In colour photographs, these giants appear yellow or red. The star now has a radius 65 times the Sun, a brightness 1,000 times the Sun, and a surface temperature of about 4,000 degrees. But although the star is very large, most of the mass of the star is contained inside a very small central sphere. While our five solar mass star was on the main sequence, heat was carried outwards from central the core by massive currents of hot plasma moving up, cooling and moving down again. This process is known as convection, and is familiar to anyone who has boiled water in a saucepan.
Convection is very important for stellar evolution because it mixes up different layers of the star.
If the region where nuclear reactions take place is convective, it increases the amount of fuel available. If the surface layers are convective, processed material can be dredged up from the deep interior to become visible at the surface. As a red giant, our star now looks quite different. Although the core is hot, not much energy is produced there, so the convection currents switch off. The outer layers of the giant star are now very cool and quite opaque to radiation.