Figure 11-22 The Sun Today and as a Giant (a) In about 5 billion years, when the Sun expands to become a giant, its diameter will increase a hundred-fold from what it is now, while its core becomes more compact. Today, the Sun’s energy is produced in a hydrogen-fusing core whose diameter is about 200,000 km. When the Sun becomes a giant, it will draw its energy from a hydrogen-fusing shell that surrounds a compact helium-rich core. The helium core will have a diameter of only 30,000 km. The Sun’s diameter will be about 100 times larger, and it will be about 2000 times more luminous as a giant than it is today. (b) This image shows a red giant star in the cluster M103 in the constellation of Cassiopea.