the Sun

	The Sun, captured with an Ultraviolet Imaging Telescope

Our Sun is by far the biggest object in the solar system: its diameter is 109 times the Earth's. Furthermore, nearly all of the mass in our Solar System is in the Sun.

Its strong gravitational pull holds all the other objects in the solar system in orbit.

The Sun is considered to be an average sort of a star, only 4.6 billion years old. Some of its material came from former stars.

How stars and planets are formed

Stars like the Sun form over long periods of time from massive clouds of dust and gas in space. Gravity pulls the dust and gas together. As the gas falls together, it gets hot. A star forms when it is hot enough for a nuclear reaction to start. This releases energy, and keeps the star hot. The outward pressure from the expanding hot gases is balanced by the force of the star's gravity. This happened about 5,000 million years ago. This is quite recent in the history of the Universe, which is currently believed to be 14,000 million years old.

Gravity pulls together smaller amounts of dust and gas that are further out. This forms planets in orbit around the star.

Nuclear fusion reactions in the Sun

The Sun is the source of nearly all the energy we receive. We now realise that its energy comes from nuclear fusion reactions. Smaller nuclei are joined together and form larger nuclei. In particular, hydrogen nuclei come together to make helium nuclei.

hydrogen nucleus + hydrogen nucleus → helium nuclei

Nuclear fusion releases enormous amounts of energy.

Once a star has run out of hydrogen fuel, it is nearing the end of it's life. However, it can generate some further energy by now using the helium produced earlier as fuel, joining together the helium nuclei to form heavier nuclei. When the helium fuel in the Sun eventually runs out, no more fusion can take place (though this does not apply to stars much heaveir than our Sun, in which heavier and heavier elements are produced. All the chemical elements in our solar system which are heavier than helium were made in stars that existed before our Sun.

[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science ]