The nearby stars are of all ages, which gives them
a broad variety of luminosities and colors.  To see stars of the
same age, to see the effects of mass and composition alone on
a star's color and luminosity, one must examine star clusters.
All of the stars in a star cluster are born at about same time.
The open clusters scattered in the Galactic disk provide us with
collections of young stars.  The ancient globular clusters that
swarm around the Galactic center provide us with collections of
old stars.  By creating Herzsprung-Russell diagrams for both types
of star cluster—plots of the colors and luminosities of
stars—astrophysicists gain insight into how stars, especially
stars more massive than the Sun, change over billions of years.