After several billion years they become a dark cinder, only slightly less massive than they were while on the main sequence, but no longer radiating. Since the star's intrinsic luminosity gives us a pretty good idea of how fast it's consuming the available hydrogen fuel, then apparently the most luminous stars are burning their fuel at a very rapid rate. On the other hand, a star with half the mass of the sun will be about 25 to 30 times less luminous and is much more conservative in the rate of consumption of its hydrogen fuel. If stars like our sun stay on the main sequence about 10 billion years, then stars with a modest gain in mass but a much greater gain in luminosity won't stay as long on the main sequence as long as we do because such stars are burning their fuel at a prodigious rate. And the fact that the more luminous stars burn their nuclear fuel at a much faster rate gives us a pretty good idea of how old a cluster of stars is.
Astronomers begin by constructing an HR-diagram of all the stars in the cluster.
The age of the cluster affects what we can see with binoculars or a small telescope. Both clusters can be detected with the naked eye at a dark site as a cloudy concentration of light. Their brightest stars can be picked off with a pair of binoculars because they're tens of thousands brighter than our sun. Stars that far away wouldn't be seen with binoculars unless they were very luminous, and stars that luminous wouldn't still be there unless the clusters were very young. On the other hand, there is another cluster in nearby Cassiopeia called NGC 7789.