The Sun is our closest star. It has a diameter of 1,392,000 kilometers. It is thought to be 4.6 billion years old. The Sun is a medium size star in the Milky Way Galaxy. The temperature at its core is estimated to be over 15,000,000 degrees Celsius. In the Sun's core, hydrogen is being fused to form helium by a nuclear fusion process. The energy created from this process radiates out into space as heat and light. Because the Sun is so massive it exerts a powerful gravitational pull on everything in our Solar System. It is because of the Sun's gravitational pull that the Earth and the other planets orbit the Sun.