What sudden ignition event powers a low-to-mid mass star like the Sun after exhausting core hydrogen, leading toward the horizontal branch?
Answer
Helium flash
When a low-to-mid mass star exhausts the hydrogen in its core, the inert helium ash causes the core to contract and heat up dramatically. For stars similar in mass to the Sun, this contraction eventually forces the helium nuclei to fuse into carbon, but this ignition occurs explosively and suddenly in what is termed the helium flash because the core is supported by pressure that does not increase linearly with temperature. After this rapid powering event, the star briefly stabilizes and proceeds to the horizontal branch phase, continuing its evolution until the core is converted into an inert carbon/oxygen structure, preceding the Asymptotic Giant Branch.
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