The event in the life of a star that begins its expansion into a giant is that almost all the hydrogen in its core that was hot enough for fusion has been turned into helium.
A fusion reaction is defined as the type of nuclear reaction whereby two or more atomic nuclei combine to form a heavier nucleus.
The stars are being transformed into giants through fusion reaction.
A higher amount of hydrogen is found in the core of the star which is being use up in the fusion reaction to form helium.
When the hydrogen in the centre of a star runs out, the star begins to use hydrogen further out from its core. This causes the outer layers of the star to expand and cool.
Therefore, the event in the life of a star that begins its expansion into a giant is that almost all the hydrogen in its core that was hot enough for fusion has been turned into helium.
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