A red giant happens when a star has burned through its hydrogen supply and combines helium in its core to produce larger atoms, like carbon and oxygen. As the star fuses helium, the outer shell greatly expands and cools (the inner core is getting smaller and denser at the same time); this expansion is what gives the red giant its name as the star greatly increases in size, while the cooling material gives off a distinctive red color.
This outer material will soon escape the star's gravitational pull and dissipate into a nebulae, where the material will eventually be used to form new stars.
Red giant stars reach sizes of 100 million to 1 billion kilometers in diameter (62 million to 621 million miles), 100 to 1,000 times the size of the sun today. Since the energy is spread across a larger area, surface temperatures are actually cooler, reaching only 2,200 to 3,200 degrees Celsius (4,000 to 5,800 degrees Fahrenheit), a little over half as hot as the sun.
This temperature change causes stars to shine in the redder part of the spectrum, leading to the name red giant, though they are often more orange-like in appearance.
There are different types of stars available. One of the stars that you have to be familiar with is the red giant. This is the type of star that has used up most of its light elements. This means that it would start collapsing from the inside. The outer layers of the stars start to become cooler.
The color of the star starts to change and becomes red. This explains why the star is called a red giant. The time will come when all of the helium that can be found in the star’s core will get burned up. The red giant is still very bright and can be seen in the night sky.