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There are two types of star clusters: open clusters and globular clusters.
Open clusters have hundreds or a few thousand stars in a volume of about 10-100 ly on a side. Thus, it's density is only a few stars per cubic light year. A good example of an open cluster is Pleiades shown below.
Open clusters are in the plane of our galaxy and vary in age from older clusters to very young clusters of only 40 million years old or so.
There are about 150 known globular clusters in our galaxy, the Milky Way. Globular clusters are much more dense with stars than open clusters with 1 thousand to 1 million stars jammed into a sphere of onlyl about 100 ly in diameter. Thus, the density of a globular cluster is on the order of 100 stars per cubic ly.
Globular clusters are about 12-13 billion years old. They date back to about the origin of the galaxy which is only a billion or so years after the Big Bang. They are located thoughout the sphere of our galaxy, mostly outside the galactic plane.
Here's a good example of a globular cluster, NGC 6093.
The biggest advantage to studying star clusters is that all of the stars in a cluster began at around the same time. Therefore, all stars are the same age, though they are at very different places in their lifecycle. Thus, star clusters help us understand the evolution of a star.
Because of the mass-luminosity relation, more massive stars have much larger luminosities. That is, if Star A has a mass M and Star B has a mass 2 M, Star B will have a luminosity 16 times the luminosity of Star A. Though it has twice as much fuel to burn, it burns it at 16 times the rate. As a result, more massive stars "use up their fuel" more rapidly and leave the main sequence before less massive stars.
Thus, you can tell how old a cluster is by how few stars are on the main sequence. The fewer the stars on the main sequence, the older the cluster. This is most easily noticed by looking at the highest luminosity stars on the main sequence. The higher the luminosity of the brightest main sequence star for a cluster, the younger the cluster. The point where the cluster turns off the main sequence is called the turnoff point. Thus, the brighter the turnoff point, the younger the cluster.
Consider the H-R diagram below that combines the H-R diagrams of various open clusters.
The table below lists the clusters from youngest to oldest. You can see that the clusters are simply listed in order of brightness of turnoff point from the brightest turnoff point (youngest cluster) to the least bright turnoff point (oldest cluster).
H-R diagrams of globular clusters are all very similar since globular clusters are all very similar in age, forming early in the life of the universe, 12 billion years or more ago.
A globular cluster's turnoff point is at a very low luminosity, meaning that globular clusters are very old.
You can see the low turnoff point for the H-R diagram for M3 shown below.
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