Photometry of Star Clusters
In this project, me and my teammates collected data from three differently aged star clusters in order to analyze the stars within and determine the relative age of each. Not only that, but we were also able to use this data to create estimates on the average solar metallicity, as well as reddening of each cluster. After studying these clusters for so long, we were then able to take just these images and compare them to images from other teams to organize them by age completely by eye! Below is the process we used to take, edit, and process these images.
For NGC 3293, the young cluster, we used the telescope PROMPT 6. We took a total of 5 exposures per filter with 30-minutes delays between each set of B, V, R, and I filters. The total observing time for each filter are as follows: B = 31.5 seconds, V = 23.6 seconds, R = 11.8 seconds, and I = 23.6 seconds. The reasoning for spreading out our exposure times is to a) prevent “ghost” images of brighter objects that may be captured in the observation and b) allow other classmates time to collect their data as well!
For NGC 3330, the intermediate cluster, we used the telescope PROMPT 6. We took a total of 5 exposures per filter with 30-minutes delays between each set of B, V, R, and I filters. The total observing time for each filter are as follows: B = 118.1 seconds, V = 59.05 seconds, R = 78.75 seconds, and I = 59.05 seconds. The exposure lengths for each filter vary because we need to collect quality data that consists of just enough time to capture the cluster’s dimmer stars without overexposing the brighter stars. This allows us to collect the unique photometric measurements through the wide array of magnitudes that each cluster contains.
For NGC 4833, the globular cluster, we used the telescopes PROMPT 5, PROMPT 6 and PROMPT-MO-1. We took a total of 5 exposures per filter with 30-minutes delays between each set of B, V, R, and I filters. The total observing time for each observation are as follows: Prompt 5: B = 176.48 seconds, Prompt 5: V = 176.46 seconds, Prompt 5: R = 176.46 seconds, Prompt 5: I = 78.44 seconds, Prompt-MO-1: I = 51.95 seconds, Prompt 6: B = 141.74 seconds, Prompt 6: V = 94.48 seconds, Prompt 6: R = 94.48 seconds, Prompt 6: I = 62.5 seconds.
For each of these star forming regions, we aligned, stacked, and color balanced them the same way:
We first loaded all twenty exposures from the observation into Afterglow. Then, using the align tool, we selected all twenty images and used WCS Calibration to align them. Next, we selected the five exposures from each of the four filters (I, V, B, R) and used the stack tool with Chauvenet rejection and propagation mask enabled. With the four total stacks, we group them into one file and change the color map to its respective filter.
Now we have a full color image. To make the colors in the image more accurate, we used the histogram fitting tool. With photometric calibration open, we measure the zero points and select our reference layer. For all three of our clusters, we used the B filter layer as our reference layer. With that, we have a full color calibrated image!
After creating the colored images, we can use the batch photometry tool in Afterglow to create a CSV file measuring the brightness of each star in the red, green, and blue wavelengths of the images. We loaded those CSV files for each cluster into the Skynet graphing tool in Cluster Pro Plus mode to generate a diagram that shows the ages of the individual stars in each cluster (an HR diagram). Using the option in the top right of the graphing tool, we can narrow down the stars that are shown in the diagram to just the ones in the cluster, completely ignoring the field stars. We can use this data to estimate the age, solar metallicity – how much metal (anything that isn’t hydrogen or helium) is in the stars on average, and reddening – how much dust is between us and the cluster – of each image. We also used data from the space telescopes GAIA and 2MASS to get the most accurate measurements possible, as well as near-to-mid infrared measurements of each star in each cluster. We then tweaked the values of age, metallicity, and reddening until the black line on the graph – the “turnoff” point, or the description of how stars change over time for stars in that cluster – lined up with the stars in our diagrams. We then compiled the data into 3 separate graphs shown below. The diagrams on the left shows how the stars change as they age in our visible wavelength (B, V, R), while the diagrams on the right shows how they change in the near-to-mid infrared wavelengths (J, H, K).
Graph credit from top to bottom: Andreas Buzan, Alyssa Manus, Ruby McGhee
Our average values for each cluster are as follows:
For NGC 3293 (the youngest cluster)
- Age: 7.1275 +- 0.1297 log(yr)
- Metallicity: -0.1525 +- 0.1717 solar metallicities
- Reddening: 0.285 +- 0.03872983346 magnitude
For NGC 3300 (intermediate cluster)
- Age: 7.67 +- 0.136381817 log(yr)
- Metallicity: -0.5875 +- 0.2399131787 solar metallicities
- Reddening: 0.18 +- 0.03741657387 magnitude
For NGC 4833 (old, globular cluster)
- Age: 10.0975 +- 0.07135591543 log(yr)
- Metallicity: -1.9075 +- 0.3863827981 solar metallicities
- Reddening: 0.2625 +- 0.02753785274 magnitude
Age is measured in log(years) (billions of years), metallicity in solar metallicities (how much more metal there is in each star on average compared to our sun), and reddening in magnitude (how much dimmer it looks from Earth than the cluster would look without any dust in the way).
In the young cluster, there are many more hot and bright stars compared to the other clusters: this is indicative of a lot of young massive stars. These stars are often blue in appearance as suggested by the diagram, and as much is evident in the image. As well, there are some redder—or at least less blue—stars in the cluster, which are likely young Sun-like stars. Massive stars burn brighter than Sun-like stars because they exhaust their fuel much more quickly than Sun-like stars, and since they use fuel more quickly, they appear bluer and have hotter surface temperatures, just like smaller flames. As such, it makes sense that a cluster roughly 10-17 million (10^(7.1275 +- 0.1297)) years old would look this way: the massive stars are quickly expending their supply of hydrogen in their youth while the smaller Sun-line stars expend their hydrogen more slowly and thus are smaller.
In the intermediate cluster, we see a much more varied mix of stars with fewer blue ones and comparatively more red ones. As well, the stars are much cooler, which makes sense given the context from the discussion of the young cluster. This suggests that there are either many Sun-like stars in this image, or that many of the massive stars are reaching the limits of their hydrogen supply and must begin fusing helium to keep from collapsing. These helium burning stars become red supergiants that are very big and burn very brightly, but they have reduced surface temperatures compared to when they were burning hydrogen. In the HR diagram, the two stars seemingly departed above the lower part of the isochrone are these supergiants. Being roughly 35-63 million (10^(7.67 +- 0.136381817)) years old, it only makes sense that the stars are getting redder, and that is exactly what we see.
Finally, in the globular cluster, we see many yellow and red stars, but some blue stars still remain. At this point, the Sun-like stars are very mature and very bright, but also very cool (as far as stars go); these red supergiants make up the top right portion of the graph. On the left side of the graph are many stars that were once red giants that have now begun to fuse helium and as such are brighter, hotter, and bluer. With time, these helium fusing stars lose mass and become the white dwarfs visible near the bottom of the diagram. At this point, there aren’t really any more massive stars in the cluster, because the life cycle of massive stars is markedly shorter than that of Sun-like stars, and being that the cluster is 10-13 billion (10^(10.0975 +- 0.07135591543)) years old, this result isn’t unexpected.