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Open Cluster

Trumpler Classification System

Degree of concentration

I. Detached clusters with strong central concentration.
II. Detached clusters with little central concentration.
III. Detached clusters with no noticeable concentration.
IV. Not well detached from surrounding star field.

Range of brightness

1. Most of the cluster stars are nearly the same apparent brightness.
2. Moderate range in brightness.
3. Cluster is composed of bright and faint stars.

Number of stars in cluster

p. Poor (less than 50 stars).
m. Medium rich (50-100 stars).
r. Rich (more than 100 stars).

The letter 'n' following the Trumpler type means that there is nebulosity associated with the cluster.

Trumpler Classification Image Reference

image sourced from: http://www.astrophoton.com/trumpler_class.htm[1]

[1] http://www.astrophoton.com/trumpler_class.htm (HTTP)