Open Cluster Classification

It may take some getting used to, and some practice to keep it fresh, but learning the classification for clusters can be a real help in identification. Gives you some idea of what to look for.

Trumpler System

The system devised by Shapley and Melotte was superseded by the Trumpler System which had it origins in 1930. It is superior than all the earlier systems because it gives an abbreviated indication of the nature of the cluster and describes details on the stellar membership. This four part structure in the system is as follows;


I - Detached with strong central condensation
II - Detached with little central condensation
III - Detached with no central condensation
IV- Not well attached, but displays a strong field concentration

2. Numerals

Numerals show the luminosity function (lmf) which is the open cluster lmf compared the nearby non-cluster region's lmf. It is the mean luminosity of all the clusters stars. This suggests that the lmf decreases in magnitude compared to the surrounding fields. For an actual cluster, this shows that lmf increases with the brighter of the cluster stars, followed by some decrease to fainter ones. Lmf follows the numerals 1, 2 and 3.

3. Star Numbers

p - Poor < 50 stars
m - Medium 50 to 100 stars
r - Rich > 100 stars

4. Nebulosity

The 'n' at the end of the classification indicates nebulosity associated with the open cluster. Globulars are classified by astronomers according to their density. A class I globular is very compact and compressed. A class XII is loosely condensed. M13 in Hercules is in the middle and has a classification of V. An enjoyable observing exercise is to estimate a globular's classification before consulting any reference and then comparing your observation with the accepted value.

Space isn't remote at all. It's only an hour's drive away if your car could go straight upwards.

Fred Hoyle