In about 5 to 6 billion years, the Sun will have depleted the hydrogen fuel in its core and will begin to expand. At its largest, its surface (photosphere) will approximately reach the current orbit of the Earth. It will then lose its atmosphere completely; its outer layers forming a planetary nebula and the core a white dwarf. The evolution of the Sun into and through the red-giant phase has been extensively modelled, but it remains unclear whether the Earth will be engulfed by the Sun or will continue in orbit. The uncertainty arises in part because as the Sun burns hydrogen, it loses mass causing the Earth (and all planets) to orbit farther away. There are also significant uncertainties in calculating the orbits of the planets over the next 5 â?? 6½ billion years, so the fate of the Earth is not well understood. At its brightest, the red-giant Sun will be several thousand times more luminous than today but its surface will be at about half the temperature. In its Red Giant phase, the Sun will be so bright that any water on Earth will boil away into space, leaving our planet unable to support life.

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