Open Cluster Classification

February 2022 - Nebula and Cluster of the Month

The criteria I use for nebulae and clusters to be included for a particular month are that they should culminate within an hour of midnight on some date during the month. This effectively means that the Right Ascension (RA) of the object must lie within a two-hour band. I also tend to favour objects north of -30° declination.

I have in the past struggled with February. There are really only two open clusters of any note in February’s RA band – Messiers 44 and 67, in Cancer. Both these objects are so well known that they fail to make my other criteria – that they should be good but obscure objects, or that I have something interesting to say about them.

Nebulae are equally absent in February. I find myself therefore in a bit of a quandary, so this month I’m going to discuss how we categorize open clusters. I frequently quote the Trumpler classification for the open clusters that I discuss, but what exactly do those classifications mean?

The first person to attempt to classify open clusters was probably our old friend William Herschel, who gathered his deep-sky discoveries into eight different catalogues. Catalogues VI, VII and VIII were for open clusters of differing visual appearance. VI was for ‘very compressed and rich clusters of stars’, VII was for ‘pretty much compressed clusters of large or small stars’, VIII was for ‘coarsely scattered clusters of stars’.

A later, though barely more complex, system was introduced by Harlow Shapley. He suggested eight classes: (a) – field irregularities, (b) – star associations, (c) – very loose and irregular, (d) – loose and poor, (e) – intermediate rich, (f) – fairly rich and (g) – considerably rich and concentrated.

A class (a) object could simply be a statistical irregularity within a certain patch of sky, slightly more stars than would be expected on average. Class (b) objects would include such systems as the Ursa Major moving group. Shapley included the Hyades and the Pleiades in his class (c), M21 and M34 in class (d), M38 in class (e), M37 in class (f) and M71 (a globular cluster) in class(g).

Enter Robert Trumpler, who recognised that open clusters had more complex morphologies than could be described in Shapley’s system. He broke his classification system into three distinct properties: concentration/detachment, the range of brightnesses of stars within the cluster and the richness (number of stars) of the cluster.

The word ‘detachment’ refers to how easily differentiated from the background star field the cluster is. Obviously, this is closely allied to, but not wholly dependent on its concentration.

The concentration or detachment part of the classification is given first, in Roman numerals.

Detached clusters with strong central condensation
Detached clusters with little central condensation
Detached clusters with no discernible central concentration
Clusters not well detached from the surrounding star field

The range of star brightnesses is given next, in Arabic numerals:

Most stars are of similar magnitudes
There is a moderate range in brightnesses
The cluster is composed of both bright and faint stars

The third part of the classification refers to the richness of the cluster, defined by the number of members. This is given as a lower-case letter:

poor (fewer than 50 stars)
medium (between 50 and 100 stars)
rich (more than 100 stars)

These categorizations are determined usually by a visual inspection of a photograph of the cluster. If there is nebulosity associated with the cluster, then an additional ‘n’ will be placed after the richness indicator.

The system is flexible but is beset with problems. How for example, does one define how strong the central condensation is? What exactly does ‘moderate range’ mean for brightnesses, and how can you be sure how many stars in the field are cluster members?

This inevitably leads to differences in quoted Trumpler types from different sources. I’ve pointed some of these out in previous months. When I make an observation of an open cluster, I tend to have a go at assigning a Trumpler classification based on my observation, rather than on what may be seen on a photograph or image. This can be a fun addition to the process, and I recommend it to you. It can improve your observing technique as you try to classify the object in your field of view, making you look ever more closely and critically.

Have a look at this image of NGC 1245 in Perseus.

An image of open cluster NGC 1245 provided by Gregg Ruppel
An image of open cluster NGC 1245 provided by Gregg Ruppel. Click this link for a larger version of this image in a new tab.

The following Trumpler classifications are given to it just in books that happen to be on my desk at the moment:

  • II2r
  • III1r
  • III2r
  • III3r

At least everyone agrees that it’s a rich cluster, but does it have little central condensation (II) or no discernible central condensation (III)? Where, for that matter, is the centre, exactly? Are the stars all the same magnitude (1), of moderate range (2) or with both bright and faint stars (3) (but the range isn’t ‘moderate’, whatever that means)?

I have my own opinion; I’d be interested to hear yours.

Patrick Maloney