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The Question

(Submitted April 02, 1998)

How many stars (or what percentage) in our galaxy are 4.5 billion years old (age of the Sun) or older?

The Answer

The Sun is about 4.5 billion years old now, but its total life is estimated to be about 10 billion years. Most stars in the Milky Way live at least this long. And experts estimate that stars have been forming at a similar rate for most of the history of Milky Way (~10 billion years). This means about half the stars are older than 4.5 billion years old.

Koji Mukai
for Ask an Astrophysicist

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