Star clusters are large groups of stars held together by the gravitational force they exert on themselves. There exist two main types of star clusters – globular clusters are tight groups of ten thousand to millions of old gravitationally bound stars, and open clusters are groups of tens to a few thousand stars formed from the same molecular cloud, thus, share many properties including age, distance, metallicity, and motion.
NGC 2158 is one of the 1,100 open clusters discovered in the Milky Way galaxy. In the constellation of Gemini, NGC 2158 is located immediately to the southwest of the open cluster Messier 35. Discovered by William Herschel on November 16, 1784, this metal-poor open cluster is believed to be about 2 billion years old. It is a genuine member of the old thin-disk population, making it an interesting enough object to get the implications of both Galactic and Stellar Evolution theories (Carraro et al. 2002). In addition to this, NGC 2158 is similar to the populous intermediate-age open clusters in the Large Magellanic Clouds (Christian et al. 1985).Â
To study this open cluster and investigate the relationships between the colours, brightnesses, and the age and chemical composition of the cluster as a whole, I collected 15 images in the B (Blue), V (Green), and R (Red) filters with the PROMPT6 telescope located at the Cerro Tololo Inter-American Observatory in Chile, with total exposure durations of 61, 36 and 26 seconds in the B, V, and R filters respectively, using Skynet.
Using Afterglow, I photo-metered and stacked the images in each filter to produce a tri-colour image of the star cluster.
Then using Skynet’s Cluster Astromancer to analyse the Afterglow photometry results, following field star removal, archival data fetching and isochrone matching steps, I found the following information:
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NGC 2158 |
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| Est. PM in RA (mas/yr) | Est. PM in RA (mas/yr) | Distance (kpc) | Log(Age) | Age (in M years) | Metallicity (solar units) | E(B-V) (mag) |
| -0.19 | -1.96 | 4.64 | 9.4 | 2511.89 | -0.15 | 0.26 |
These HR diagrams correspond to the values above.
The calculated E(B-V) value was used to produce the de-reddened image of the cluster.
The final step in this cluster analysis involved a comparison with the Milky Way Star Clusters Catalog values. The MWSCÂ is the result of a global survey of clusters in our Galaxy, published in 2012-2015, with cluster parameters estimated through isochrone modelling. The MWSC Catalogueâs values and graphs for NGC 2158 are as follows:
| NGC 2158 | |||
| Distance (kpc) | Log(Age) | Metallicity (solar units) | E(B-V) (mag) |
| 4.770 | 9.33 | -0.25 | 0.333 |
Comparing these previously catalogued values with the values calculated using Cluster Astromancer for NGC 2158, it can be noted that these values are pretty close to each other (the difference in the values presented in the table above and the found values in this lab is 0.13, 0.07, 0.1, 0.73 respectively). Also, visually analysing the graphs, we can say that they are comparable with only slight variations in the isochrone model fit line.
My interest in this project stemmed from the role that star clusters play in unravelling mysteries related to stellar evolution and galactic dynamics. It provided an opportunity to learn about photometry and hands-on and comparison-based analysis; producing coloured images of the cluster was a remarkable experience. Even though this project was tedious and challenging, it was ultimately an enriching experience that broadened my knowledge of stars and their properties.