The Solar System

OCR

GCSE

The Solar System comprises the Sun, eight planets, dwarf planets, natural satellites, asteroids, and comets, all governed by gravitational forces. Orbital motion requires a centripetal force provided by gravity, where velocity changes continuously despite constant speed in circular orbits. Stellar evolution describes the life cycle of stars, determined by their initial mass, involving nuclear fusion processes that balance gravitational collapse. Cosmological evidence, specifically red shift, supports the Big Bang theory and the concept of an expanding universe.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

What You Need to Demonstrate

Key skills and knowledge for this topic

Award 1 mark for stating that the geocentric model places Earth at the centre, whereas the heliocentric model places the Sun at the centre
Credit responses that identify gravity as the centripetal force acting perpendicular to velocity to maintain circular orbits
Award 1 mark for linking the stability of a main sequence star to the equilibrium between gravitational collapse and outward radiation pressure from fusion
Candidates must correctly sequence the high-mass star life cycle: nebula → protostar → main sequence → red supergiant → supernova → neutron star/black hole

Example Examiner Feedback

Real feedback patterns examiners use when marking

"You correctly identified the stages of the star's life, but you must specify *why* the star expands into a Red Giant (fusion changes)"
"Be careful with vectors—remember that speed is scalar but velocity is vector, so an orbiting planet has changing velocity"
"Good recall of the heliocentric model; to improve, explain the observational evidence (like retrograde motion or Galileo's observations) that supported it"
"When discussing the Big Bang, ensure you mention Cosmic Microwave Background Radiation (CMBR) as key evidence alongside Red Shift"

Marking Points

Key points examiners look for in your answers

Award 1 mark for stating that the geocentric model places Earth at the centre, whereas the heliocentric model places the Sun at the centre
Credit responses that identify gravity as the centripetal force acting perpendicular to velocity to maintain circular orbits
Award 1 mark for linking the stability of a main sequence star to the equilibrium between gravitational collapse and outward radiation pressure from fusion
Candidates must correctly sequence the high-mass star life cycle: nebula → protostar → main sequence → red supergiant → supernova → neutron star/black hole

Examiner Tips

Expert advice for maximising your marks

💡When explaining orbital motion, explicitly link 'gravitational force' to 'centripetal force'—do not just say 'gravity holds it in place'
💡For 6-mark questions on stellar life cycles, structure your answer chronologically and clearly separate the 'low mass' and 'high mass' pathways to avoid mixing stages
💡Ensure you are comfortable using standard form calculations, as questions on orbital radii and light years often involve manipulating very large powers of ten

Common Mistakes

Pitfalls to avoid in your exam answers

Stating that planets in circular orbits have constant velocity; examiners require recognition that velocity changes due to changing direction, even if speed is constant
Confusing the two stellar evolution pathways, such as suggesting the Sun will become a black hole or undergo a supernova explosion
Describing the 'Big Bang' as an explosion in space rather than the expansion of space itself from a singularity

Study Guide Available

Comprehensive revision notes & examples

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Key Terminology

Essential terms to know

Structure and components of the Solar System

Orbital motion, gravity, and satellites

Stellar evolution (Life cycle of stars)

Red shift and the Big Bang theory

Likely Command Words

How questions on this topic are typically asked

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