<P> In the long term, the greatest changes in the Solar System will come from changes in the Sun itself as it ages . As the Sun burns through its supply of hydrogen fuel, it gets hotter and burns the remaining fuel even faster . As a result, the Sun is growing brighter at a rate of ten percent every 1.1 billion years . In one billion years' time, as the Sun's radiation output increases, its circumstellar habitable zone will move outwards, making the Earth's surface too hot for liquid water to exist there naturally . At this point, all life on land will become extinct . Evaporation of water, a potent greenhouse gas, from the oceans' surface could accelerate temperature increase, potentially ending all life on Earth even sooner . During this time, it is possible that as Mars's surface temperature gradually rises, carbon dioxide and water currently frozen under the surface regolith will release into the atmosphere, creating a greenhouse effect that will heat the planet until it achieves conditions parallel to Earth today, providing a potential future abode for life . By 3.5 billion years from now, Earth's surface conditions will be similar to those of Venus today . </P> <P> Around 5.4 billion years from now, the core of the Sun will become hot enough to trigger hydrogen fusion in its surrounding shell . This will cause the outer layers of the star to expand greatly, and the star will enter a phase of its life in which it is called a red giant . Within 7.5 billion years, the Sun will have expanded to a radius of 1.2 AU--256 times its current size . At the tip of the red giant branch, as a result of the vastly increased surface area, the Sun's surface will be much cooler (about 2600 K) than now and its luminosity much higher--up to 2,700 current solar luminosities . For part of its red giant life, the Sun will have a strong stellar wind that will carry away around 33% of its mass . During these times, it is possible that Saturn's moon Titan could achieve surface temperatures necessary to support life . </P> <P> As the Sun expands, it will swallow the planets Mercury and Venus . Earth's fate is less clear; although the Sun will envelop Earth's current orbit, the star's loss of mass (and thus weaker gravity) will cause the planets' orbits to move farther out . If it were only for this, Venus and Earth would probably escape incineration, but a 2008 study suggests that Earth will likely be swallowed up as a result of tidal interactions with the Sun's weakly bound outer envelope . </P> <P> Gradually, the hydrogen burning in the shell around the solar core will increase the mass of the core until it reaches about 45% of the present solar mass . At this point the density and temperature will become so high that the fusion of helium into carbon will begin, leading to a helium flash; the Sun will shrink from around 250 to 11 times its present (main - sequence) radius . Consequently, its luminosity will decrease from around 3,000 to 54 times its current level, and its surface temperature will increase to about 4770 K . The Sun will become a horizontal giant, burning helium in its core in a stable fashion much like it burns hydrogen today . The helium - fusing stage will last only 100 million years . Eventually, it will have to again resort to the reserves of hydrogen and helium in its outer layers and will expand a second time, turning into what is known as an asymptotic giant . Here the luminosity of the Sun will increase again, reaching about 2,090 present luminosities, and it will cool to about 3500 K . This phase lasts about 30 million years, after which, over the course of a further 100,000 years, the Sun's remaining outer layers will fall away, ejecting a vast stream of matter into space and forming a halo known (misleadingly) as a planetary nebula . The ejected material will contain the helium and carbon produced by the Sun's nuclear reactions, continuing the enrichment of the interstellar medium with heavy elements for future generations of stars . </P>

Where does the process of solar energy formation begins