P8 Space physics — Section Overview

Space physics is a Physics-only topic (not required for Combined Science). It covers the structure of the universe, the life cycles of stars, and our understanding of the origin and ultimate fate of the cosmos.

What this section covers

Sub-topic	Key ideas
P8.1 Our solar system	Sun, eight planets, dwarf planets, moons, asteroids, comets
P8.2 Life cycle of a star	Nebula → main sequence → red giant/supergiant → white dwarf/neutron star/black hole
P8.3 Orbital motion	Gravity as centripetal force; orbital speed vs radius
P8.4 Red-shift	Light from distant galaxies redshifted; further = faster recession
P8.5 Origin of the universe	Big Bang theory; cosmic microwave background (CMB)

Our place in the universe

The Solar System is held together by the Sun's gravity. Planets orbit in ellipses (approximately circular). Beyond the Solar System lies the Milky Way galaxy, and beyond that hundreds of billions of other galaxies. The universe is vast and almost entirely empty space.

Stellar life cycle (briefly)

Stars form in nebulae (clouds of gas and dust) under gravity. In the main sequence phase, hydrogen fusion in the core balances gravity. When hydrogen runs out, the star expands to a red giant/supergiant. The endpoint depends on mass:

• Small stars → white dwarf (slowly cool)
• Massive stars → supernova → neutron star or black hole

Expanding universe

Red-shift of light from galaxies shows they are moving away from us. The further the galaxy, the greater the red-shift (Hubble's Law). This is evidence that the universe is expanding. Projecting backwards leads to the Big Bang model: the universe began about 13.8 billion years ago from an extremely hot, dense state.

Exam focus

• Know the star life cycle path clearly — small star route vs massive star route.
• Red-shift: wavelength increases (light appears redder) when source moves away.
• CMB: uniform microwave radiation from all directions — strong evidence for the Big Bang.