Combined Science Physics P5: Space Physics

Cambridge IGCSE Combined Science 0653 Physics P5 scales from the Solar System to stars, galaxies and the Universe. It uses gravitational attraction, light-travel time and stellar mass to explain observations. Redshift is not an explicit P5 outcome in the 2025-2027 syllabus.

P5.1 The Solar System

The Solar System contains one star, the Sun, and eight planets in order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Minor planets orbit the Sun, including dwarf planets such as Pluto and asteroids concentrated in the asteroid belt. Moons orbit planets.

Do not classify every small body as a moon. The object it orbits matters. A moon orbits a planet, while an asteroid or dwarf planet in this treatment orbits the Sun.

P5.2 The Sun as a star

The Sun is the closest star to Earth and contains most of the Solar System's mass. Its gravitational attraction keeps planets in orbit. Orbiting objects are continually accelerating because their velocity direction changes, even when their speed is constant.

The Sun is a medium-sized star made mostly of hydrogen and helium. It emits much of its energy in infrared, visible and ultraviolet regions. Stable stars release energy through nuclear fusion of hydrogen into helium.

Gravity is not absent in orbit. It provides the inward force that changes the direction of motion. Without that force, a moving planet would not continue around the Sun.

Light-years and travel time

A light-year is the distance light travels through a vacuum in one year. It is a unit of distance, not time. Light takes time to cross significant distances, so observing a distant object shows it as it was when the light left.

For a supplied distance, use:

$$t = \frac{d}{v}$$