IGCSE Physics Astrophysics Notes
Astrophysics covers orbits, the life cycle of stars and evidence for the Big Bang. Learn the key ideas and the role of gravity throughout.
Key concepts
- Orbits
- Gravity provides the centripetal force that keeps planets, moons and satellites in orbit.
- Life cycle of a star
- Nebula → protostar → main sequence → red giant → white dwarf (or supernova → neutron star/black hole for massive stars).
- Red shift & Big Bang
- Light from distant galaxies is red-shifted, showing the universe is expanding — evidence for the Big Bang.
- The closer a planet is to the Sun, the faster it orbits.
- Stars form when gravity pulls together gas and dust in a nebula.
- The greater the red shift, the faster and further away a galaxy is.
Key formulas
Orbital speed
v = (2 × π × r) ÷ T
r = orbit radius, T = orbital period.
Worked example
Explain why a comet speeds up as it approaches the Sun.
- 1Gravity is stronger closer to the Sun.
- 2The gravitational force does work on the comet, transferring energy to its kinetic store.
- 3Its speed therefore increases as it gets closer.
- 4It slows again as it moves away.
Answer: Stronger gravity nearer the Sun increases the comet's kinetic energy and speed.
Common mistakes
✗ Saying there is no gravity in space.
✓ Gravity acts everywhere; it provides the centripetal force for orbits.
✗ Confusing the life cycles of small and massive stars.
✓ Only massive stars end in supernovae, neutron stars or black holes.
✗ Thinking red shift means galaxies move through space away from us only.
✓ Red shift shows space itself is expanding, carrying galaxies apart.
Quick check
Try these, then reveal the answers.
What force keeps planets in orbit?Reveal answer
Gravity (acting as the centripetal force).
What does red shift tell us about the universe?Reveal answer
It is expanding.
What is the first stage in a star's life cycle?Reveal answer
A nebula (cloud of gas and dust).
Exam tips
- Always link orbits to gravity providing the centripetal force.
- Learn both star life cycles (low-mass and high-mass) separately.
- Use red shift as evidence for both expansion and the Big Bang.
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Frequently asked questions
What is red shift?+
The stretching of light from distant galaxies to longer (redder) wavelengths as the universe expands.
How does a star stay stable on the main sequence?+
The outward pressure from fusion balances the inward pull of gravity.
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