To remain in hydrostatic equilibrium, a star has to keep its gas very hot. The gas near the Sun's surface is about 6,000K (10,292°F; 5,700°C), while deeper in its interior the temperature reaches millions of degrees Kelvin. Clearly, a star needs a potent power source to keep all this gas so hot. And if we continued our imaginary trip from Figure 1 still deeper into the star, we would eventually find this power source, the star's core.
Stars generate energy in their cores, their central and hottest part. The Sun's core has a temperature of about 15,000,000K (15,000,000°C), and this is hot enough for thermonuclear fusion reactions to take place. Many different kinds of reactions are possible, but for stable stars, including the Sun, the primary reaction is one in which four hydrogen atoms are converted into one helium atom. Accompanying this transformation is an enormous release of energy, which streams out from the star's core and supplies the energy needed to heat the star's gas. (This is the same reaction, by the way, that occurs in a modern-day ICBM, the so-called "H-bomb." The ultimate human weapon of destruction is, for a very brief instant, a tiny star.) The Sun converts about six hundred million tons of hydrogen into helium every second, yet it is so massive that it has been maintaining this rate of fuel consumption for five billion years, and will continue to do so for another five.
Science EncyclopediaScience & Philosophy: Spectroscopy to Stoma (pl. stomata)Star - Energy Generation, Stellar Models, Mass: The Fundamental Stellar Property, Four Stars, Variable Stars - The nature of the stars