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16.1 Formation of the Solar System

GCSE Physics · AQA · Chapter 16: Space

A free GCSE Physics lesson for AQA (8463) from OBA School: what is in the Solar System, how the Sun and planets formed, nuclear fusion and why the Sun is stable (physics only). Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Name the different kinds of object in the Solar System and describe their orbits
  • Describe how the Sun formed from a cloud of dust and gas pulled together by gravity
  • Explain what a protostar is and how it becomes a star when nuclear fusion starts
  • Explain how energy is released in the core of the Sun
  • Explain why a main-sequence star such as the Sun is stable

Key words

  • Solar System: The Sun and everything that orbits it: planets, dwarf planets, moons, asteroids and comets. It is a tiny part of the Milky Way galaxy.
  • Nebula: A huge cloud of dust and gas (mostly hydrogen) in space. Stars form from nebulae.
  • Protostar: A hot, dense ball of gas pulled together by gravity that is not yet hot enough for nuclear fusion: a star-to-be.
  • Nuclear fusion: Two light nuclei joining to make one heavier nucleus. In a star, hydrogen nuclei fuse to make helium nuclei and energy is released.
  • Main sequence star: A stable star that is fusing hydrogen into helium in its core. This is the longest stage of a star’s life.
  • Equilibrium: A state in which the forces on an object are balanced, so it does not change.
  • Natural satellite: A body that orbits a planet naturally, such as the Moon. Also called a moon.

What is in the Solar System?

The Solar System has one star, the Sun, at its centre. Everything else orbits the Sun, held by the Sun’s gravity. The Solar System is a very small part of a much bigger collection of stars called the Milky Way galaxy.

  • Planet; What it is Large body orbiting the Sun (8 of them); Its orbit Almost circular, all in nearly the same plane and the same direction
  • Dwarf planet; What it is Smaller body orbiting the Sun, e.g. Pluto, Ceres; Its orbit Around the Sun; often more stretched (elliptical) than a planet’s
  • Moon (natural satellite); What it is Body orbiting a planet; Its orbit Around its planet
  • Asteroid; What it is Lump of rock and metal, up to hundreds of km across; Its orbit Around the Sun, mostly in a belt between Mars and Jupiter
  • Comet; What it is Lump of ice and dust; Its orbit Very elliptical: far from the Sun for most of the orbit; grows a glowing tail only when near the Sun

A meteor (shooting star) is a small piece of rock that burns up as it enters the Earth’s atmosphere. The Earth orbits in a zone where the temperature allows liquid water on the surface – not too hot like Venus, not too cold like Mars.

Worked example: how long light takes to travel from the Sun to Mars

Distance from the Sun to Mars = 2.3 × 10¹¹ m; speed of light = 3.0 × 10⁸ m/s
time = distance ÷ speed = 2.3 × 10¹¹ ÷ 3.0 × 10⁸ = 767 s
in minutes: 767 ÷ 60 = 12.8 minutes

Answer: about 770 s, or about 13 minutes

How the Sun and planets formed

About 4.6 billion years ago a nebula – a vast cloud of dust and gas, mostly hydrogen – began to collapse. Gravitational attraction between the particles pulled them together. As the cloud shrank it began to spin and flattened into a disc.

  • As the particles fall inwards they speed up, so energy is transferred from the gravitational potential energy store of the cloud to the thermal energy store of the gas. The centre gets denser and hotter, forming a protostar.
  • In the inner disc, heat from the young Sun drove away gas and ice, leaving rock and metal: the rocky planets formed there.
  • Further out it was cold enough for ice and gas to remain, so the giant planets grew there.
  • Lumps of matter too small to become stars were pulled into orbit and became planets, moons, asteroids and comets.

Key fact: Because the planets formed from one spinning disc, they all orbit the Sun in the same direction and in almost the same plane.

From protostar to star: nuclear fusion

As the protostar keeps collapsing, its particles move faster and collide more often, so its temperature rises. When the core reaches about 10 million °C, hydrogen nuclei fuse together to form helium nuclei. Fusion releases a huge amount of energy, so the protostar gets hotter and brighter and starts to shine: a star is born.

Fusion in the core: hydrogen nuclei → helium nuclei + energy (released mainly as electromagnetic radiation)

Nuclei are positively charged and repel each other. Only at enormous temperatures and pressures do they move fast enough to get close enough to fuse.

Exam tip: The Sun does not burn. Burning is a chemical reaction that needs oxygen. The Sun’s energy comes from nuclear fusion, which changes the nuclei themselves. Do not confuse fusion (joining light nuclei) with fission (splitting heavy nuclei).

Why the Sun is stable

A star that is fusing hydrogen into helium in its core is a main-sequence star. This is the longest, steadiest stage of a star’s life. The Sun has been on the main sequence for about 4.6 billion years and has enough hydrogen for roughly another 5 billion years.

  • Gravity acts inwards, trying to make the star collapse.
  • The energy released by fusion in the core heats the gas and produces radiation, which push outwards, trying to make the star expand.
  • In a main-sequence star these forces are balanced – the star is in equilibrium – so it stays the same size and gives out energy at a steady rate.

Exam tip: For full marks, name both forces, give their directions (inwards / outwards) and say that they are balanced.

Clues in the rocks of the Earth

The Earth contains heavy elements such as gold and uranium. These elements can only be made in the explosion of a massive star (a supernova – see section 16.2). So the cloud that formed the Solar System must have contained debris from earlier stars that exploded.

Uranium-238 is radioactive with a half-life of about 4.5 billion years – about the age of the Earth. So roughly half of the uranium-238 the Earth started with is still here. Heavy elements with much shorter half-lives were also made, but they decayed away long ago.

Worked example: how much uranium-238 is left?

A rock contained 6.0 mg of uranium-238 when it formed 4.5 billion years ago.
4.5 billion years = 1 half-life, so the mass has halved once.
mass left = 6.0 ÷ 2 = 3.0 mg

Answer: 3.0 mg of uranium-238 remains

Common mistakes

  • Saying the Sun “burns” hydrogen or gas. The energy comes from nuclear fusion, not combustion.
  • Mixing up fusion and fission: stars join light nuclei (hydrogen → helium).
  • Saying “the forces in a star cancel out” without naming them. Give gravity inwards and outward pressure from fusion energy.
  • Calling a comet a planet or a meteor. Comets are icy bodies on very elliptical orbits around the Sun; meteors burn up in our atmosphere.

Revision questions

Try these, then check your answers below each one.

Question 1. What pulled the cloud of dust and gas together to form the Sun? (Choose from: gravitational attraction, magnetic attraction, nuclear fusion, the pressure of radiation.)

Answer: gravitational attraction

Working: Gravitational attraction between the particles pulled the cloud (nebula) together. Fusion only started later, once the protostar was hot enough.

Question 2. What name is given to the hot, dense ball of gas that forms before nuclear fusion starts – a “star-to-be”?

Answer: protostar

Working: A protostar: gas and dust pulled together by gravity that is heating up but is not yet hot enough for fusion.

Question 3. Which process releases the energy in the core of the Sun? (Choose from: hydrogen nuclei fusing to form helium nuclei, helium nuclei splitting to form hydrogen nuclei, hydrogen burning in oxygen, uranium nuclei splitting (fission).)

Answer: hydrogen nuclei fusing to form helium nuclei

Working: Nuclear fusion: hydrogen nuclei join to form helium nuclei, releasing energy. It is not burning (a chemical reaction) and not fission.

Related lessons

Continue in the full course

This lesson is section 16.1 (AGCPH1601) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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