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.
Cat's Eye nebula in the constellation Draco is something that the remains of our Sun could be after 6 billion years from now.
