Describe: More than 90 percent of all stars in the universe, including the Sun, are main sequence stars. As main sequence stars age, they move up and to the right on the H-R diagram and become giants or supergiants. Main sequence stars produce heat by fusing hydrogen atoms into helium in their core. Giants and supergiants form when the star's core runs out of hydrogen and begins to collapse due to the force of gravity. As this occurs, hydrogen-rich outer layers of the star heat up enough to start forming helium, causing the star to rapidly expand. What are the characteristics of giants and supergiants?