Open Star Cluster Observations

One of the joys of being a visual  observer is in describing how an object appears. This description will vary according to the light (dark) conditions, the magnification, the field width and the dark adaption of the observer. It will also vary based on the amount of time an observer spends directly observing the object. It is pretty safe to say that no two observations will be exactly alike, but each are equally valuable as a description. So, this week, I want to talk a bit about open star cluster descriptions and how I go about recording the visual image

My first step in observing an open cluster is to inspect the cluster under very low magnification and a wide field of view. For me this is my 30mm wide field which yields 102 arc minutes at 40X magnification. With this eyepiece, I inspect how the cluster is situated within the star field. Is it highly concentrated with respect to the surrounding field, or is it fairly loose and almost indistinguishable against the field stars? Robert Trumpler developed a classification system for describing this. Trumpler assigned a Roman Numeral to describe the concentration and detachment of the cluster. Here is the classification that Trumpler used:

     I Detached with strong central concentration
    II Detached with little central concentration
   III Detached with no central concentration
   IV Not detached but apparently like a strong field concentration

At this point, I increase the magnification so that the cluster fills the majority of the field of view of the eyepiece. Using this higher magnification, I record the dimensions of the cluster. If the cluster is not circular then I describe the shape and which direction it is elongated in.

At this point, I estimate the number of stars that appear to be associated with the cluster. If the cluster is not detached from the background, or foreground stars then this can be difficult. Trumpler divided clusters according to the number of stars:
 
    p     less than 50 stars
    m    50 - 100 stars
    r      more than 100 stars

Trumpler used the Lick Observatory equipment for his survey, so don't expect to come up with the same number of stars that he did. Often if the cluster is very rich in faint stars, then it appears more as a mottled effect. If this is the case then I just record it as such.

The next aspect to observe is the variety of brightnesses and the colors. The higher proportion of red stars indicate that the cluster may be older than a cluster with more blue stars.  Observe the brightness ranges. Are they all the same magnitude, or is there a significant difference in magnitudes. Trumpler used a scale of 1 to 3 to classify brightness differences 1 being very little difference in brightness and 3 being a large difference of several magnitudes.

So now for some examples:
NGC 1647 is an open cluster in  Taurus. I have described this cluster as having many stars, barely concentrated with a density higher than the background field. The stars are mostly the same brightness.

Trumpler classified this cluster is II-2-r which translates to detached with little central concentration, minimal range in brightness, and more than 100  members.




NGC 2360 is an open cluster in Canis Major. I described this as " Highly concentrated open cluster, 2 dozen brighter stars, many,many fainter stars. density higher than surrounding area"

Trumpler classified this as II-2-m - Detached with little concentration, 50 - 100 stars, minimal range in brightness.






NGC 457 is a really nice OC in Cassiopia. My description goes like this: Nice irregular shaped open cluster. Elongation N-S direction with 34 Cas and its companion on South side. The double is a yellow and green pair. Many stars of different brightness, slightly concentrated with density higher than surrounding area.

Trumpler classification is I-3-r - detached with strong central concentration, large brightness differences with more than 100 stars.




The trick with the descriptions is to give as much detail as possible. You will find that the more descriptive you are, the more you will remember these little gems.

Next time I will talk about getting ready for a star party....

Clear skies,

rw