Our Solar System [Physics only]

What is in the solar system

• Within our solar system there is one star, the Sun, plus the eight planets and the dwarf planets (such as Pluto) that orbit around the Sun
• Natural satellites, the moons that orbit planets, are also part of the solar system, along with asteroids and comets
• The eight planets in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
• Our solar system is a small part of the Milky Way galaxy. A galaxy is a collection of billions of stars held together by gravity, and the universe contains billions of galaxies

How the Sun formed

• The Sun was formed from a cloud of dust and gas (a nebula) pulled together by gravitational attraction
• As the cloud collapses under gravity, the particles gain speed and the temperature and density of the core rise. This stage is a protostar
• When the core becomes hot and dense enough, nuclear fusion starts: hydrogen nuclei fuse to form helium, releasing huge amounts of energy. A star is born

Stable stars: the main sequence

• Fusion reactions lead to an equilibrium (balance) between the gravitational collapse of the star (gravity pulling inwards) and the expansion of the star due to the energy released by fusion (pushing outwards)
• While these two effects are balanced the star is stable. This is the main sequence stage, the longest part of a star's life. The Sun is a main sequence star and has been for about 4.6 billion years