Since positive charges repel each other, nuclei must be moving at very high velocities in order to overcome this repulsion and undergo fusion. Velocity depends on temperature, and that is the reason why very high temperatures are required for fusion. After this stage, the star becomes a red giant. In about 5 billion more years, the Sun will leave the main sequence and evolve to a red giant. Simply, a red giant is a large cool star of high luminosity.
More massive stars live a shorter lifetime on the main sequence than lower mass stars.
As with the lower mass stars, the main sequence lifetime for higher mass stars ends when the hydrogen in the core is used up. The core begins to contract, and the temperature for helium fusion for heavier elements is quickly reached. As the luminosity of the core increases, the outer layers of the star expand. The atmosphere cools with the expansion, but the size increases sufficiently for the luminosity to increase. The star then becomes a red supergiant.