Deep in the interior of the Sun, 600 million tons of hydrogen are converted to helium every second, with the simultaneous conversion of about 4 million of these tons to energy. During the interval of about 4.5 billion years since its formation, the Sun has depleted much of the hydrogen at its center, but a pure helium core has not yet had time to form.

The Sun is a typical star. The initial stages of evolution of all stars are similar. On the main sequence, all stars derive energy from the fusion of 4 hydrogen atoms to form He. After the hydrogen in the core of a star is exhausted, the core shrinks under the weight of the star's outer layers. As the core contracts, it releases gravitational potential energy and heats the region surrounding it. Hydrogen then begins to burn in the region that surrounds the core. The nuclei in the interiors of stars are all completely ionized and are positively charged. 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. At this point, the star has a helium-burning core with a hydrogen-burning shell around it.

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.

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