The Pleiades (credit NASA, ESA, AURA/Caltech, Digitized Sky Survey )
A familiar example of what is called an "open cluster" is the Pleiades , or Seven Sisters which can be seen with the naked eye. When a Hertzprung-Russell diagram (H-R diagram) is done for this cluster, stars of all colors (spectral classes) are seen. The stars spread out along the main sequence with an excess of hot B-type (young) stars. None of the stars has migrated off the main sequence to the red giant phase. All this is consistent with the cluster being relatively young, since the O and B type stars are short-lived. An age of 25 million years or less is estimated. Containing about 3000 stars, the Pleiades cluster is about 135 pc or 440 light years from the Sun and spans about 4 pc.
Chaisson & McMillan