Star Clusters
Open Clusters Globular Clusters

M45 - The Plieades

M13 - The Great Hercules Cluster
Open clusters are physically related groups of stars held together by mutual gravitational attraction.
The classification system  goes back to Harlow Shapley, which describes richness and concentration.
The Shapley Scheme
 a very loose and irregular
d loose and poor
e intermediately rich
f fairly rich
 g considerably rich and concentrated

A more sophisticated scheme was introduced by R.J. Trumpler.
The Trumpler Scheme
  • 4Concentration
I Detached; strong concentration toward center
II Detached; weak concentration toward center
III Detached; no concentration toward center
 IV Not  well detached from surrounding star field
  • Range in Brightness
1 Small range in brightness
2 Moderate range in brightness
3 Large range in brightness
  • Richness
p Poor: Less than 50 stars
m Moderately rich: 50 to 100 stars
r Rich: More than 100 stars

Next, Take a look at some of the best star clusters in the night skies.




Globular clusters are gravitationally bound concentrations of approximately ten thousand to one million stars.
4The distribution of the globular clusters in our Milky Way galaxy is concentrated around the galactic center in the Sagittarius -- Scorpius -- Ophiuchus region. Fifty percent are located there.