The Sun.

98% of our total solar systems mass is taken up by the Sun, making it the largest object in the solar system. The Sun is around 70% Hydrogen, and 28% Helium. The last 2% is made up of "metals". This can slowly change over time as the Sun converts Hydrogen to Helium in its core (The inner 25% of its radius). The Sun produces power by Nuclear Fussion Reactions.

The Sun is approximately 4.5 billion years old, and has used up to half of the Hydrogen in its core. For the next 5 billions years it will continue to radiate, and it's luminosity will double in that time. Eventually, the Sun will run out of Hydrogen fuel and be forced into radical changes. This means that through commonplace by stellar standards, it will result in the total destruction of the Earth.