What is a red giant star? Why is it called so?

What is a red giant star? Why is it called so?

     A star of medium size, such as the Earth's Sun, burns hydrogen in its early years, fusing the nuclei of this most abundant element to form helium and creating the energy that makes the star shine. This fusion process usually takes place in the core of the star; temperatures in the outer layers are generally too low to allow fusion to occur. After eons, the hydrogen in the core of the star is converted completely to helium.

     As hydrogen burning comes to an end, the pressures and temperatures in the core decrease, because there is not enough gas and radiation pressure to keep the core in its former state of equilibrium. The pull of gravity now

becomes greater than the push of pressure, and the core contracts. This heats not only the core but the layer above the core. The hydrogen in that layer ignites, providing the fuel for another period of fusion. As the hydrogen in this layer burns, the radiation it creates puffs up the star's outermost layers, so that it increases greatly in size. The expansion of the gases in the star's outer laver has temperature reducing effect, so that the cool stellar atmosphere shines with a red or orange colour. The star is now a red giant.

     A typical red giant has a diameter 100 times greater than that of our Sun, with only half its mass. Eventually, the surface temperature of the red giant will increase as fusion activity creates carbon and oxygen from helium. The surface temperature of the star increases, so that its colour becomes yellow or white and it ceases to be a red giant.