The Solar System & Lifecycles of StarsEdexcel GCSE Physics: Revision notes

Our Solar System consists of:

• The Sun — our star, at the centre
• Eight planets and their natural satellites (moons)
• Dwarf planets
• Asteroids and comets

The eight planets, in order of increasing distance from the Sun, are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

Ideas about the structure of the Solar System have changed over time — from early geocentric models (Earth at the centre, with the Sun and planets orbiting it) to the modern heliocentric model (the Sun at the centre, with the planets, including Earth, orbiting it), which is supported by observational evidence.

Moons, planets, comets and artificial satellites all move in orbits held by gravitational attraction.

• For a body in a circular orbit, gravity provides a centripetal force directed towards the centre of the orbit.
• This force continuously changes the direction of the body's velocity, but not its speed — so velocity changes (as it is a vector) even though speed stays constant.
• Weight and the value of gravitational field strength, $g$, differ between the surface of the Earth and the surface of other bodies in space (such as the Moon), because $g$ depends on the mass and radius of the body — an object's weight changes accordingly, even though its mass stays the same.
• For a stable orbit, if the orbital speed of a body changes, the radius of its orbit must also change to keep the orbit stable (a faster orbital speed requires a larger radius, and vice versa).

Stars of similar mass to the Sun evolve through these stages:

1. Nebula — a cloud of dust and gas pulled together by gravity
2. Star (main sequence) — nuclear fusion of hydrogen into helium begins, releasing energy; the star is stable for billions of years
3. Red giant — once hydrogen fuel runs low, the star expands and cools, glowing red
4. White dwarf — the outer layers are shed and the core collapses into a small, dense, hot remnant that gradually cools

This life cycle is controlled by a balance between two opposing effects:

• Gravity pulling matter inwards
• Thermal expansion (from the energy released by fusion) pushing matter outwards

While these two effects are balanced, the star remains stable on the main sequence. When fusion can no longer balance gravity, the star changes stage.

Stars with a mass much larger than the Sun follow a different path:

1. Nebula
2. Star (main sequence)
3. Red supergiant — expands to an even larger size than a red giant
4. Supernova — a massive explosion as the core collapses rapidly
5. After the supernova, the remnant becomes either a:

• Neutron star, or
• Black hole (if the remaining mass is large enough)

Massive stars have shorter lifetimes than stars like the Sun because they fuse fuel much faster, and they end in a violent explosion rather than gently shedding their layers.

Methods of observing the Universe have changed considerably over time:

• Early astronomy relied on naked-eye observation, then optical telescopes on Earth's surface.
• Modern astronomy uses telescopes that detect a wide range of electromagnetic radiation (radio, infrared, ultraviolet, X-ray, gamma).
• Some telescopes are placed outside the Earth's atmosphere (in space) because the atmosphere absorbs or distorts many wavelengths of radiation (such as X-rays, ultraviolet and infrared) before they reach the ground, and also causes distortion ('twinkling') of visible light.