Why Do Red Giant Stars Get Bigger? During the main sequence phase of solar mass stars they are in hydrostatic equilibrium. This is where the gravitational forces trying to collapse the star are balanced by outward radiative pressure created by fusion in the core. When the core is depleted of hydrogen the now helium core contracts and allows a shell of hydrogen fusion to occur around it. This shell of hydrogen fusion has a large volume and creates a larger outward radiative pressure. The gravitational forces do not change so this is able to cause an expansion of the outer layers, which in turn moves the star into the Red Giant phase of its evolution. Share this:TwitterFacebookLike this:Like Loading... Related Leave a Reply Cancel reply Enter your comment here... Fill in your details below or click an icon to log in: Email (required) (Address never made public) Name (required) Website You are commenting using your WordPress.com account. ( Log Out / Change ) You are commenting using your Twitter account. ( Log Out / Change ) You are commenting using your Facebook account. ( Log Out / Change ) Cancel Connecting to %s Notify me of new comments via email. Notify me of new posts via email. Δ This site uses Akismet to reduce spam. Learn how your comment data is processed.