The Thermonuclear Bomb

The first atomic bombs generated their output from the release of energy from splitting large nuclei in a chain reaction. Hydrogen bombs generated energy by fusing two small nuclei together. Both reactions create a large amount of energy, but initiating a fusion reaction is much more difficult.

Before small nuclei will combine, the temperatures need to be incredibly hot, over a million degrees Fahrenheit -- comparable to the temperatures inside stars. That's why these bombs are called "thermonuclear." To create these high temperatures, a fission reaction is initiated to raise the temperature and pressure of the fusionable material, usualy tritium or deuterium.