This mini-lesson walks you through the core of CCEA Space Physics: the Solar System, how gravity holds objects in orbit, the life cycle of stars, and the Big Bang model with its evidence — 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 (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
The Solar System
What the Solar System contains
Our Solar System is held together by the Sun's gravity. CCEA expects you to recognise each kind of object:
the Sun — a star at the centre;
eight planets — Mercury, Venus, Earth, Mars (small, rocky), then Jupiter, Saturn, Uranus, Neptune (large gas planets);
dwarf planets (such as Pluto) — too small to have cleared their orbits;
moons — natural satellites that orbit planets;
asteroids — lumps of rock and metal, mostly in the belt between Mars and Jupiter;
comets — balls of ice and dust on highly elliptical orbits.
Planets follow near-circular orbits; comets swing in on long elliptical paths.Quick check
Name that object
?A body in the Solar System made mostly of ice and dust follows a very stretched, highly elliptical orbit around the Sun. What is it?
Gravity & orbits
Gravity provides the orbital force
An object moving in a circle is always changing direction, so it is always accelerating — which means a force must constantly pull it towards the centre. That inward force is the centripetal force.
For planets, moons, comets and satellites, gravity supplies that centripetal force, pulling them towards the body they orbit.
The velocity points along the orbit (a tangent); gravity pulls inward at right angles to it, bending the path into a circle.
Watch out: there is no outward force flinging the planet away. The only force is gravity pulling inward. Remove it and the planet would fly off in a straight line along the tangent.
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?
Orbital speed & radius
Closer orbits move faster
The strength of gravity falls with distance, so the speed needed to stay in a stable orbit depends on the radius:
objects in smaller (closer) orbits need a faster orbital speed and complete an orbit in less time;
objects in larger (distant) orbits move more slowly and take longer to go round.
That is why Mercury races round the Sun in 88 days while distant Neptune takes about 165 years, and why the Moon (far out) orbits Earth far more slowly than a low satellite skimming just above the atmosphere.
Satellites are objects that orbit a planet — natural ones (moons) or artificial ones we launch for observing the Earth, weather monitoring, astronomy and communications (plus navigation, GPS). Two common orbits for artificial satellites are coming up next.
Geostationary vs polar
Two kinds of satellite orbit
A geostationary satellite sits high over the equator; a polar satellite is low and passes over the poles (not to scale).
Geostationary: high above the equator (~36 000 km), orbiting once every 24 hours — so it stays fixed above the same point on Earth. Ideal for communications and TV (a dish can point at one spot in the sky).
Polar: a much lower orbit passing over the poles, going round in about 1½–2 hours. As Earth spins beneath it, it scans the whole surface in detail — ideal for weather and imaging.
Quick check
Why doesn't it fly off?
?The Moon travels in a near-circular orbit around the Earth at a steady speed. Which statement is correct?
Quick check
Pick the right orbit
?A TV company wants a satellite that stays fixed above one point on the equator so a home dish can point at it permanently. Which orbit do they need?
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?
Stars & nuclear fusion
The Sun is a star
A star is a huge ball of hot gas — mostly hydrogen and helium. Our Sun is an ordinary, middle-aged star.
A star's energy comes from nuclear fusion: in its incredibly hot, dense core, hydrogen nuclei fuse together to form helium, releasing enormous amounts of energy.
While a star is stable it is in balance: the inward pull of gravity is exactly matched by the outward push of radiation/thermal pressure from fusion.
A stable (main-sequence) star: gravity pulling in balances the outward pressure from fusion.Quick check
What keeps a star stable?
?During the main-sequence stage a star neither collapses nor expands. Why is it stable?
Life cycle of stars
How a star lives and dies
Every star is born in a nebula (a cloud of gas and dust), collapses into a protostar, then ignites fusion to join the main sequence. What happens after the main sequence depends on the star's mass:
After the main sequence the path forks by mass: Sun-like stars end as white → black dwarfs; massive stars explode as a supernova, leaving a neutron star or a black hole.
Key split: only massive stars go red supergiant → supernova → neutron star/black hole. A Sun-mass star just swells to a red giant, sheds its outer layers and is left as a slowly-cooling white dwarf (eventually a black dwarf).
Put it in order
Life cycle of a Sun-like star
Tap the stages in the correct order, starting from the very beginning.
Quick check
The end of a massive star
?A star far more massive than the Sun reaches the end of its life. Which sequence of events is correct?
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
Light from galaxies is red-shifted
When we split the light from a distant galaxy into a spectrum, the dark absorption lines are shifted towards the red (longer-wavelength) end compared with a lab source. This is red-shift.
It happens because the galaxy is moving away from us — the wavelengths are stretched. The key observation:
The more distant a galaxy, the greater its red-shift — so the faster it is receding.
The same absorption lines appear shifted towards the red end — evidence the galaxy is receding.
Misconception: red-shift is not just the galaxy speeding through space like a car. It is space itself stretching, which stretches the light's wavelength as it travels. More distant galaxies recede faster because there's more expanding space between us and them.
Quick check
Reading the red-shift
?Galaxy A shows a large red-shift; galaxy B shows a small red-shift. What can you conclude?
The Big Bang
An expanding universe
Red-shift is seen for galaxies in every direction, and the further away they are the faster they recede. The whole universe is expanding.
Run that expansion backwards in time and everything was once squeezed into an extremely small, hot, dense state. The Big Bang theory says the universe began from that state about 13.8 billion years ago (CCEA rounds this to 14 billion years) and has been expanding ever since.
Galaxies aren't flying into empty space — the space between them is expanding.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?
Cosmic microwave background
The afterglow of the Big Bang
The early universe was extremely hot and glowed with high-energy radiation. As space expanded over billions of years, that radiation was stretched to much longer wavelengths — it is now faint microwave radiation reaching us from every direction in the sky.
This is the cosmic microwave background radiation (CMBR).
The Big Bang is currently the only model that successfully predicts and explains the CMBR — which is why it is the accepted model.
Two pillars of evidence: (1) the red-shift of distant galaxies shows the universe is expanding; (2) the CMBR is the leftover, cooled-down radiation from when the universe was young and hot. Together they make the Big Bang the best-supported model.
Quick check
Why the CMBR matters
?How does the cosmic microwave background radiation support the Big Bang theory?
Sort it
Evidence or not?
Tap a statement, then tap the box: is it evidence that supports the Big Bang, or not?
✅ Supports the Big Bang
✋ Not evidence for it
Recap
The big ideas to know
Solar System: Sun + 8 planets, dwarf planets, moons, asteroids (Mars–Jupiter belt), comets (elliptical orbits)
Our place: Sun is a star in the Milky Way galaxy, one of many galaxies in the universe
Orbits: gravity provides the centripetal force; closer orbits are faster
Satellites: geostationary (high, over equator, 24 h) vs polar (low, scans whole Earth)
Life cycle: nebula → protostar → main sequence → (Sun-mass: red giant → white → black dwarf) or (massive: red supergiant → supernova → neutron star / black hole)