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[sci.astro] Cosmology (Astronomy Frequently Asked Questions) (9/9)
Section - I.09. How can the oldest stars in the Universe be older than the Universe?

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Obviously, the Universe has to be older than the oldest stars in
it. So this question basically asks, which estimate is wrong:

   * The age of the Universe?
   * The age of the oldest stars? or
   * Both?

The age of the Universe is determined from its expansion rate: the
Hubble constant, which is the ratio of the radial velocity of a
distant galaxy to its distance. The radial velocity is easy to
measure, but the distances are not. Thus there is currently a 15%
uncertainty in the Hubble constant.

Determining the age of the oldest stars requires a knowledge of their
luminosity, which depends on their distance. This leads to a 25%
uncertainty in the ages of the oldest stars due to the difficulty in
determining distances.

Thus the discrepancy between the age of the oldest things in the
Universe and the age inferred from the expansion rate is within the
current margin of error.

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