Stars form through the gravitational collapse of a vast cloud of hydrogen gas. More on this in the section about nebulae. As the gas cloud collapses it typically forms a number of young hot stars which emit intense amounts of ultravoilet radiation and visible light. Over many millions of years, the ultraviolet radiation will slowly burn away the remnants of the nebula it formed from leaving behind a loose grouping of stars that we see as an open cluster. The stars within the cluster will have their own velocity and over millions of years the cluster will slowly disperse.
Since open clusters are the result of stellar formation, we tend to find them in the locations with the highest concentration of nebulae. This meatop they are usually found within a galaxy and in particular within the spiral arms. Interestingly, it was by studying the location of open clusters that astronomers were able to deduce the shape of our galaxy, the Milky Way as a vast rotating spiral galaxy.
A great example of an open cluster is the Seven Sisters or Pleiades star cluster. To the unaided eye it looks like perhaps 7 stars (hence its name) but through a telescope there are in the region of 250 young hot stars. Photographs of the region even show what looks like some of the original nebulosity the stars formed out of but further studies have shown its just a passing cloud of gas and dust.
At the opposite end of the scale are the aging globular clusters which seem to have formed at the same time as the galaxy itself. Due to the lack of star forming gas within the clusters there isn’t a great deal of star formation going on and so we are left with clusters of extremely old stars heading toward the ends of their lives. Indeed some of the oldest stars in the Universe have been found in globular clusters.
Unlike the open clusters, globular clusters are found in a huge halo surrounding our Galaxy. We can tell they are not unique to the Milky Way though as we can detect them in other galaxies too. The Andromeda Galaxy which is the nearest major galaxy to our own also has them and they can just be detected in large amateur telescopes. The largest globular cluster in our skies is known as Omega Centauri and plays host to several million stars, compared to the typical few hundred of open clusters.
Mark’s Observing Tip
Clusters are easy to observe and there are examples that suit any instrument. One of the benefits of a telescope is that you can not only see fainter objects but you can also see finer levels of detail. This is called resolution and a larger telescope will have a higher resolution than a smaller one. With higher resolution you will be able to separate more individual stars in any given cluster than you can with a telescope with lower resolution. This is of greatmbenefit with globular star clusters when a larger telescope will allow you to separate stars right into the core.