NGC 2158 is a 2-billion-year-old open cluster in the northern constellation Gemini. It was initially thought to be a globular cluster but is now known to be an intermediate-age, metal-poor open cluster. The reason astronomers believed NGC 2158 was a globular cluster, was due to its unusually large number of stars in the cluster and their symmetrical distribution, but since the cluster does not show the central density that a globular cluster typically shows, suggests that this is an intermediate age cluster, transitioning from an open cluster to a globular cluster. Because this cluster is roughly 2 billion years old, the hottest stars left in the main sequence are F0-type and it is heavily populated by orange and red giants. This, combined with the fact that most of NGC 2158’s bright blue stars have self destructed, leaves the cluster light to be dominated by older yellower stars, giving yellow glow.
NGC 2158 was first discovered by William Herchel in November of 1784, with his self-made 18.7 inch reflecting telescope. It is a fairly well-known open cluster, situated very close to M35 (NGC 2168).  It is also a very dense cluster with several thousand stars in an area of about five arcminutes. From Saskatoon at this time of year, NGC 2158 becomes visible around 9pm (CST), 52° above the south-western horizon, it then sinks towards the horizon, setting around 3am.
To analyze the relationships and characteristics between different physical properties of the cluster, I collected 15 images of the cluster using Skynet software and telescopes. The images were taking using the B(blue filter), V(green filter), and R(red filter) filters and the PROMPT6 telescope located in Cerro-Tololo, Chile. The total exposure durations were 47.24s for images in the B filter, 27.56s for images in the V filter, and 25s for images in the R filter. Once I collected my images, I used Afterglow to create a tri-coloured image of NGC 2158. The images first have to go through a series of operations including calibrating, stacking, aligning, and colouring before the tri-colour calibrated image can be obtained. After performing all those necessary steps, I created the colour calibrated image below:
(Reddened Image)
After collecting this image, I used Cluster Astromancer combined with my previous data to do a full analysis of NGC 2158. After going through another multi-step process of removing field stars and importing data from GAIA, 2MASS, and WISE, I was able to determine certain characteristics about my star cluster through Isochrone Maching. The basic results I found for NGC 2158 included: RA: 91.86, Dec: 24.2, pmRA: -0.2, pmDec: -1.96, Distance: 3.5kpc, Log (age): 9.8, Metalicity: -0.35, and E(B-V): 0.43. I then used these values, along with the Isochrone Matching, to create an RP vs (BP-RP) HR diagram of NGC 2158:
(RP vs (BP-RP) HR Diagram of NGC 2158)
The next step of this analysis was to apply the data determined through Cluster Astromancer, to the colour image of the cluster previously developed through Afterglow. Using the determined E(B-V) value, I returned to Afterglow to once again use photometric calibration to de-redden the previous image. By applying my E(B-V) value of 0.43, I got this de-reddened image of NGC 2158:
(De-reddened Image)
Lastly, my analysis consisted of comparing my data with the data previously obtained by the Milky Way Star Clusters Catalogue. Substituting the given MWSC parameters into Isochrone matching, (same way I made my previous HR diagram), I was able to make the RP vs (BP-RP) HR diagram (MWSC version) below:
(RP vs (BP-RP) HR diagram using MWSC data)
Now that I had both HR diagrams, one using my determined data and one using previously determined MWSC data, I was able to compare them. My initial comparison was that the values used in both diagrams were quite similar. I had used a slightly older age, but the metallicity and E(B-V) values were the same or very close. Looking closely, there is an extra little bend in the MWSC diagram which is caused by their data having a younger age.
Overall, I found this project to be very interesting and enjoyable. Having studied a star cluster from start to finish, I have a deeper understanding of the meaning behind certain parameters and an appreciation for this line of work. It has been a memorable experience and definitely sparked my curiosity to explore astronomical objects further!