The Sun

Our Sun is a star. Its diameter is 1,400,000 km (840,000 miles) which is more than 100 times the diameter of the Earth. Solar mass is more than 300,000 times that of Earth. The Sun is a very hot gaseous body composed of nearly 75% hydrogen, 25% helium, less than 1% oxygen and all the other elements constituting less than 1%. The temperature at the Sun's core is 15 million degrees Celsius (27 million degrees Fahrenheit), and the pressure there is 340 billion times Earth's air pressure at sea level. The source of energy in the Sun is the fusion of hydrogen nuclei (protons) into helium nuclei. In this process a small amount of mass is lost and transformed into energy. This nuclear reaction can only take place in the very hot (15,000,000 C) and dense centre of the Sun. The Sun appears to have been active for 4.6 billion years and has enough fuel to go on for another five billion years or so. The Sun loses half a million tons every second in this destruction of mass in exchange for energy but should maintain its present energy output for another 5,000 million years. The Sun also produces a strong magnetic field and streams of charged particles, both extending far beyond the planets.

The Sun is called a main-sequence star but eventually the hydrogen in the centre will all have been converted into helium. The balance between the force of gravity pulling all the Sun's mass towards its centre, and the force due to the energy in the Sun which pushes matter outwards, will then be upset. The centre will contract and become even hotter while the outer part will expand and become cooler. The Sun will then be brighter, cooler and bigger -- a red giant star. Ultimately all sources of energy production will come to an end and the Sun will collapse to become a very small hot object called a white dwarf.