The night sky is full of stars. Many of them are united in accumulations of hundreds or thousands of stars. They are what is known as star clusters. But among all of them we must differentiate between two types of clusters: open clusters and globular clusters.
Open cumulus clouds
The open clusters are groups of stars formed from the same nebula. These types of clusters are also known as galactic clusters, since they can be found throughout our galaxy.
Its structure is random and, in general, asymmetric. But the stars that form them are gravitationally bound to each other. They are normally made up of several hundred or thousands of stars, which we can perceive individually through the telescope.
Origin and evolution of open clusters
Open clusters are ideal tools for studying stellar evolution theories . We can observe clusters with very different ages and with stars of different masses that, therefore, evolve differently. Thus, we can analyze the behavior of the stars through the HR diagram from the formation of the cluster to its disintegration. In this way, at the origin of the cluster the stars are all distributed in the main sequence. But in the middle of the squares, the most massive stars will evolve more rapidly and the diagram of the cluster as a whole will vary. This allows comparing the HR diagrams of the clusters with the theoretical models, thus helping to refine and refine them, better understanding the evolution of the stars.
However, the end of the open cumulus clouds is more bleak. Due to the limited number of stars in these clusters, their structure is vulnerable to the gravitational effects of our galaxy. Thus, throughout their life these clusters are suffering disturbances in orbit around the galaxy. These alterations cause the stars to separate and, therefore, the clusters end up disintegrating with the passage of time.
The clearest example of an open cluster is found in the Pleiades. However, it is estimated that their stars will only stay together 200 million more years.
Pleiades by Marco Lorenzi – Reflection Nebula
A globular cluster is a collection of hundreds of thousands of stars held together gravitationally . Unlike open clusters, these clusters are quite old and are found in outer parts of the galaxy. About 150 clusters are known in our galaxy, but for example it is estimated that the Andromeda Galaxy contains about 500.
Due to their large number of stars, it is sometimes difficult to resolve them (distinguish individual stars) through the telescope. A large aperture telescope will allow us to distinguish a whitish sphere made up of thousands of tiny dots. Without a doubt, they are spectacular images that are often reminiscent of spider webs or cities seen from space. A very characteristic example of this type of cluster is M13, the Cluster of Hercules.
M13, known as the Hercules Cluster
Origin and evolution of globular clusters
Their great density means that they have a spherical distribution and that they are normally in orbit around the galaxy, in what we know as the galactic halo. These agglomerations, which can reach up to a million stars, seem to have a common origin. However, this origin is still not very clear.
These clusters contain stars between 11,000 and 13,000 million years old. These ages are similar to those of the galaxies themselves, so it seems that their formation took place simultaneously. Furthermore, no globular clusters are known to be forming stars today, which is consistent with the view that globular clusters are typically the oldest objects in the Galaxy and were among the first collections of stars to form.
Different types of star clusters
Now you know the main differences between the different types of clusters that we can find in the sky. And we hope that when you look at them through the telescope you will remember that both clusters are very different. In any case, whether they are open clusters or globular clusters, their image through the telescope is always spectacular.
We recommend you observe the open clusters : M37, M38, M44 (Manger), M45 (Pleiades), M46, NGC 869 and 884 (Double Cluster), NGC 457 (ET or the Líbelula), Hyades.
As for globular clusters , our favorite clusters and that we usually see in our astronomical observations are: M2, M3, M4, M5 M12, M13, M22, M53 and M71.