The red giant stage occurs after the main sequence stage in a star’s lifetime. When a star runs out of its nuclear fuel, it grows to be many hundreds of times its original size and cools, making it appear red.
A red giant is a star that has come to the end of its main sequence stage. The red giant stage is the first stage of the end of the star’s life. The name comes from the fact they are larger than a main sequence star and they emit more red light. The reason for the change in color is that the surface of the expanded star is cooler, giving an orange or red appearance.
Stars enter the red giant phase when they have used up the hydrogen in their cores. This means that the stars start to fuse hydrogen or other elements together in the area surrounding their core. This alters the equilibrium between the pressure of the nuclear reactions and the force of gravity holding the star together. The outer layers expand increasing the size of the star. After fuel runs out and the nuclear reactions end, the star dies.
The star then enters a stage known planetary nebula then the white dwarf phase. Stars that are much more massive than our sun also have a red giant phase. They are known as red supergiants. These stars have different ending to stars with a similar mass to that of our Sun. After the red supergiant phase, the star explodes in an event known as a supernova, and then becomes a smaller dense neutron star. If the star is extremely large, it has the possibility to become a black hole.
In about five billion years, when our own Sun becomes a red giant it will grow, swallowing the orbits of Mercury and Venus. Even if the Sun does not absorb the Earth, the extra radiation from the red giant will vaporize all life on Earth.
(a star with a similar mass to our Sun)
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