Such an aging, swollen star is called a red giant. The first sign of a star's old age is a swelling and reddening of its outer regions. When a star begins to exhaust its hydrogen supply, its life nears an end. The onset of these reactions marks the birth of a star.
At this temperature, the hydrogen within the star ignites and burns in a continuing series of nuclear reactions. When this process has continued for some millions of years, the temperature reaches about 20 million degrees Fahrenheit. Individual hydrogen atoms fall with increasing speed and energy toward the center of the cloud under the force of the star's gravity. Astronomers think that a star begins to form as a dense cloud of gas in the arms of spiral galaxies.
