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The Question

(Submitted March 11, 1999)

Is it possible to determine the age of a star cluster?

The Answer

Yes, it's actually much easier to determine the age of a star cluster than of an isolated star.

A star cluster includes many stars that were created at about the same time, but of different masses (sizes), all at the same distance (more or less). By observing the brightness and color of each star, astronomers can construct a color-magnitude, or Hertzsprung-Russell (HR) diagram:

http://imagine.gsfc.nasa.gov/docs/science/know_l2/stars.html

Stars like the Sun spend most of their life on the 'main sequence', a dense concentration of stars on a narrow belt going from the upper left to the lower right of the HR diagram. The mass of the star determines where on the main sequence it is located; the mass of the star also determines how soon the star will move away from the main sequence. By looking at the HR diagram of a cluster of stars, in particular where the main sequence ends, you can estimate the age of the cluster fairly accurately --- much more so than for single stars.

Best wishes,

Koji Mukai & Maggie Masetti
for Ask an Astrophysicist

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