At the Sun's core, or very center, hydrogen atoms are collided together to form helium. This process is called nuclear fusion. These helium atoms then travel from the core to the surface of the sun, called the photosphere, where they release their energy. Helium atoms can take thousands of years to reach the Sun's surface because other atoms get in their way.

The Sun is about halfway through its main-sequence evolution, during which nuclear fusion reactions in its core fuse hydrogen into helium. Each second, more than 4 million tonnes of matter are converted into energy within the Sun's core, producing neutrinos and solar radiation; at this rate, the Sun will have so far converted around 100 Earth-masses of matter into energy. The Sun will spend a total of approximately 10 billion years as a main sequence star.

The core of the Sun is considered to extend from the center to about 0.2 solar radii. It has a density of up to 150,000 kg/m³ (150 times the density of water on Earth) and a temperature of close to 13,600,000 kelvin (by contrast, the surface of the Sun is around 5,800 kelvin). Recent analysis of SOHO mission data favors a faster rotation rate in the core than in the rest of the radiative zone. Through most of the Sun's life, energy is produced by nuclear fusion through a series of steps called the p–p (proton–proton) chain; this process converts hydrogen into helium. The core is the only location in the Sun that produces an appreciable amount of heat via fusion: the rest of the star is heated by energy that is transferred outward from the core. All of the energy produced by fusion in the core must travel through many successive layers to the solar photosphere before it escapes into space as sunlight or kinetic energy of particles.

Scientists called astrophysicists say our Sun is a main sequence star in the middle of its life. In about another 4-5 billion years, they think it will get bigger and become a red giant star. The sun would be up to 250 times its current size, as big as 1.4 AU. Earth's fate is still a bit of a mystery. Previous calculations show due to the stellar wind, which drops 30% of the sun's mass, Earth could escape to a higher orbit. But a newer study shows that due to the tidal forces, Earth would possibly vanish itself while the sun continues to get bigger, although the sun will lose mass. Anyway, Earth's ocean and air would long have worn out, even if the sun is still on its main sequence stage. After the Sun reaches a point where it can no longer get bigger, the Sun will literally explode, but not like a supernova, but rather, it will expand rapidly and lose its layers, forming a planetary nebula. Eventually the sun will shrink into a white dwarf, and over several hundred billion, even trillion years (100,000,000,000 to 1,000,000,000,000 years), fade to a black dwarf.

See also: The Sun

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