 As stars of at least 0 @.@ 4 M ☉ exhaust their supply of hydrogen at their core , they start to fuse hydrogen in a shell outside the helium core . Their outer layers expand and cool greatly as they form a red giant . In about 5 billion years , when the Sun enters the helium burning phase , it will expand to a maximum radius of roughly 1 astronomical unit ( 150 million kilometres ) , 250 times its present size , and lose 30 % of its current mass . 
