Star Clusters: An Introduction

Till now, we have studied about binary stars, neutron stars, black holes, evolution of different stars and a lot more. So, now it's high time to move towards another important topic related to stars: Star Clusters. The 22nd article of the series, Basics of Astrophysics, aims to explore a bit about different types of Star clusters known till date.

Click here to read all articles of Basics of Astrophysics series

What are Star Clusters?

Our observable Universe contains about 200 billion to 2 trillion galaxies. At an average, our milky way alone is home to about 300 billion stars. So, the total number of stars in this universe is immensely very huge. Most of these stars are wandering in this large universe all alone. However, some are enjoying their journey in the universe along with their companions in binary or multiple star systems. But, apart from this scenario, there exist some stars, that are associated with groups of different shapes and sizes. These groups of stars where the stars are gravitationally bounded to each other are referred to as Star Clusters. One of the most astonishing fact about these associations is that all the members possess a common motion, no matter what the motion of their surroundings is!

Types of Clusters

Depending upon their sizes, shapes, galactic distribution and number along with the physical characteristics of the stellar content, Star Clusters can be basically divided into two distinct categories, i.e. Galactic Clusters and Globular clusters.


Galactic clusters are also known as open clusters. The name open clusters is given due to the fact that the individual component stars can easily be resolved through a telescope. They are generally small in size, flattened in shape and are mostly distributed close to the galactic plane. A single Open cluster consists of up to a few thousand stars that formed from the same giant molecular cloud. These stars have roughly the same age and chemical composition. These clusters can contain as low as a few dozen stars to few thousands.

Open clusters are found only in spiral and irregular galaxies where active star formation is occurring. The stars in such clusters are loosely gravitationally bound to each other. Eventually, as the cluster rotates around the galaxy , it disperses due to gravitational interactions with other objects in the galaxy. Open clusters are therefore usually young objects. The stars here belong to Population I, as they are young and have high metallicity. The famous examples of galactic clusters include "The Hyades", "The Pleiades" and many more.

A color-composite image of the Pleiades Cluster
Image Courtesy: NASA/ESA/AURA/Caltech


Unlike the Galactic clusters, Globular clusters contain several thousands to one million stars in a spherical, strongly gravitationally bound system. Our Milky Way has about 200 Globular clusters, These clusters are usually located in the halo surrounding the galactic plane and are home to the oldest stars in the universe. Unlike the stars in Galactic clusters, the stars here belong to Population II as they have relatively low metallicity and have highly evolved.

Distribution of Globular Cluster in our galaxy
Image Courtesy: Brooks/Cole Thomson Learning

There is negligible free gas and dust available in globular clusters, so no new star formation is taking place in them. The inner regions of the globular clusters possess high stellar densities. Unlike Galactic clusters, Globular clusters mostly remain gravitationally unified throughout their journey in this Universe. The prominent examples include M56, M15, Omega Centauri along with many others.

Star Clusters
Omega Centauri
Image Credits: ESO

Some other types:

Apart from these two fundamental classifications, there is one another kind of star clusters evolving in our Universe. This type of Star clusters is termed as "The OB Associations". The OB Associations contain OB stars. These are hot, massive stars of spectral type O and early type B and lie in loosely organized groups. These stars enjoy a short life span. Hence, they don't move far from where they formed during their life time. However, during their short life time , they emit many UV radiations. These radiations ionise the surrounding interstellar gas of the giant molecular cloud, eventually forming H II regions.

Star Clusters: OB Association
Scorpius Centaurus Association ( an OB association)
Image Courtesy- Akira Fujii

Moreover, in 2005, Astronomers discovered a new type of star clusters in Andromeda galaxy. These share a lot in common with globular clusters. But, these are comparatively less dense and are much larger , about several hundred light years across. The three discovered such clusters in Andromeda galaxy are M31WFS C1, M31WFS C2 and M31WFS C3.

Also Read: 3 Types of Black Holes

Author's message:

Nearby open clusters play a key role in calibrating our measure of cosmic distances. Hence, star clusters are of a great importance to the study of our universe. This is all about the 22nd article of our series. I hope it gave you an overview about different kinds of Star Clusters existing in our Universe!

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