What makes up the solar system, and how is a star kept stable?
The structure of the solar system, the difference between stars, planets and other bodies, and the balance of forces in a stable star.
A focused answer to WJEC GCSE Physics topic 2.5 on stars and planets, covering the structure of the solar system, the difference between stars, planets, moons and other bodies, how a star forms, and the balance of gravity and radiation pressure that keeps a star stable.
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What this topic is asking
WJEC wants you to describe the solar system, distinguish stars from planets and other bodies, and explain the balance of forces that keeps a star stable. This is part of topic 2.5 Stars and planets in Unit 2 of WJEC GCSE Physics (3420).
The structure of the solar system
The Sun is one ordinary star among the billions in our galaxy, the Milky Way, which is itself one of billions of galaxies in the Universe. Gravity is the force that holds the whole system together: it keeps planets orbiting the Sun and moons orbiting planets. An orbit is a balance between the body's tendency to travel in a straight line and the gravitational pull bending its path into a curve, which is why a planet neither flies off into space nor falls into the Sun.
It helps to keep the scales straight: a moon orbits a planet, a planet orbits a star (our Sun), the Sun is one star in a galaxy, and galaxies make up the Universe. Distances in space are so large that light, the fastest thing there is, still takes minutes to reach us from the Sun and years from the nearest stars, which is why astronomers measure star distances in light years.
Stars versus planets
The balance of forces in a star
Try this
Q1. State what force keeps the planets in orbit around the Sun. [1 mark]
- Cue. Gravity (the Sun's gravitational attraction).
Q2. Name the process that produces a star's light and heat. [1 mark]
- Cue. Nuclear fusion (of hydrogen into helium) in the core.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC 20183 marksDescribe the difference between a star and a planet.Show worked answer →
A topic 2.5 Describe question. A star is a huge ball of hot gas that gives out its own light, produced by nuclear fusion in its core (1 mark). A planet does not produce its own light; it is seen because it reflects light from a star (1 mark) and it orbits a star held by gravity (1 mark). Markers reward fusion producing light in a star, reflection for a planet and the orbit. A common error is to say planets give out light.
WJEC 20214 marksExplain why a stable star does not collapse under its own gravity.Show worked answer →
A topic 2.5 Explain question. Gravity pulls the material of a star inwards (1 mark). Energy released by nuclear fusion in the core heats the gas, creating an outward radiation (thermal) pressure (1 mark). In a stable star these two effects are balanced (1 mark), so the star stays the same size, in equilibrium (1 mark). Markers reward gravity inwards, fusion pressure outwards and the balance. A common error is to ignore gravity and only mention fusion.
Related dot points
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Sources & how we know this
- WJEC GCSE Physics specification (3420) from 2016 — WJEC (2016)