Edexcel GCSE Physics Topic 7 Astronomy: a complete overview of the Solar System, orbits and gravity, the life cycle of stars, and red-shift and the Big Bang

What Topic 7 actually demands

Astronomy is a separate-physics Paper 1 topic that is mostly descriptive but rewards precise explanation: why gravity gives a changing velocity at constant speed, the ordered stages of a star's life by mass, and how red-shift and the cosmic microwave background distinguish the Big Bang from the Steady State theory.

This guide walks through all four dot points of the topic, then sets out the exam patterns Pearson repeats. Each dot point has a matching page with practice questions; this overview ties them together.

The Solar System

Our Solar System contains the Sun (our star), eight planets and their moons, plus dwarf planets, asteroids and comets. The planets in order from the Sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Models changed from the geocentric (Earth-centred) to the heliocentric (Sun-centred) model as better evidence (such as telescope observations) became available.

Orbits and gravity

Gravity provides a force towards the centre of an orbit, continually changing the direction of motion, so an orbiting object has a constant speed but a changing velocity. For a stable orbit, a faster speed means a smaller radius. Weight and gravitational field strength differ between bodies ($g$ is smaller on the Moon), but mass is unchanged.

The life cycle of stars

A star forms from a nebula under gravity until hydrogen fusion begins (main sequence), where gravity is balanced by the outward pressure from fusion. A Sun-sized star becomes a red giant then a white dwarf. A much more massive star becomes a red supergiant, explodes as a supernova, and leaves a neutron star or a black hole.

Red-shift and the Big Bang

Red-shift is the increase in observed wavelength of light from a receding source. Distant galaxies are red-shifted, and more distant ones recede faster, showing the Universe is expanding. The Big Bang theory is accepted because of red-shift and especially the cosmic microwave background (CMB), which the Steady State theory cannot explain.

How Topic 7 is examined

A typical Edexcel profile for Astronomy:

• Recall. The contents and order of the Solar System and the changing models.
• Explanations. Why gravity gives a changing velocity at constant speed, and the orbital speed-radius link.
• Sequencing. The life cycle of stars by mass.
• Evidence. Red-shift and the CMB as evidence for the expanding Universe and the Big Bang.

Check your knowledge

A mix of recall and explanation questions covering Topic 7. Attempt them under timed conditions, then check against the solutions.

1. Name the eight planets in order from the Sun. (2 marks)
2. State which model of the Solar System is accepted today. (1 mark)
3. Explain why a planet's velocity changes at constant speed in a circular orbit. (2 marks)
4. State what happens to an astronaut's mass and weight on the Moon compared with Earth. (2 marks)
5. State what a star forms from. (1 mark)
6. State the final stage of a star about the size of the Sun. (1 mark)
7. State what red-shift tells us about distant galaxies. (2 marks)
8. State the evidence that makes the Big Bang the accepted model. (1 mark)