Add Trumpler classification

[Gabriel-p]

The Trumpler classification system classifies open clusters according to concentration, brightness and number of stars/members in cluster.

This could be obtained by the code. A possible scheme for obtaining the concentration could be:

I: CI < 0.25 or dens_max > 4*dens_field
II: 0.25 < CI < 0.5 or 3*dens_field < dens_max < 4*dens_field
III: 0.5 < CI < 0.75 or 2*dens_field < dens_max < 3*dens_field
IV: 0.75 < CI or dens_max < 2*dens_field

Also applicable to globular clusters via the Shapley–Sawyer Concentration Class.

Separation between open and globular clusters, from the ENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICS.

There also exists the concentration parameter (used for example here) defined as the logarithmic relation between the tidal and core radii:

c = log(r_t/r_c)

This manual details the Trumpler Classification System as:

Trumpler identified three features in an open cluster:

• Degree of Concentration: I. Detached clusters with strong central concentration. II. Detached clusters with little central concentration. III. Detached cluster with no noticeable concentration. IV. Clusters not well detached, but has a strong field concentration

• Range of Brightness

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

• Number of Stars in Cluster p. Poor clusters with less than 50 (fifty) stars. m. Medium rich cluster with 50-100 stars. r. Rich clusters with over 100 stars.

[Gabriel-p]

Nobody really uses this classification anymore