What happens for a star to enter old age?

Because the interstellar medium is 97% hydrogen and 3% helium, with trace amounts of dust, etc., a star primarily burns hydrogen during its lifetime. A medium-size star will live in the hydrogen phase, called the main sequence phase, for about 50 million years. Once hydrogen fuel is gone, the star has entered “old age.”

for a star to start the process of become an old star it has to burn all the hydrogen in the star.

Explanation:

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