Stellar Clusters

There are two distinct types of stellar cluster ;
Open clusters and Globular clusters


  • a star cluster is a group of stars that share a common origin and
    are gravitationally bound for some length of time.
  • a stellar cluster is a group of stars (a stellar system) where the constituent stars are located much more closely together than the surrounding stars.

Open Clusters

Formation of Open clusters

  • open clusters contain between a few hundred and several thousand fairly young stars, and they are mainly located in the same place (ecliptic) as the spiral arms of the galaxy.
  • most open clusters are less than 200 million years old, and in some clusters the stars are so young (only a few million years old) that they are still embedded in the clouds from which they emerged.
  • the most famous open cluster is the Pleiades known as Messier 45 (M45) – although there are nine stars in the cluster that are visible to unaided eye.

Close up
The Pleiades, which is also known as Messier 45 9M45), and the Seven Sisters
in the contellation of Taurus (The Bull).


Globular Clusters

Formation of globular clusters

  • globular clusters are much bigger than open clusters and contain up to one million stars.
  • they are also much older, with an average age of at least 10 billion years.
  • the globular cluster M92 had an estimated age of 13-14 billion years, making it almost as old as the Universe itself.

Messier 92
Globular cluster in the constellation of Hercules.

  • in our galaxy, the globular clusters are mainly located above and below the galactic plane, and they are concentrated around the centre in a sphere cloud that is called the galactic halo.

Globular clusters are mainly located above and below
the galacticplane of the galaxy.

  • stars in globular clusters are sometimes described as metal-poor because they were  formed so long ago, before the Universe contained many metal atoms.
  • younger stars, like our Sun and the stars in open clusters, are considered to be metal-rich.


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