How do astronomers estimate the age of a galaxy?
Quick Answer
They combine the ages of its stars, its chemical makeup, and how far its light has been stretched by expansion. For nearby galaxies, detailed star-by-star studies can give ages of about 10 to 13 billion years.
The Full Story
Astronomers estimate a galaxy’s age by studying the ages of the stars inside it and by measuring its chemical composition. Older galaxies tend to have more “metals” — elements heavier than helium — because earlier stars made them and spread them into space. For distant galaxies, redshift helps show how early in cosmic history we are seeing them, since light from far away takes billions of years to arrive. The Milky Way’s oldest known stars are about 13 billion years old, so our galaxy formed very early in the universe’s history.
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