Betelgeuse is a red giant star.
It was a star like the Sun which burns hydrogen in its core.
Betelgeuse has exhausted all its hydrogen and is now burning helium in its core.
The extra heat from this more energetic reaction has caused the star to swell enormously.
However, the outer atmosphere of this massive star is so far removed from the
hot inner core that it has cooled to a dull red.
When all its fusable core material is gone,
Betelgeuse will spend its remaining days as a white dwarf.
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