The solar system and the life cycle of a star
The Sun is one of billions of stars in the Milky Way. Every star forms from a cloud of dust and gas, spends most of its life fusing hydrogen, and then ends in a way that depends on its mass.
Part 1 of 3: Learn it
In short
- Our solar system is one star (the Sun), eight planets, dwarf planets such as Pluto, moons and smaller bodies.
- A star is stable on the main sequence while outward pressure from fusion balances gravity pulling in.
- Sun-sized stars end as white dwarfs; much more massive stars explode as supernovae.
Where this is in your specification
Spec points: AQA 4.8.1.1 and 4.8.1.2 (Physics only), Edexcel 1PH0 Topic 7, OCR J249 P8.2
| Board | Topic: Space physics |
|---|---|
| AQA 8463 | 4.8 (8463) |
| Edexcel 1PH0 | Topic 7 (1PH0) |
| OCR J249 | P8.2 (J249) |
| Eduqas C420 | 10.1, 10.2 |
| Cambridge IGCSE 0625 | 6.1, 6.2 |
| National 5 C857 75 | Space: space exploration, cosmology |
The solar system
Eight planets travel round the Sun, and moons travel round planets. The solar system is part of the Milky Way galaxy. The Sun formed from a cloud of dust and gas (a nebula) pulled together by gravity.
Birth of a star
- Gravity pulls dust and gas in a nebula together.
- As it collapses it gets hotter and forms a protostar.
- When it is hot and dense enough, hydrogen nuclei fuse into helium, releasing energy: a star is born.
The main sequence
For most of its life, a star is in equilibrium. Gravity pulls inwards, and the pressure from the energy released by fusion pushes outwards. Our Sun is about halfway through this stage.
How stars end
| Star about the size of the Sun | Star much more massive than the Sun |
|---|---|
| red giant | red supergiant |
| white dwarf | supernova (an enormous explosion) |
| black dwarf (once it has cooled) | neutron star, or a black hole if the mass is great enough |
Fusion in stars made all the naturally occurring elements up to iron. Elements heavier than iron are made in supernovae, which also spread the elements through the universe, where they can form new stars and planets.
What is a protostar?
Show the answer
A collapsing, heating cloud of dust and gas that has not yet started fusion.
Part 2 of 3: See it worked
Worked examples
Example 1
Light travels at 3 × 10⁸ m/s and takes about 500 s to reach Earth from the Sun. Estimate the distance from the Sun to the Earth.
- distance = speed × time
- 3 × 10⁸ × 500 = 1500 × 10⁸
- = 1.5 × 10¹¹ m
Answer: About 1.5 × 10¹¹ m.
Example 2
Explain why a main sequence star neither collapses nor expands.
- Gravity pulls the star's material inwards
- Energy released by fusion creates pressure that pushes outwards
- The two forces are balanced
Answer: The inward pull of gravity is balanced by the outward pressure from fusion, so the star is in equilibrium.
Common mistakes
- Saying the Sun will become a black hole or a supernova. It is not massive enough.
- Calling the energy source of a star burning. It is nuclear fusion.
- Putting the white dwarf stage before the red giant.
- Saying supernovae only destroy material. They also make and scatter heavy elements.
Name the stage after the red giant for a star like the Sun.
Show the answer
White dwarf.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
What is a protostar?
A collapsing, heating cloud of dust and gas that has not yet started fusion.
Name the stage after the red giant for a star like the Sun.
White dwarf.
Where are elements heavier than iron made?
In supernovae.
Jobs that use this
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Diese Lernzettel sind auf Englisch, weil sie britischen Prüfungslehrplänen folgen.
Full lessons and marked practice for this course are coming soon to Brainlag Learn. See courses