Astronomers think that a star begins to form as a dense cloud of gas in the arms of spiral galaxies. Individual hydrogen atoms fall with increasing speed and energy toward the center of the cloud under the force of the star's gravity. The increase in energy heats the gas. When this process has continued for some millions of years, the temperature reaches about 20 million degrees Fahrenheit. At this temperature, the hydrogen within the star ignites and burns in a continuing series of nuclear reactions. The onset of these reactions marks the birth of a star.