Clusters are unique in that their stars are pretty much guaranteed to all have the same birthday. They all formed together from the same cloud of gas at roughly the same time. This helps us learn how stars change as they age.

Above is the textbook figure of the HR diagram of the Pleiades, the open cluster you can see in the constellation of Taurus. You can see that just about all stars on the main sequence are included, but that there is a "turnoff" of the most massive stars.

Above is the textbook figure of the HR diagram of a globular cluster. You can see that the "turnoff" has happened for stars with a mass around that of our own Sun!

The meaning of this is that hot massive stars evolve faster, so they leave the Main Sequence first. In a younger cluster like the Pleiades, the massive stars are still on the Main Sequence. In an old cluster like a globular cluster, stars down to the mass of our Sun have all used up their hydrogen fuel.

Above is an HR diagram, from the book, comparing several different clusters. Most of the data for these HR diagrams comes from the Hipparcos satellite.

This excellent NASA site discusses the evolution of stars on a basic level.

Stars are born in nebulas called molecular clouds. They are cold nebulas, about 10-30 K. Hotter nebulas would resist gravity more--the first act in the long battle between gravity and pressure!

These nebulas are called molecular clouds because they are cold enough for molecules to form, mostly H₂, but also CO and H₂O and others.

Orion is full of molecular clouds. Some clouds are dark splotches, and we have to use radio telescopes to really learn about what they are made of. The Eagle Nebula also shows star birth. We talked about this a little when we talked about the formation of the Solar System.

Here is a complete image of the constellation of Orion with all its nebulas:

As a star is born, it forms a disk (which may or may not form planets). Paradoxically, a star as it's pulling together also ejects a lot of material, either through a stellar wind or through jets. Jets are very common in astrophysics but we don't agree on what causes them.

Some young stars that give off jets that hit into surrounding gas--these are called Herbig-Haro objects:

From the textbook:

Here is how a protostar "moves across the HR diagram" as it is being born (figure from the book):

More massive stars evolve faster at every stage, including birth:

There are also several other differences. On the Main Sequence, a low mass star will have convection in its outer layers, like the Sun. If it's really low mass it will have convection all the way in! A high mass star will have convection only in its core.

The difference between stars under and above 8 M_Sun is that the heavy stars will fuse elements in their core all the way up to Iron. Then fusion doesn't work any more, and the star implodes, bounces, and explodes as a supernova. A lower mass star will fuse only until Carbon is created in its core, and end life more passively as a white dwarf. The difference is that the degenerate pressure in the white dwarf can hold it together, but the degenerate pressure is not enough for the more massive stars.

Above is an HR diagram showing the entire life track of a low mass star.

What happens, in summary, is this. The p-p chain fusion turns hydrogen in the core into helium. The reduction in number of particles causes the core to shrink. Shrinkage means heating up. As all the hydrogen is used up, the core can no longer fuse. Instead, there is fusion in a thin layer of hydrogen around the core. The rest of the star expands into a subgiant, and then a red giant. The core becomes degenerate. Then when more helium is produced from hydrogen in the shell drops down to the core, shrinking it further, the core gets hot enough for fusion of helium into carbon. But because the core is degenerate, it can't expand. It fuses very fast in a "helium flash"--the core is no longer degenerate and the star goes down into subgiant status again. This can last for only a few seconds! Now there's hydrogen fusion surrounding helium fusion surrounding an inert carbon core.

Coming down from the helium flash, a star finds itself on the "horizontal branch" in the HR diagram--because stars that start similar can lose mass in different amounts through a stellar wind, they end up at different places horizontally on the diagram.

Open cluster: Pleiades