A nonburning helium core surrounded by a shell of hydrogen-burning gas characterizes the subgiant stage of stellar evolution. As time goes on, the star continues to evolve, and eventually, it becomes a red giant. Rank the stages a star goes through as it evolves from a subgiant into a red giant, from latest to earliest. Rank the stages a star goes through as it evolves from a subgiant into a red giant, from latest to earliest. a. The shell of hydrogen surrounding the star's nonburning helium core ignites. b. Pressure in the star's core decreases. c. Pressure from the star's hydrogen-burning shell causes the nonburning envelope to expand. d. The star's nonburning helium core starts to contract and heat up. e. The star becomes a red giant. f. The surface of the star becomes brighter and cooler.