Here's an explaination. The best I could do.An evolved star that has greatly expanded in size and undergone a change in its surface temperature so that it appears red. A star becomes a red giant at a phase in the course of stellar evolution where the hydrogen fuel for nuclear fusion in the central core has been exhausted. In the internal adjustment that follows, the core collapses until sufficient gravitational energy is released to cause hydrogen burning to restart, but in a shell around the now inert core. The energy generated by shell burning of hydrogen causes the great expansion of the star's outer layers. As the gas expands, it cools. Regardless of the star's original spectral type, its surface temperature drops until it reaches 4,000 K. When the Sun becomes a red giant, it will expand until its diameter is roughly the diameter of the Earth's orbit. Though the light emitted per unit surface area of a star decreases sharply with temperature, a red giant compensates for this effect by the enormous increase in surface area. Thus, red giant stars are relatively very luminous. All the bright red stars visible to the naked eye are giants or supergiants, such as Aldebaran or Betelgeuse. The spectra of red giants show different characteristics according to whether the stellar atmosphere is rich in carbon or oxygen. If oxygen is dominant, carbon monoxide (CO) and metallic oxides, such as titanium oxide (TiO) are evident. If carbon predominates, carbon compounds such as C2, CH and CN are formed; such giants are called carbon stars. The different compositions observed in red giants are presumed to be the result of processes that bring to the surface the products of nuclear reactions that have taken place in the interior. I hope this helps a little.