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The key to this is that NO star uses up ALL of its hydrogen.

In fact, they only use up the Hydrogen in their cores, where pressure and temperature are highest.

In the case of a red giant, the star is at the stage of burning Helium into Beryllium, Boron and Carbon, which requires much more heat (the heat is "borrowed" from the previous collapsing of the star at the end of the main sequence phase) and continues with a much denser core. When a star expands into a red giant, it blows some of its Hydrogen mass into space.

Later, when it simply cannot sustain any fusion reactions in its core, it still contains a considerable amount of hydrogen in the outermost layers, and the subsequent collapse causes a rebound (supernova) that blows about 50% of the original star's mass - most of the hydrogen plus some of the heavier elements into space.

When that blown away material collides with a large gas cloud in space which is mostly hydrogen, and compresses that cloud enough that the material can then collapse and form another new star.

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17y ago

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