Once a star has formed, nuclear reactions in its core supply it with energy.
For most of a star’s life, its energy is supplied by the fusion of hydrogen atoms into helium atoms.
When a star uses up the hydrogen in its core, the core shrinks and the outer layers of the star expand and cool. This makes the star turn into a red giant.
Eventually a star consumes all its available fuel and collapses.
Stars much more massive than the Sun end their lives by exploding.
Stars like the Sun may lose material from their outer layers; their core then becomes a white dwarf.
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