Star Clusters

Open Clusters

Globular Clusters

M45 - The Plieades

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.