NGC 4833
NGC 4833 is an old globular cluster located in the southern constellation Musca. It was discovered in 1751-1752 by the French astronomer Nicolas-Louis de Lacaille during his two-year journey to South Africa. NGC 4833 is one of the oldest clusters known in the Milky Way. Its age is estimated to be around 12.54 billions of years and it is a metal-poor cluster.
NGC4833 contains an important population of variable stars such as RR Lyrae and eclipsing binaries. Some of them have been studied by Kopacki G. in 2014. Using the 40-inch telescope at Siding Spring Observatory in Australia, Kopacki used 740 V-filter and 220 IC-filter CCD frames to confirm the existence and the nature of variable stars already discovered by Darragh A. N. and Murphy B. W. in 2012. Furthermore, he also discovered “two new SX Phoenicis stars, one new RR Lyrae Star, twelve new eclipsing systems mostly of the W Ursae Majoris type, nine new variable red giants at the tip of the RGB, and ten field-stars showing irregular variations” (Kopacki G., 2014).
The goal of this project was to collect photometric data from images taken with telescopes and use them to identify different characteristics of one chosen cluster. In order to do so, I collected a total of 15 images using the Prompt-6 telescope located in Cerro-Tololo with B, R and V filters. The exposure lengths were 70.87s, 47.24s and 47.24s for B, V and R filters respectively. 5 images were taken for each filter with at least a one hour gap between each image.
To study the cluster and make a coloured image of NGC4833, the 5 images in the B and V filter were used but only 4 out of the 5 taken in the R filter could be used. Images were processed and stacked using the Afterglow software which permitted me to obtain a set of photometric data and stacked images of each B, V and R filter.
The three images grouped altogether gave the following apparent coloured image of NGC 4833 :
The photometric data obtained in Afterglow was used in Astromancer to determine the different characteristics of NGC 4833 such as its age, its distance from Earth and its metallicity for example. Using the data collected in Afterglow as well as the GAIA and 2MASS catalogues, I could obtain these 4 HR Diagrams :
And from these, the following basic results for NGC 4833 :
| Proper Motion RA
(mas/yr) |
Proper Motion Dec
(mas/yr) |
Distance
(kpc) |
Log Age
(yrs) |
Age
(Gyrs) |
Metallicity
(solar) |
Reddening/E(B-V)
(mag) |
| -8.4 | -0.97 | 6.45 | 10.1 | 12.589 | -1.85 | 0.32 |
Analysing these diagrams, one can deduct the presence of red giants and the horizontal branch suggests the presence of numerous blue stars : some of them can easily be spotted in the following de-reddened image of NGC 4833.
This de-reddened image was obtained by adding the E(B-V) coefficient determined in Astromancer into the photometric calibration tool in Afterglow.
Finally, I compared my final results with the data collected between 2012 and 2015 in the Milky Way Star Clusters Catalogue by Kharchenko & al.
The data used to build these models were the following :
| Distance (kpc) | Log Age (yrs) | Metallicity (solar) | Redenning (mag) |
| 6.59 | 10.1 | -1.75 | 0.32 |
The Isochrone models obtained are really similar with mine since the values used to build differ only for the metallicity and the distance. One might spot a slightly different curve near the RGB part of the model but overall, both models look alike. Consequently, my analysis did not really improve upon the previously published values for NGC 4833.
This project has been very interesting and provided a lot of knowledge about star evolution and the process to study star clusters. This reinforced my idea to study Astronomy and Astrophysics and work in this field later on.