This mini-lesson walks you through the whole of AQA Topic 4.8 — Space Physics: our solar system, the life cycle of a star, why things orbit, and how red-shift tells us the universe is expanding.
Work through each screen, answer the questions as you go (most are reasoning, a couple are calculations) and collect ⭐ stars. Press Start when you're ready.
Our solar system is held together by the gravity of the Sun — which is itself a star. Orbiting it we find:
Watch out — the Sun IS a star. It only looks different from the night-time stars because it is so much closer. Every other star you can see is a distant sun, far too far away to have planets we can easily detect. Our solar system is just one tiny part of a much larger galaxy, the Milky Way.
A star begins as a nebula — a giant cloud of dust and gas. Gravity pulls this cloud together:
A main-sequence star (like the Sun) is stable for a long time because the outward pressure from fusion exactly balances the inward pull of gravity.
Picture it — a cosmic tug-of-war. A main-sequence star is two giant forces pulling against each other: gravity hauling everything inward, and the outward pressure from fusion pushing everything outward. While the two teams are evenly matched the star holds its size and stays stable. When the fuel starts to run out, fusion weakens, gravity "wins" the next tug, and the star begins to change.
Watch out — stars don't "burn" like a fire. A fire is a chemical reaction with oxygen. A star releases energy by nuclear fusion — joining light nuclei (hydrogen → helium) in its core. There is no oxygen and nothing is "on fire".
When a star runs low on hydrogen its future depends on its mass. The shared early stages — nebula → protostar → main-sequence star — then branch into two paths:
Sun-sized star: red giant → sheds its outer layers as a planetary nebula → leaves a white dwarf. Much more massive star: red supergiant → supernova → neutron star, or a black hole if the remnant is massive enough.
For each ending, tap the kind of star it belongs to.
Fusion in a star builds heavier elements from lighter ones — but only up to iron:
Watch out — fusion stops at iron. A common mistake is to think stars fuse all the way up to gold and uranium. They don't: ordinary fusion can only build elements up to iron. Everything heavier than iron — gold, silver, uranium, the iodine in your thyroid — was forged in the violence of a supernova and scattered across space long ago. You really are made of stardust.
Gravity provides the centripetal force — a force pointing toward the centre — that keeps a planet, moon or satellite in its orbit.
In a circular orbit the speed stays the same, but the velocity is always changing direction. Because velocity changes, the object is constantly accelerating — toward the centre.
Picture it — whirling a ball on a string. Swing a ball round your head on a string: the string tension pulls the ball into a circle, just as gravity pulls a planet into its orbit. Notice the string always pulls inward, never forward. And if you let go (cut the string), the ball flies off in a straight line along its velocity — exactly the direction it was heading. Remove gravity and a planet would do the same.
Watch out — constant speed still means accelerating. Students often say "the speed is constant, so there's no acceleration." But acceleration is any change in velocity, and velocity includes direction. The orbiting object's direction is changing every instant, so it is accelerating — toward the centre — even though its speed never changes.
For a stable circular orbit at a given radius there is only one possible speed. The link to remember:
If a satellite speeds up, it can move out to a larger orbit; if it slows down it falls to a smaller one. There are two kinds of satellite:
Watch out — closer means faster, not slower. It feels natural to think an inner orbit is "slower" because it is shorter, but the opposite is true: a smaller orbit radius needs a greater speed. Closer to the Sun, gravity is stronger, so the planet must travel faster to stay in a stable orbit. That is why Mercury (innermost) races round the Sun far faster than Neptune (outermost).
Tap an object, then tap the box it belongs in.
When we look at light from distant galaxies, the dark lines in its spectrum are shifted toward the red (longer-wavelength) end. This is called red-shift. The further the galaxy, the bigger the shift.
Red-shift means the galaxy is moving away from us. The further away a galaxy is, the bigger its red-shift and the faster it is receding.
Picture it — a passing siren, but for light. An ambulance siren drops in pitch as it speeds away: its sound waves get stretched to a lower frequency. Light from a receding galaxy is stretched the same way — to longer (redder) wavelengths. The faster the galaxy recedes, the more its light is stretched, just as a faster ambulance shows a bigger drop in pitch.
Because nearly all galaxies are red-shifted (and the more distant ones more so), the whole universe is expanding — every galaxy is moving away from every other.
This supports the Big Bang theory: the universe began from a very small, hot, dense point and has been expanding ever since.
Remember: red-shift is the key evidence that the universe is expanding, which is in turn the key evidence for the Big Bang.
Solar system: the Sun (a star) + 8 planets, dwarf planets, moons, asteroids & comets.
Star birth: nebula → protostar → main sequence (fusion balances gravity).
Sun-like death: red giant → planetary nebula → white dwarf.
Massive death: red supergiant → supernova → neutron star or black hole.
Elements: fusion up to iron; heavier elements made & scattered by supernovae.
Orbits: gravity = centripetal force; smaller radius → faster speed.
Red-shift: distant galaxies recede; the universe is expanding → Big Bang.
You've covered all of AQA 4.8 — the solar system, the life cycle of a star, orbital motion and red-shift. Press Finish to see your score.
You've worked through Space Physics for AQA GCSE Physics. 🎉
Your stars: 0 / 0
Next: test yourself in the Evaluate stage Confidence Quiz, then lock it in with Verify.