What our solar system contains

1. Our solar system contains one star, the Sun, at its centre.
2. Eight planets orbit the Sun, in order out from it: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
3. Dwarf planets, such as Pluto, also orbit the Sun.
4. Natural satellites, such as moons, orbit the planets.
5. The Sun's gravitational attraction holds the planets and dwarf planets in orbit around it.
6. In the same way, a planet's gravitational attraction holds its moons in orbit around that planet.
7. A natural satellite is a moon that formed naturally, whereas an artificial satellite is a human-made object placed into orbit.

Our solar system is only a small part of the Milky Way

1. Our solar system sits inside the Milky Way galaxy.
2. A galaxy is a huge collection of stars held together by gravitational attraction.
3. The Milky Way contains the Sun and billions of other stars, many of which have their own planets.
4. The solar system is only a tiny part of the Milky Way, so the two are not the same thing.
5. The solar system is the Sun plus everything that orbits it, while the galaxy is a vast group of many stars, one of which is the Sun.

How the Sun formed from a nebula

1. The Sun began as a nebula, a cloud of dust and gas in space.
2. Gravitational attraction pulled the dust and gas together.
3. As more material was pulled in, the cloud collapsed and became denser.
4. Squeezing the material together made the centre much hotter.
5. The centre eventually became hot and dense enough for nuclear fusion to begin.
6. In fusion, hydrogen nuclei join to form helium, releasing energy.
7. Once fusion was releasing energy steadily, the Sun became a stable star, also called a main sequence star.

Why a star stays stable

1. In a stable star, two effects act in opposite directions.
2. Gravitational attraction pulls the star's material inwards, tending to make it collapse.
3. The energy released by fusion pushes the star's material outwards, tending to make it expand.
4. While these two effects are balanced, the star stays the same size and is stable; this balance is called equilibrium.
5. Neither effect stops: fusion keeps releasing energy and gravity keeps pulling inwards, and the star is stable only because they cancel out.