How intermediate-mass stars evolve
Intermediate-mass stars, such as the Sun consume their hydrogen faster than low mass stars. When their hydrogen runs out, the core collapses. As the core contracts, the temperature increases and the outer layers begin to expand. The expanded layers are cooler and appear red called a red giant. In about 5 billion years the Sun will become a red giant and it will become so large that it's diameter will extend to the current orbit of Mars. Eventually, the layers will disappear into space, and the Sun will become a white dwarf.