Physics · Both
Space Physics
The solar system, gravity and orbits, life cycle of stars, and evidence for an expanding universe.
Gravity holds planets in orbit and shapes the life cycles of stars. Observations of galaxies support the idea that the universe is expanding.
Solar system and orbits
Planets orbit the Sun; moons orbit planets.
Gravity provides the centripetal force for orbits.
Weight is less on the Moon because gravitational field strength is smaller – mass stays the same.
Orbital speed and radius are related; closer orbits generally mean stronger gravity and higher speed for stable orbits.
Life cycle of stars
Stars form from clouds of gas and dust (nebulae).
**Average star** (like the Sun):
nebula → protostar → main sequence → red giant → white dwarf
**Massive star**:
nebula → protostar → main sequence → red supergiant → supernova → neutron star or black hole
Main sequence: hydrogen fusion in the core balances gravity.
Expanding universe
Light from distant galaxies is **red-shifted** (wavelength stretched).
More distant galaxies show greater red shift → moving away faster.
This supports the Big Bang model – the universe began from a hot, dense state and has been expanding.
Practice questions
Try these without looking at the answers first.
Q1.What force keeps Earth in orbit around the Sun?
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Gravity
Q2.What is the next stage after main sequence for the Sun?
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Red giant
Q3.What does red shift of galaxies suggest?
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Universe is expanding / galaxies moving away