WJEC GCSE Physics 2.5 Stars and Planets Exam Questions

About WJEC GCSE Physics 2.5 Stars and Planets

Stars and planets is the first of the two space topics in Unit 2. You need to describe the Solar System, including the order, size and composition of the planets, dwarf planets, moons, asteroids and comets, and use distance units such as the astronomical unit and light year. You must describe the life cycles of stars of different masses, explain why a main sequence star is stable, and explain how fusion makes heavier elements that are returned to space. The formation of the Solar System from a collapsing cloud of gas and dust and the Hertzsprung-Russell diagram complete the topic. Examiners often ask for life cycle sequences and stability explanations.

What you need to know

• Solar System The Sun, the four inner rocky (terrestrial) planets, the asteroid belt, the four outer gas giants, dwarf planets such as Pluto, moons and comets in highly elliptical orbits. Order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
• Distances 1 astronomical unit (AU) is the mean distance from the Earth to the Sun. A light year is the distance light travels in one year, about 9.5 x 10^15 m, used for distances to stars and galaxies.
• Stable stars A main sequence star is stable because the inward force of gravity is balanced by the outward pressure from hot gas and radiation produced by fusion in the core.
• Life cycles All stars form from a nebula (cloud of gas and dust) that collapses into a protostar. A Sun-like star becomes a main sequence star, then a red giant, then a white dwarf. A much more massive star becomes a red supergiant, explodes as a supernova and leaves a neutron star or black hole.
• Fusion and elements Stars fuse hydrogen into helium, and later heavier elements. Elements heavier than iron are made in supernovae, which spread elements into space where they can form new stars and planets.
• Hertzsprung-Russell diagram Plots luminosity against surface temperature (hot on the left). Main sequence stars form a diagonal band; red giants are top right and white dwarfs bottom left.

Common mistakes in 2.5 Stars and Planets

• Saying stars burn hydrogen. Stars release energy by nuclear fusion of hydrogen nuclei, not by chemical burning.
• Writing that a light year is a unit of time. It is a unit of distance.
• Saying all stars become black holes. Only the most massive stars end as black holes; Sun-like stars end as white dwarfs.
• Describing stability as gravity being greater than pressure. A stable star has balanced forces: gravity inwards equals pressure outwards.

Try three 2.5 Stars and Planets exam-style questions