From http://www.space.com/scienceastronomy/astronomy/goldilocks_zone_020528-2.html Part 2 of 2
Kevin Zahnle, an astrobiologist at NASA's Ames Research Center, said our Sun has gotten significantly brighter during its roughly 4.6 billion-year life. It emits 30 to 40 percent more radiation than when Earth was born.
Within the next 5 billion years or so, the aging Sun will have swollen so much that it envelops and vaporizes Earth. In just a billion years, the Sun could be 11 percent brighter than now, turning the planet into an inhospitable greenhouse.
Part 1 of the article is at http://www.space.com/scienceastronomy/astronomy/goldilocks_zone_020528-1.html