<Ul> <Li> Mira (ο Ceti) </Li> <Li> χ Cygni </Li> <Li> α Herculis </Li> </Ul> <Li> Mira (ο Ceti) </Li> <P> In about 5 to 6 billion years, the Sun will have depleted the hydrogen fuel in its core . It will shrink, with the hydrogen outside the core able to compress enough for hydrogen there to fuse, and will begin to expand into a subgiant . Eventually, the pressure builds up so much that the core will begin to fuse helium, and will expand even more into a red giant . At its largest, its surface (photosphere) will approximately reach the current orbit of 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 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 Earth (and all planets) to orbit farther away . There are also significant uncertainties in calculating the orbits of the planets over the next 5--6.5 billion years, so the fate of 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 . </P>

When does the sun turn into a red giant