Final answer:
Globular clusters are over 11 billion years old and lack luminous blue stars, while open clusters and associations are much younger, featuring bright blue main sequence stars.
Step-by-step explanation:
The general trend in the ages of the two types of star clusters, globular clusters and open clusters, indicates a significant age difference between the two. Globular clusters, containing some of the oldest stars in the galaxy, are typically older than 11 billion years, with their stars distributed in a halo around the Galaxy. These clusters lack luminous blue stars due to their advanced age and the evolutionary state of their stellar populations.
On the other hand, open clusters and associations are relatively young, featuring highly luminous blue stars on the main sequence and lower-mass stars that are still contracting towards the main sequence. The age of these clusters is determined using the H-R diagram, where astronomers can gauge an open cluster's relative youthfulness by identifying the luminous main sequence stars that have yet to evolve into red giants. Stellar associations, which are even younger, are found in regions rich in gas and dust, indicating recent star formation.