<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>
<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
when does the sun turn into a red giant
In about 5 to 6 billion years
In about 5 to 6 billion years