Waves and space · GCSE Physics
Space physics
The solar system, gravity and orbits, the life cycle of stars, red shift and the Big Bang — GCSE Physics rather than Combined Science on most specifications.
Gravity holds orbits. Bigger mass, bigger pull; bigger distance, much smaller pull. Receding galaxies show red shift that increases with distance — the evidence that the Universe is expanding.
The important bits
What you need to know
- 1
This topic is part of GCSE Physics on most specifications, not Combined Science. The solar system: star (the Sun), planets, moons, dwarf planets, asteroids and comets, all held by gravity.
- 2
Weight on other planets changes because g changes; mass does not. Orbital motion is a continual free-fall: gravity provides the centripetal force that changes the direction of velocity.
- 3
A stable orbit: if a satellite speeds up it moves to a higher orbit where gravity is weaker. Polar orbits and geostationary orbits are chosen for different jobs (mapping versus communications).
- 4
Stars form from collapsing clouds of gas and dust. Fusion of hydrogen to helium begins and a main-sequence star is in equilibrium: fusion outward, gravity inward.
- 5
A Sun-like star: main sequence → red giant → planetary nebula → white dwarf. A massive star: main sequence → red supergiant → supernova → neutron star or black hole. Fusion in massive stars makes elements up to iron; a supernova makes heavier elements and distributes them.
- 6
Red shift: light from receding galaxies has increased wavelength. More distant galaxies recede faster (Hubble’s pattern). This supports an expanding Universe and the Big Bang model: the Universe began from a very hot, dense state.
- 7
CMBR — cosmic microwave background radiation — is leftover radiation from the early Universe, now stretched into microwaves, and is further evidence for the Big Bang.
Go deeper
Orbits are gravity plus a sideways velocity
If you could throw a ball sideways fast enough, the ground would curve away as fast as the ball fell — that is an orbit, not a magic floating force. Gravity still acts, so the satellite is accelerating even if its speed is constant, because direction is changing. A higher orbit has a longer period and a lower orbital speed. Students often write that there is no gravity in space. There is; astronauts feel weightless because they are falling at the same rate as their spacecraft. Mass of the planet sets g at the surface. That is why you would weigh less on Mars but your mass, and your inertia, would be unchanged.
Go deeper
Red shift is a stretching of wavelength, not a colour filter
Absorption lines in a galaxy’s spectrum sit at longer wavelengths than the same lines in a lab. That is red shift. Hubble found that the further the galaxy, the greater the red shift, so the faster it recedes. Reverse that expansion and the Universe was once hotter and denser — the Big Bang. CMBR is the cooled glow of that early time. Alternative models struggled once CMBR was measured. In an exam, do not say galaxies are “red because they are hot” or that red shift proves the Earth is at the centre. Every distant galaxy recedes from every other as space expands. Nearby galaxies can be gravitationally bound and not follow the Hubble pattern; the question will usually mean distant ones.
See the idea in action
A line that is 656 nm in a laboratory is measured at 662 nm in a distant galaxy. The wavelength has increased, so the galaxy is receding — red shift. A more distant galaxy showing 670 nm for the same line is receding faster, which matches Hubble’s pattern and supports expansion. Separately, a star like the Sun will leave the main sequence when hydrogen in the core runs low, swell to a red giant, shed outer layers, and leave a white dwarf. Gold in the Earth’s crust cannot have been made in the Sun; it required a supernova (or similar high-energy event) before the solar system formed.
Exam technique
Turn knowledge into marks
Say whether the question is Combined or Physics: space is Physics on AQA-style routes. For stars, learn two pathways — Sun-like and massive — with the end products. For cosmology, link red shift to recession speed, distance, expansion and the Big Bang, then mention CMBR.
Common mistakes
Do not give these marks away
- 01
Saying there is no gravity in space, or that mass changes on the Moon.
- 02
Putting a Sun-like star ending as a supernova or black hole.
- 03
Explaining red shift as light getting tired, or as galaxies being red-hot.
What does the red shift of distant galaxies provide evidence for?
AThat the Sun will become a black hole
BThat the Universe is expanding, consistent with the Big Bang model
CThat light travels faster in space than in air
DThat planets orbit because there is no gravity
Show the answer
That the Universe is expanding, consistent with the Big Bang model. More distant galaxies show greater red shift and recede faster. Running that expansion backwards supports a hot, dense beginning, alongside evidence from CMBR.
Quick questions
If this is the bit you searched
Is space physics on Combined Science?
Usually not. On AQA-style routes it sits in GCSE Physics. Combined still includes gravity, fusion in the Sun as an energy resource context, and EM waves from space such as CMBR may appear in passing — check your specification.
What is the life cycle of a star like the Sun?
Nebula, protostar, main sequence, red giant, planetary nebula, white dwarf. Massive stars go red supergiant, supernova, then neutron star or black hole.