The Sun is one of 100 billion stars in our Milky Way galaxy. Scientists estimate that the Sun is about 4.5 billion years old, or approximately halfway through its life cycle.
The Sun will continue to live on the hydrogen in its center for a few billion more years with its temperature and size staying about the same—so no surprises in our lifetimes!
When the Sun eats up its nuclear fuel, it will start to collapse. As it compacts, the Sun will generate enough heat to conserve the remaining hydrogen in its outer shell. This will cause the Sun to inflate and "puff up," turning the star into a red giant.
As the Sun runs out of hydrogen, it will start consuming helium which it transforms into carbon and other heavier elements. The heavy core will contract from gravity, resulting in a huge outward release of energy. What will remain is a dense, small core called a white dwarf surrounded by a huge gaseous envelope called a nebula.
Eventually, the white dwarf will completely cool, blow off its nebula, and fade into black lumps of carbon. The Sun will quietly become a black dwarf, producing no heat or light.