Earth and space

The solar system

Eight planets orbit the Sun: Mercury, Venus, Earth, Mars (rocky inner four), then Jupiter, Saturn, Uranus, Neptune (gas/ice giants). Dwarf planets, asteroids and comets also orbit. Our Sun is one star among ~100 billion in the Milky Way galaxy.

Gravity and orbits

For a circular orbit, gravity provides the centripetal force pulling the body towards the centre. Closer orbits = faster speed and shorter period (Kepler).

Life cycle of a star (low- and high-mass)

1. Nebula — cloud of gas and dust collapses under gravity.
2. Protostar — heats up as gravitational potential energy converts to thermal.
3. Main sequence — hydrogen fuses to helium; gravitational collapse balances radiation pressure. Sun is here today, ~5 billion years in. 4a. Low/medium mass (≤ 8× solar): red giant → planetary nebula → white dwarf. 4b. High mass (> 8× solar): red supergiant → supernova → neutron star (or black hole if very massive).

Heavy elements (carbon, oxygen, iron) are forged inside stars. Elements heavier than iron form only in supernova explosions, which scatter them into space — these atoms are in your body.

Red shift

Light from distant galaxies has its spectral lines shifted towards the red (longer wavelength) end. Doppler interpretation: galaxies are moving away from us. The further away they are, the greater the red shift, so the faster they are receding (Hubble's law).

The Big Bang theory

Universe began ~13.8 billion years ago in a hot, dense state and has been expanding since. Evidence:

• Red shift of distant galaxies (uniform expansion).
• Cosmic microwave background CMB — uniform low-temperature radiation predicted by Big Bang and observed in 1964.

CCEA tip

For Big Bang evidence, name two distinct pieces — red shift AND CMB. One alone scores B1 of B2.