Our solar system
- Our solar system has one star (the Sun), eight planets, dwarf planets such as Pluto, and the natural satellites (moons) that orbit planets. Artificial satellites also orbit the Earth.
- Our solar system is a small part of the Milky Way galaxy.
- The Sun formed from a cloud of dust and gas (a nebula) that was pulled together by gravitational attraction.
The life of a star
- A star forms when a nebula is pulled together by gravity to form a protostar. As it gets hotter and denser, nuclear fusion of hydrogen begins and it becomes a main sequence star.
- A main sequence star is stable for billions of years, because the inward force of gravity is balanced by the outward pressure from the energy released by fusion.
- When the hydrogen runs low, a star about the size of the Sun swells into a red giant, then collapses into a white dwarf, which cools to a black dwarf.
- A star much bigger than the Sun becomes a red supergiant, then explodes as a supernova, leaving a neutron star or, for the largest stars, a black hole.
Making the elements
- Fusion in stars produces all the naturally occurring elements. Elements up to iron are made during the life of a star.
- Elements heavier than iron are made in supernovae.
- The explosion of a massive star (a supernova) distributes the elements throughout the universe, where they can form new stars and planets.
Orbits
- Gravity provides the force that keeps planets orbiting the Sun and moons and satellites orbiting planets, in circular orbits.
- (Higher) In a circular orbit, the object's speed stays the same but its direction keeps changing, so its velocity changes: it is accelerating towards the centre.
- (Higher) For a stable orbit, the radius must change if the speed changes: an object in a smaller orbit must move faster.
Key terms
- Solar system
- The Sun and everything that orbits it.
- Galaxy
- A huge group of billions of stars; ours is the Milky Way.
- Nebula
- A cloud of dust and gas in space where stars form.
- Protostar
- A hot, dense ball of gas that is becoming a star.
- Main sequence star
- A stable star fusing hydrogen into helium.
- Red giant
- A large, cooler star that forms when a Sun-sized star runs low on hydrogen.
- White dwarf
- The hot, small, dense remains of a Sun-sized star.
- Supernova
- The explosion of a massive star.
- Neutron star
- The very dense core left after some supernovae.
- Black hole
- A region where gravity is so strong that nothing, not even light, can escape.
Practise The solar system and the life cycle of stars: 10 questions