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Loading contentWhen a star exhausts the hydrogen in its core, fusion moves to a shell around an inert helium core. The envelope swells enormously and cools to a red glow, and the star climbs the red-giant branch — brightening as it ascends until helium ignites at its tip.
Facts on this topic will be cited from these primary and reference sources.
Mission data, planetary science, space telescopes, and public-domain imagery.
Most NASA-produced imagery is in the public domain; individual items are checked for usage terms before publication.