Birth

A cluster forms from a huge cloud of gas. As the gas cloud begins to shrink under its own gravity, smaller star-sized clumps form inside it. These individual clumps continue to contract and eventually become hot enough and dense enough to fuse hydrogen into helium. These are the stars of the cluster.

Life on the main sequence

The properties of the newly born stars follow a very specific relationship between mass, temperature and luminosity. This relationship is called the main sequence. Where each star falls on the main sequence depends on the amount of mass it got in its original "clump". Massive stars are very luminous and hot - they are at the top of the main sequence. Low mass stars are not very luminous and are cool - they are at the bottom of the main sequence. Since all the stars form at around the same time, their collective evolution is synchronized. Thus, by studying the H-R Diagram we can learn about how the different stars in the cluster evolve relative to each other. From their collective evolution we can determine the age of the cluster. The longest part of a star's life is spent on the main sequence where the star happily fuses hydrogen into helium. During this time the star does not move significantly in the H-R Diagram, which is another way of saying that its luminosity and temperature are not changing. When a star runs out of hydrogen its properties begin to change and consequently its position on the color magnitude diagram changes. As a result, the star "leaves the main sequence" toward the red giant part of the color magnitude diagram.

Massive stars die faster

Massive stars are hotter and they go through their fuel more quickly than less massive stars. Think of a big truck - it goes through gas more quickly than a smaller car. The fuel on the main sequence is hydrogen. The more massive the star the sooner it leaves the main sequence.

When stars run out of hydrogen

When a star runs out of hydrogen in its core and starts to burn it in a shell around the core, it starts to expand and cool down. We call this a red giant.  Eventually the helium in the core does burn into carbon and oxygen, but that too will soon end.  After that, the star is powered by burning hydrogen and helium in shells around the dead core.  At this point, it is extremely luminous and large, and we call it a supergiant.

When stars run out of helium

What happens next depends of the mass of the star. A star with a mass close to the mass of our sun will not burn elements heavier than helium. The heavier an element is, the hotter the core of the red giant needs to be in order for the star to burn it as fuel. The core never gets hot enough to burn carbon or oxygen.  But the outer layers of the star get blown away because the shell fusion becomes unstable: the result is an expanding planetary nebula and the remnant core at the center which we call a white dwarf. If you recall, this is how our Sun will die. A massive star will continue to fuse heavier and heavier elements. Eventually it will try to fuse the element iron (Fe), but this cannot be done because fusion of iron actually absorbs energy instead of providing it. With no more fuel left, the core of the star collapses resulting in a violent explosion called a supernova.

Depending on the age of the cluster, we see evidence for all these stages in its H-R Diagram.