The Sun is the dominant object in the solar system because it is the largest object. It is positioned at the centre of the solar system and its gravitational pull holds all the planets in orbit. The Sun is an average-sized star about 4.5 billion years old. Unlike planets, stars produce their own light and heat by 'burning' fuels like hydrogen and helium in a process known as nuclear fusion. Stars have a limited life and the Sun is no exception - it is about half way through its lifespan of about 10 billion years.

The Sun is one of over 100 billion stars that make up the galaxy called the Milky Way. The Milky Way galaxy is spiral in shape, and the Sun is positioned in the plane of the galaxy about half-way out from the centre. The Milky Way is about 100 000 light years in diameter and 15 000 light years thick. You can see parts of the Milky Way as a band or cloud of stars that stretches across the night sky. Within the Milky Way, the Sun is moving at 210 km/s, and takes 225 million years to complete one revolution of the galaxy's central mass of stars.

Solar system

Milky Way (edge on)


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Milky Way (from top)

Figure 3.1 Position of the Sun in the Milky Way galaxy.

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