Dispersed throughout the Milky Way are thousands of galactic (or ‘open’) star clusters, so named because of their prevalence along the galactic plane of our galaxy. Open clusters contain tens to thousands of stars and are distinguished from the more tightly bound globular clusters whose millions of stellar constituents form a dense and spherical swarm. By comparison, open clusters are much looser, and on cosmological time scales, are much shorter lived. Within 10-50 million years, most star clusters disperse due to the differential rotation of the galaxy and interactions with interstellar clouds, though some denser clusters are believed to continue to last much longer. The most prominent examples of open clusters visible to the naked eye are the Pleiades (M45) and the Hyades cluster, both in the constellation Taurus. Our own solar system began its existence within a cloud-forming open cluster region that has now long since dispersed.
The identification of open clusters is of great interest to Astronomers because the stars within a cluster are approximately the same age and share the same chemical composition (metallicity), providing a mechanism for calibrating color-luminosity relationships that are the foundation for certain distance metrics, as well as leading to better insights into the evolution of stellar populations. Cataloging the positional distribution of open clusters also yields a clearer understanding of the spiral-arm structure of the Milky way and galactic dynamics in general. While it is estimated that as many as 100,000 open clusters exist within our own galaxy, fewer than 2,000 have actually been catalogued.
The history of this effort to catalogue open clusters is rather interesting in and of itself. Messier’s catalogue of stellar objects (published 1784) contains 27 open clusters (or between 26-28 depending on whether you include certain nebulous star-forming regions.) As is well known, Messier intended his catalogue as a guide to fellow comet-hunters who might otherwise mistake such objects for comets, many of them appearing as mere patches of light when viewed with a small telescope. The New General Catalogue published by J.L.E. Dreyer in 1888, including the supplemental index catalogues of 1895 and 1908 brought the number of identified open-clusters up to over 700. (See the NGC/IC project for further discussion on efforts to resolve on-going discrepancies within the catalogue, and further historical background.) Throughout the 20th century, new clusters have been identified in small handfuls, and this process is still going on today as evidenced by recent publications found on ArXiv.org. Thus by 1987, we have the Lynga catalog of open cluster data (5th ed.) containing data for 1,151 clusters, subsequently updated by the New catalog of optically visible open clusters and candidates (Dias et al., 2002) currently containing 1,776 data records, and on it goes.
Now you might imagine that recent sky surveys would lead to an explosive growth in the number of catalogued open clusters, but from an automated image-processing point of view, the problem is actually not so easy. First, you have to identify groups of stars whose apparent concentration exceeds the background average, taking care not to ‘discover’ a mere random fluctuation. Secondly, because open clusters represent locations of recent star formation, they are, by their very nature, found amongst the light-obscuring dust clouds of the galaxy’s spiral arms, and are thus totally obscured at visible wavelengths. With regard to this later issue, near-infrared surveys, like the 2-Micron All-Sky Survey (2MASS) conducted over the past decade that can detect radiation at wavelengths transparent to these obscuring effects become a critical resource for discovery.
The Dias open cluster catalog mentioned above and almost 7,000 other astronomical catalogues can be accessed directly from the Centre de Donnees Astronomiques de Strasbourg (Strasbourg astronomical data center or CDS). Clicking on VizieR in the main menu will bring up the catalog search form:
Searching for “Dias Open Cluster” and selecting the top result “Catalogue Data File (V2.8) (1776 rows)” brings you to a data search page where you can query the specific catalog and specify which data elements you wish to display.
For purposes of this demonstration, we do the following:
- Change “Maximum Entries per Table” to “unlimited” so that we can extract the entire table at once.
- Under output preferences for Position, also check “Galactic”
- On the left hand column, we also check Metallicity, [Fe/H], and color excess in BV, E(B-V)
And now press the “Submit Query” button. The full table with all 1,776 records appears in HTML format.
At the bottom of the table is a link: “Plot the results with the VOPlot utility”. This is where the fun really begins! The basic idea is to plot various columns, looking for interesting patterns, correlations, or outliers that are indicative of a particularly unusual cluster. Here we show the positional location (galactic latitude/longitude) of the clusters, verifying that clusters are predominantly located within the galactic plane.
Plotting distance (parsecs) on the Y axis -vs- diameter (arcmin) on the X axis shows, not surprisingly that more distant clusters appear smaller. The outlier in the group, at the lower right-hand corner is Collinder 285. The stars of the big dipper are part of an open cluster! This was actually discovered in the late 1800′s when it was realized that these stars share the same spatial motion. The cluster includes Sirius and some other prominent stars – our sun is not a part of the cluster but rather appears to be just ‘passing by.’
Finally, we plot color excess, E(B-V) ,on the Y axis –vs- galactic latitude on the X axis. An astronomical object’s color excess is basically a measure of the reddening of an object due to the absorption of light from interstellar gas and dust that are particular prevalent along the galactic plane. Thus, E(B-V) values are highest for objects that are a) close to the galactic plane and b) very distant. The point at the sharp peak of the distribution is Westerlund 1, shown below from a showcase image produced by the aforementioned 2MASS survey – a great example of a distant object barely observable in the visible part of the spectrum, but highly prominent in the infrared. Discovered in 1961 by Bengt Westerlund, it is now recognized as being the most massive young open cluster ever observed.
This exercise in arm-chair data-mining showcases the great potential for amateur astronomers to access and analyze astronomical data with a view towards understanding both the typical properties of a particular class of object as well for identifying outliers whose unusual properties warrant further investigation.
Posted by John Rachlin 
