One classification of open clusters is the Trumpler classification. It consists of 3 parameters, the first is the 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.
The second parameter is the 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.
The last parameter categorizes the number of stars in the cluster.
p - Poor clusters with less than 50 stars.
m - Medium rich cluster with 50-100 stars.
r - Rich clusters with over 100 stars.
If the cluster any type of nebulosity, the letter "n" is added to the last parameter.
First up is Messier 34, located in the constellation of Perseus. Consisting of over 100 stars, in a space of a diameter of 14 light years. It is some 1,500 light years distance and shines at an apparent magnitude of 5.5 at an estimated age of 200 million years.
It has a Trumpler classification of I 3 m (Detached clusters with strong central concentration, Cluster is composed of bright and faint stars, Medium rich cluster with 50-100 stars)
Right ascension: 02h 42.1m
Declination: +42°46′
Next is Messier 35, located in the constellation of Gemini. A large open cluster, it contains over 500 stars in a space of a diameter of 24 light years. It's distance is around 2,800 light years away and shines at an apparent magnitude of 5.3. It is estimated to be 110 million years old.
It has a Trumpler classification of III 3 r
Right ascension: 06h 09.1m
Declination: +24°21′
Messier 36 Auriga
Then we have Messier 36, located in the constellation of Auriga. A smaller cluster with 60+ members in a diameter of 14 light years. At a distance of 4,100 light years, its one of the fainter Messier open clusters at an apparent magnitude of 6.3. A younger cluster with an estimated age of 25 million years.
The cluster is classified as Trumpler type I 3 m
Right ascension: 5h 36m 12s
Declination: +34°08’04”
