Within our solar system there is one star, the Sun, plus the eight planets and the dwarf planets that orbit around the Sun. Natural satellites, the moons that orbit planets, are also part of the solar system. • Our solar system is a small part of the Milky Way galaxy. • The Sun was formed from a cloud of dust and gas (nebula) pulled together by gravitational attraction. • Students should be able to explain: how, at the start of a star's life cycle, the dust and gas drawn together by gravity causes fusion reactions • that fusion reactions lead to an equilibrium between the gravitational collapse of a star and the expansion of a star due to fusion energy.