<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 and will begin to expand . 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>

How will the location of sun on the h-r diagram change when the sun becomes a red giant
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