Open and Globular Star Clusters

Star Clusters Fact

• Stars can cluster together in groups rather than be spread out like the Sun is to its neighbours.
• There are two main groups of star clusters :-
  • Open Cluster
  • Globular Cluster

What is a star Cluster?

The term refers to groups of stars that are either tightly clustered together (Globular) or loosely (Open). The most well known and famous of clusters must be the Seven Sisters or Pleiades in the constellation of Taurus. They should not be considered as being unique to our galaxy, the Milky Way. A global cluster has been found in NGC 4874 Springer

What is a Globular Star Cluster?

Globular clusters tend to be old clusters of stars that can range in numbers from 10,000 to anything up to several million. The shape of Globular Clusters are roughly spherical in nature hence where its name comes from (Globe). As mentioned, they tend to be old stars, born not that long after the the beginning of the universe. Although the vast majority of Globular Clusters contain white and yellow stars, they have been a few blue stars in amongst clusters.

Globular Clusters tend to be found nearer the Galactic Centre of the galaxy whereas Open ones tend to be further out. The Pleiades Cluster is a good example of an open Cluster, they are in the opposite direction to the centre of the Galaxy, Sagittarius A*. For the opposite, Messier 80 is a Globular Cluster in the constellation of Scorpius.

Some galaxies do not have a Supermassive black hole at its heart, some will have a Globular Cluster at the heart instead.

Omega Centauri, Largest Globular Cluster

The largest Globular Cluster in the galaxy is Omega Centauri located in Centaurus galaxy. The Globular Cluster is estimated to have at least 10 millions stars of different colours and sizes. The combined mass is equivalent to four million solar masses. The picture below is taken from the Hubble space telescope. Some of the blue stars are likely to be blue stragglers, or vampire stars.

What is an Open Star Cluster?

Stars in an Open Cluster tend to be younger stars compared to Globular Clusters and they don't tend to be as tightly compact. Open Cluster stars all tend to be made of the same material and can be of different ages. At the birth of the Open Cluster, all the stars will be of the same age but they can die at different rates. All the stars aren't the same size but be varying sizes. They don't all stay clustered together as they can different away from one another.

Of the two types of cluster, these are the more common of the two types with as many as 1000 having been discovered. An Open Cluster whilst it can loose stars as they move away, it can replace the lost stars with new stars from the same material. This is something that a Globular Cluster is unable to do, that is replace a star which is lost. When a Globular Star is lost, its lost and irreplaceable thereafter.

Hyades, Local Star Cluster

The Hyades open star cluster is a local star cluster to us at around 153 light years. The cluster is located in the constellation of Taurus, the bull. The Hyades includes some of the well know Taurian stars such as Ain also known as Epsilon Tauri. Although located near Aldebaran, Aldebaran is not a member of the Hyades. Ain is of particular interest because has an orbiting exoplanet.

Age of a Cluster

You can determine the age of the star cluster by looking at the colour of the stars that exist in the cluster. All the stars should be roughly the same age. If the cluster is relatively blue then the stars are hot and young and will only last millions of years. If the cluster is yellow then the cluster will last billions of years. If the cluster is red then the cluster is old and can last trillions of years.

If the cluster is predominantly red but there are some blue stars in amongst the stars then its a good sign that there are Vampire Stars in amongst the cluster. These vampire stars are referred to as Blue Stragglers. In short, vampire stars suck material and fuel from a close star and as they feed and grow, the Vampire Star can appear blue and younger than other stars in the cluster. Ref: Science Channel