Figure 19-4: Red Giants (a) The present-day Sun produces energy in a hydrogen-fusing core about 100,000 km in diameter. Some 7.6 billion years from now, when the Sun becomes a red giant, its energy source will be a shell only about 30,000 km in diameter within which hydrogen fusion will take place at a furious rate. The Sun’s luminosity will be about 2000 times greater than today, and the increased luminosity will make the Sun’s outer layers expand to approximately 100 times their present size. (b) The cluster M103 contains a red giant star. M103 is 14 ly across and lies around 8000 ly from Earth in the constellation Cassiopeia.

(NOAO/Science Source)