bigger picture: the Sun is just one of hundreds of billions of stars in our galaxy, the Milky Way. Ideas about the Solar System have changed over time — from an Earth-centred (geocentric) model to today's Sun-centred (heliocentric) one, as telescopes and evidence improved.
Orbits & circular motion
Gravity keeps things in orbit
Moons, planets, comets and artificial satellites all follow orbits. For a roughly circular orbit, gravity pulls the orbiting body towards the central body. This inward pull is the centripetal force.
It constantly changes the direction of motion (so the velocity changes), but keeps the speed the same — the body is forever "falling" towards the centre without ever reaching it.
Gravity (red) points inward; velocity (green) is along the path — at right angles to it.
Analogy: it's like a ball whirled on a string. The string's tension pulls the ball inward (centripetal force) and the ball keeps circling. Cut the string — remove gravity — and it would fly off in a straight line.
Misconception: orbiting bodies are not "weightless because there's no gravity". Gravity is exactly what holds them in orbit — they are continually falling around the central body.
Quick check
What gravity does in an orbit
?A satellite travels in a steady circular orbit around Earth. What does the force of gravity do to its motion?
Mission 1 check
Why does the Moon never fall?
?A satellite circles Earth with its engines switched off. Earth's gravity pulls on it all the time. Why does it not fall to the ground?
Stable orbits
Speed depends on radius
For a stable orbit of a given radius, there is only one orbital speed that works — fast enough that the body keeps missing the central body, but not so fast that it flies away.
The closer the orbit, the faster it must travel: a smaller radius → higher orbital speed; a larger radius → slower speed.
Mercury (close in) races round the Sun far faster than Neptune (far out).
Why? Closer in, gravity is stronger, so the centripetal force is bigger — the body must move faster to keep "missing" the central body instead of falling in. If a body in a stable orbit sped up, it would move to a larger radius (and vice versa).
Reasoning
Closer means…
?Two satellites are in stable circular orbits around Earth. Satellite A orbits at a smaller radius than satellite B. Which travels faster?
Mission 2 of 3
What holds a star up, and for how long?
Make a guess: what stops the Sun collapsing under its own gravity?
The life cycle of stars
How a star lives and dies
Stars form in a nebula — a giant cloud of dust and gas. Gravity pulls it together into a protostar; when it is hot and dense enough, hydrogen fusion begins and it becomes a stable main-sequence star.
What happens next depends on the star's mass. The two branches:
Left branch: stars like the Sun. Right branch: stars much more massive than the Sun.
Why it changes: a main-sequence star is a balance between gravity pulling inward and the outward pressure from fusion (thermal expansion/radiation) pushing outward. When the hydrogen in the core runs low, the balance is upset: the core contracts and heats up while the outer layers swell, and the star moves to the next stage.
Massive stars
The dramatic ending
A star much more massive than the Sun lives fast and dies spectacularly:
nebula → main-sequence star → red super giant;
it then explodes as a supernova;
the core left behind becomes a neutron star, or — if massive enough — a black hole.
Why bigger stars go further: the more massive the star, the stronger the inward gravity, so its core gets hotter and it fuses heavier elements before gravity finally wins in a supernova. The Sun is not massive enough to do any of this — it ends quietly as a white dwarf.
Misconception: a star is not constantly "burning like a fire" — its energy comes from nuclear fusion, not combustion. Its size is a tug-of-war: gravity inward vs fusion pressure outward. Each new stage happens when that balance tips.
Order it
Life cycle of a Sun-sized star
Tap the stages in order, from birth to death, for a star about the same mass as our Sun.
Quick check
End of a massive star
?A star far more massive than the Sun reaches the end of its life. What is the correct sequence for its final stages?
Mission 2 check
What holds a star up, and for how long?
?A Sun-like star has stayed stable for billions of years. Now the hydrogen in its core is running low. What happens next, and why?
Mission 3 of 3
Are we at the centre of the universe?
Make a guess: does red-shift mean we are at the centre?
Red-shift
Stretched light from galaxies
When a wave source moves relative to an observer, the observed frequency and wavelength change. Light from distant galaxies arrives with its wavelength stretched — shifted towards the red (longer-wavelength) end of the spectrum. This is red-shift.
The dark lines sit at the same pattern but are shifted towards the red end for the distant galaxy.
And there's a pattern: the more distant a galaxy, the greater its red-shift — so the faster it is moving away from us.
Analogy (only an analogy!): like the falling pitch of a passing siren as it speeds away. But red-shift isn't really about sound — it's space itself stretching, lengthening the light's wavelength on its journey to us.
Quick check
What greater red-shift means
?Galaxy X shows a greater red-shift than galaxy Y. What can you conclude?
The Big Bang & the CMB
How the Universe began
Red-shift shows that almost all distant galaxies are moving apart — the Universe is expanding. There are two theories that account for this:
Big Bang: the Universe began as a tiny, hot, dense region about 13.8 billion years ago and has been expanding and cooling ever since.
Steady State: the Universe has always existed and always looks the same, with new matter created as it expands.
The deciding evidence was the discovery of the cosmic microwave background (CMB) — faint microwave radiation reaching us from every direction. The Big Bang predicts it (the cooled-down "afterglow"); the Steady State theory does not explain it well.
Both theories can explain red-shift, but only the Big Bang explains the CMB. Because it has more supporting evidence, the Big Bang is now the accepted model for the origin of the Universe.
Observing the Universe has improved over time — and some telescopes are placed outside Earth's atmosphere (in space) to avoid the atmosphere absorbing and distorting the radiation they collect.
Quick check
Why the Big Bang wins
?Both the Big Bang and Steady State theories explain red-shift. Which piece of evidence makes the Big Bang the currently accepted model?
Mission 3 check
Are we at the centre of the universe?
?A pupil says: 'Almost every distant galaxy is red-shifted, and the furthest move away fastest. So we must be at the centre of the universe.' What is wrong with this?
Recap
The key ideas to know
Gravity & weight: W = m × g; g (and so weight) differs between worlds — mass does not.
Solar System: Sun + 8 planets & moons, dwarf planets, asteroids, comets — within the Milky Way.
Orbits: gravity provides the centripetal force; it changes direction (velocity), not speed.
Stable orbit: smaller radius → faster orbital speed.
Sun-sized star: nebula → main sequence → red giant → planetary nebula → white dwarf.
Massive star: nebula → main sequence → red super giant → supernova → neutron star / black hole.
Red-shift: distant galaxies are red-shifted; more distant = greater shift = Universe expanding.
Big Bang: supported by red-shift and the CMB — the accepted model.
You've covered the key ideas in Edexcel Topic 7 — Astronomy. Press Finish to see your score.
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Mini-lesson complete!
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You've worked through Astronomy for Edexcel GCSE Physics. 🎉