Our Solar System Notes
AQA GCSE Physics: Revision notes
Key facts
- The Solar System has one star (the Sun), eight planets, dwarf planets and natural satellites (moons); it is part of the Milky Way galaxy.
- The Sun formed from a nebula pulled together by gravity.
- In a star, fusion pushing outwards balances gravity pulling inwards.
- Gravity keeps planets, moons and satellites in orbit.
- In a circular orbit the speed is constant but the velocity changes (Higher Tier).
What makes up the Solar System
One star, eight planets, dwarf planets and natural satellites, all within the Milky Way galaxy.
Our Solar System is a small part of a much larger galaxy, the Milky Way, which contains billions of stars.
Dwarf planets orbit the Sun but have not cleared their orbital path. A natural satellite is a moon; an artificial satellite is human-made, for example for communications.
The Sun
- The one star
- Everything else orbits it
Planets
- Eight, orbiting the Sun
Dwarf planets
- Orbit the Sun
- Have not cleared their orbital path
Natural satellites
- Moons, orbiting planets
What is the Milky Way?
How the Sun formed
The Sun formed from a cloud of dust and gas pulled together by gravity, until fusion began.
A nebula is a huge cloud of dust and gas. Gravitational attraction pulled it together; as it contracted it heated up. At the start of the Sun’s life this heating began fusion reactions, which release enormous amounts of energy.
The Sun then reached equilibrium: the inward pull of gravity is balanced by the outward push of fusion energy, so the star stays a stable size.
- 1
Nebula
A huge cloud of dust and gas.
- 2
Gravity pulls matter together
The cloud contracts and heats up.
- 3
Fusion begins
Releasing enormous amounts of energy.
- 4
Equilibrium
Gravity inwards balances fusion energy outwards.
What stops a star from collapsing?
What keeps things in orbit
Gravity provides the force that keeps planets, moons and satellites moving in roughly circular orbits.
Without gravity an orbiting object would travel off in a straight line. Gravity constantly pulls it towards the centre of the orbit, changing its direction of travel.
All three types of orbiting object share the same physics: gravity supplies the force needed for a circular orbit.
| Planet | Moon | Artificial satellite | |
|---|---|---|---|
| Orbits | The Sun | A planet | A planet (usually Earth) |
| Natural or artificial? | Natural | Natural | Artificial (human-made) |
Planet
- Orbits:
- The Sun
- Natural or artificial?:
- Natural
Moon
- Orbits:
- A planet
- Natural or artificial?:
- Natural
Artificial satellite
- Orbits:
- A planet (usually Earth)
- Natural or artificial?:
- Artificial (human-made)
What would happen to the Moon if gravity suddenly stopped acting?
Constant speed, changing velocity
Gravity acts at right angles to the motion, so it changes direction but not speed.
This is Higher Tier. In a circular orbit, gravity acts towards the centre at all times. It does not speed the object up or slow it down, because it is at right angles to the direction of travel, so the speed stays constant.
It continuously changes the direction, and velocity is a vector, so the velocity is always changing.
If the speed changes, the radius must change too: a stable orbit at greater speed has a smaller radius.
A satellite orbits at constant speed in a circle. Its velocity is:
Try an exam question
Describe how the Sun was formed.
[4 marks]
- [1]From a nebula: a huge cloud of dust and gas.
- [1]Gravitational attraction pulled the matter together.
- [1]The cloud contracted and heated up.
- [1]Fusion reactions began, and the Sun reached equilibrium between gravity and the outward push of fusion energy.
That's the notes covered.
Carry on to the next subtopic.