(Submitted March 11, 1999)
Is it possible to determine the age of a star cluster?
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:
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.
Koji Mukai & Maggie Masetti
for Ask an Astrophysicist