The core starts to become unstable and it starts to contract. The outer shell of the star, which is still mostly hydrogen, starts to expand. As it expands, it cools and starts to glow red. The star has now reached the red giant phase. It is red because it is cooler than the protostar phase and it is a giant because the outer shell has expanded outward.
All stars evolve the same way up to the red giant phase.
The amount of mass a star has determines which of the following life cycle paths the star will take. As a red giant, the hydrogen gas in the outer shell continues to burn as the temperature in the core continues to rise. At 200,000,000 degrees Celsius, the helium atoms fuse to form carbon atoms in the core. The last of the hydrogen gas in the outer shell is blown away to form a ring around the core. This ring is called a planetary nebula.