The Future of the Sun
When the hydrogen resources have been used up, fusion energy will run out of fuel. As the fusion energy will no longer inflate the Sun, gravity will take over and cause the core of the Sun to compress and heat up again. Once the fusion reaction has transformed hydrogen in the Sun's core to helium, the intense pressure will then initiate nuclear reactions of helium. Three helium atoms are there combined into one carbon atom, increasing the intensity of the reaction. As a consequence, the Sun will expand and becomes a red giant. It will become so huge that its halo will reach the orbit of Mars. The Sun will remain a red giant for only 700 million years. As helium runs out, the few exterior parts of the Sun will flow to surrounding space. This is called a planetary nebula. Later the interior of the Sun will contract to a white dwarf, about the size of Earth, and finally cool down to a black dwarf. The Sun will then extinguish.
 


Point A is where the Sun starts Hydrogen fusion
Point B is where about half of the supply of the hydrogen in the core has been used up. This is where the Sun is in its lifetime right now.
Point C is reached when there is no more hydrogen in the core and the fusion of hydrogen starts in the shell around the core. The radius of the Sun will swell to 40% larger than its present size and twice its present luminosity.
At point D, approximately one and a half billion years later, the surface of the Sun will be 3.3 times the size it is now and have a temperature of about 4300 degrees. The temperature on Earth will increase by 100 degrees, causing all the seas to evaporate and destroying life on Earth as we know it. Within another 250 million years, the Sun will grow 100 times larger than it is now and 500 times more luminous.
At point E the core temperature of the Sun will rise so high that in one bang, all the rest of the helium will fuse into carbon, causing one third of the solar envelope to be thrown out into space.