When the Sun first became a main sequence star, it radiated only 70% of the current luminosity. The luminosity has increased in a nearly linear fashion to the present, increasing by 1% every 110 million years. In three billion years the Sun is expected to be 33% more luminous. The hydrogen fuel at the core will finally be limited in 4.8 billion years, when the Sun will be 67% more luminous than at present. Thereafter the Sun will continue to burn hydrogen in a shell surrounding its core, until the increase in luminosity reaches 121% of the present value. This marks the end of the Sun's main sequence lifetime, and thereafter it will be into a red giant.