The Sun is a star, which glows due to the thermonuclear or fusion reactions taking place at its core. These reactions constantly produce heat and light. The Sun is made up of 90% hydrogen atoms, about 9% helium atoms (helium is a very light gas used to lift balloons), and 1% other elements such as oxygen and nitrogen. In its centre, at a temperature of about 20 million degrees celsius, hydrogen atoms combine to form helium atoms. Four hydrogen atoms (H) are required to make one helium (He) atom. This is called a fusion reaction. In this process, a burst of energy (heat and light) is given out. The Sun’s hydrogen will last for thousands of millions of years from now, at least as long as it has already existed. As long as fusion reaction continues, the Sun will keep shining.