Simplify the following text for a Grade 7 native English-speaking person.
Original: Once the hydrogen fuel at the core is exhausted , those stars having at least 0.4 times the mass of the Sun expand to become a red giant , in some cases fusing heavier elements at the core or in shells around the core .
Simplified: Once the hydrogen fuel at the core is exhausted , those stars with at least 0.4 times the mass of the Sun expand to become a red giant , in some cases fusing heavier elements .