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.
Work through each screen, answer the questions as you go (some are reasoning, some are calculations) and collect ⭐ stars. Press Start when you're ready.
Every body in space has a gravitational field around it. Its strength, g, is the gravitational force on each kilogram of mass:
Your mass (kg of matter) never changes, but your weight does — because g is different on every world. A more massive planet has a stronger field at its surface.
Our Solar System is held together by the gravity of the Sun — our nearest star. Orbiting it are:
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.
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.
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.
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.
Why? Closer in, gravity is stronger, so a bigger centripetal force is needed — which a faster speed provides. If a body in a stable orbit sped up, it would move to a larger radius (and vice versa).
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:
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 fuel runs low, gravity wins and the star collapses, triggering the next stage.
A star much more massive than the Sun lives fast and dies spectacularly:
Why bigger stars go further: the more massive the star, the stronger the inward gravity, so it burns hotter and 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". It is a tug-of-war: gravity inward vs fusion pressure outward. Each new stage happens when that balance tips.
Tap the stages in order, from birth to death, for a star about the same mass as our Sun.
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.
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.
Red-shift shows that almost all galaxies are moving apart — the Universe is expanding. There are two theories that account for this:
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.
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 whole of Edexcel Topic 7 — Astronomy. Press Finish to see your score.
You've worked through Astronomy for Edexcel GCSE Physics. 🎉
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Next: test yourself in the Evaluate stage Confidence Quiz, then lock it in with Verify.