NGC 5139 is a large globular cluster, given the more common name Omega Centauri and is the most massive-known globular cluster in the Milky Way Galaxy. It is located in the constellation of Centaurus in the southern sky, at a distance of around 4.8 (+- 0.34) kiloparsecs from Earth, and contains approximately 10 million stars. Unlike most globular clusters, Omega Centauri contains a variety of star ages, and is 10 times the mass of other common globular clusters.

Since this cluster is visible to the naked eye, it isn’t properly documented who first discovered the cluster, but early evidence of this object was written around 150 AD by astronomer Claudius Ptolemy in his writing the Almagest. Omega Centauri gained its name from Johann Bayer, a German cartographer who used the writings of Ptolemy to designate NGC 5139 as “Omega Centauri” in his publication Uranometria, written in 1603. Omega Centauri was coined to be a nebulae by Edmond Halley in 1677, until John Herschel properly identified it as a globular star cluster in the 1830s.

Using the Prompt6 telescope on Skynet I exposed 15 images in total, with 5 images each in the B,R and V filters. By aligning and stacking the images in each respective filter on top of each other in the Afterglow software, I was given these images in the B,R and V filters respectively:

B:                                                                      R:                                                                        V:

Exposure length in seconds (s):

47.24s                                                              23.62s                                                                11.81s

Combining these filters together to create a realistic image of the cluster through our atmosphere gives you the following colour image:

Like stated earlier, this is the view of Omega Centauri through our own atmosphere, that discolours the image true to how it really looks. To fix the ‘reddening’ on the image that appears due to outside factors such as the Earth’s atmosphere, I used Cluster Astromancer to determine the appropriate reddening value E(B-V) for Omega Centauri.

This tool is used to crossmatch the stars that I photometred in Afterglow with the most recent catalogue datasets from GAIA, 2MASS, WISE and APASS, and uses their distance and PM data to allow for the removal of field stars. In my case I only used the datasets from Gaia and 2MASS. Astromancer allows you to set the appropriate PM RA and PM DEC range for the star cluster, so you are able to remove the field stars that are not apart of the globular cluster. Combining this ability with the Isochrone Matching tool allowed me to determine the appropriate Proper Motion values, giving me an isochrone line that is used to determine the proper values for distance, log age, metallicity, and the reddening value E(B-V). By referencing 4 plots, and choosing a line of best fit amongst all of them, I determined the appropriate values below

Values determined by my best fit isochrone lines:

Proper Motion RA: -3.31 mas/yr

Proper Motion Dec: -6.74 mas/yr

Distance: 5.53 kiloparsecs

Log Age: 10log(yrs)

Metallicity: -1.35 [Fe/H]

E(B-V): 0.12 mag

 

Plots used in order: (RP vs BP-RP), V vs B-V, R vs B-R, and H vs J-H

Looking at the first graph, we can see that there are white dwarfs near the bottom left-most of the graph, as well as a horizontal branch and some blue stragglers, and different giants in NGC 5139. Their appearence on the graph makes sense because Omega Centauri has an interesting population of several different star ages, and contains 10 million stars ranging from 10-12 billion years old.

These graphs allowed me to determine the reddening value E(B-V) of 0.12, and by adjusting the previous colour image of Omega Centauri above in Afterglow using this value, the ‘true’ appearence of Omega Centauri looks like the following:

As you can tell, the ‘reddened’ images makes the cluster appear more warm and yellow/red than it actually is.

A similar full resolution image of Omega Centauri can be found here, posted by GoldfieldAstro

 

Professor William Harris (Harris 1996 (2010 edition)) also studied this star cluster and submitted his information to the Milky Way Stars Clusters (MWSC) Catalogue, and found relatively similar results to what I had concluded.

Distance: 5.199 kiloparsecs

Log Age: 10.1 log(yrs)

Metallicity: -1.445 [Fe/H]

E(B-V): 0.125 mag

 

Side-by-side comparison of plots using my values (left) vs the MWSC’s (right)

RP over (BP-RP) Models:

H over (J-H) Models:

Visually the MWSC Catalogue isochrone line fits more accurately than what I had concluded, which concludes that some if not all values are more correct than the ones that I had determined. However, comparing the values that I had determined to the MWSC values, there are some similarities. The reddening values are quite similar, with a difference of 0.05 magnitude, similarly the log age is a tenth off of eachother.

So while I conclude that my findings were not the true values for NGC 5139, I feel that my analysis is adequate, and supports the current findings of the ‘correct’ data for NGC 5139.

My cluster project was very interesting, and I’m glad I picked such an interesting looking cluster without knowing much about it. It gave me a new appreciation for all the other different types of globular clusters, many of which aren’t quite as big or well-researched as Omega Centauri.