Complex sentence: When the hydrogen fuel at the core is exhausted, a star with at least 0.4 times the mass of the Sun expands to become a red giant, in some cases fusing heavier elements at the core or in shells around the core.
Split into simpler sentences: When the hydrogen fuel at the core is exhausted, a star of mass 0.4 times greater than the Sun's will expand to become a red giant. In some cases, it will fuse heavier elements at the core or in shells around the core.